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Softrock Lite 6.2
Adventures in Electronics and Radio
Elecraft K2 and K3 Transceivers
Notes on the Softrock
Lite V 6.2 Receiver Kit
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11 Feb 2010. Added measurements of phase and amplitude
comparison of 4 Triad SP-70 transformers
02 November 2009. Added frequency stability section.
13 July 2008. Pete, N4ZR, provided a Radio Shack PN 273-1374 transformer which
21 August 2007. Added information about using E-MU 0202 with Windows Vista.
19 August 2007. Added a note about E-MU 0202 drivers only available for Windows
28 July 2007. Added measurement data for Bourns LM-NP-1001-B1 600Ω : 600Ω
25 July 2007. Added measurement data for Triad MIL-T-27E-SP68 transformer
23 July 2007. Added details for broadband antenna isolation transformer.
22 July 2007. Added frequency response plot for Triad audio transformer.
11 July 2007. Quick comments on two software defined radio programs from I2PHD, Winrad and SDRadio appear below.
2007. Intermodulation performance analysis completed on Softrock Lite 6.2 30
meter band receiver. Details in the IP3 section below.
2007. I've installed M0KGK's SDR Decoder software. With my SX260, it's almost
impossible to run due to CPU consumption. Details below.
06 July 2007. I've completely rewritten this page to reflect
my experience with an E-MU 0202 sound card. My SX260 computer is still
CPU-bound, but there's a world of improvement with this sound card compared with
the built-in SoundMAX device.
One architectures I'm considering for the DSP-based panadapter is an I&Q
down-converting design. Conventional swept spectrum analyzer front end with a
first IF of perhaps 21.4 MHz, with the second IF selectivity and log conversion
via DSP. This has some similarity to the Softrock receiver, so I purchased
Softrock 6.2 Lite kits for 7 MHz and
10 MHz bands and built them over the last couple days. This page is collection
of notes and comments after very limited exposure to the Softrock.
For background on software defined radios, such as the Softrock, you might
start with the ARRL.
http://www.arrl.org/tis/info/sdr.html has a collection of relevant articles.
For an overview of quadrature sampling, check
I built both a 30 meter and 40 meter receiver. Due to a poor solder joint, I
thought I had damaged the 30 meter receiver's FST3253 mixer. That's not the case
and both receivers are now working correctly.
I've used two computers with the Softrock receivers:
- 2.8 GHz Pentium 4
- 2 GB RAM
- Graphics resolution run at 1600 x 1200 with 32 MB video RAM, shared from
the main 2 GB RAM.
- Internal 16-bit SoundMax soundcard and
- USB 2.0 E-MU 0202 sound card
Gateway NX860XL laptop:
- Intel® Core™ 2 Duo Processor T7200 (2.00GHz, 667MHz FSB, 4MB L2 Cache)
- 2048MB 667MHz DDR2 SDRAM
- 1920 x 1200 graphics with built-in 17" display
- Intel® Centrino® Duo Mobile Technology NVIDIA GeForce Go 7900 GS Graphics
w/ 256MB DDR Video Memory
- Built-in audio, which is not further described in Gateway's documentation.
Dell's SX260 is, for all practical purposes, a laptop board in a very small
desktop enclosure. It shares many components with Dell's Latitude series
machines of the same vintage. More importantly, it has no expansion slots—the
mother board has both the graphics display and sound chips and it's impossible
to add a new graphics card or internal sound card. I've standardized on the
SX260 machines and have three in my network, along with the Gateway laptop and
an older COMPAQ used as a file server. All run one version or another of Windows
I've used five software packages (all free) with the Softrock:
These programs have significantly different behavior and different pluses and
minuses, at least with my computers.
The kits are not projects for beginners. The PCB is
approximately 1.5" x 1.5" and in that 2.25 square inches per side are a lot of
parts. Resistors are vertically mounted to save space and the two inductive
components you must wind are on 0.25" cores. Parts are a mix of surface mount
and through-hole mount and are on both sides of the board. The photos below show
one of my completed boards.
I added the right angle headers to permit swapping boards
and for easier experimenting. I also mounted the crystal in a socket for the
same reason. If I were doing it again, I would use straight headers, not right
30 Meter band Softrock Lite 6.2. Top View
Assembly time is about 3 to 3.5 hours per board. The surface
mount capacitors are 1206 size parts, so they are not too hard to work with.
Likewise the four surface mount integrated circuits are 20 pins/inch, which is
within normal home workshop capabilities.
Builder's Notes do not provide "Heathkit type" step-by-step, "solder R1,"
instructions. Rather they are of the form "install the bottom surface mount 0.1
uF capacitors." This is not a problem for an experienced builder but may be
inadequate if this is your first kit. The Builder's Notes could benefit from
proofreading and editing, but these flaws are not critical.
I found the surface mount parts easier to install than the
through hole parts, by the way. This is partly a matter of having the right
tools and partly a matter of experience. If you have never built a kit
with surface mount parts, you might wish to start with one of the inexpensive
Norcal dummy loads. This Norcal kit has 46 surface mount parts (44 resistors, one capacitor and one
diode) and costs only $7.50, so by the time you finish building it, you'll have
plenty of surface mount practice.
I made two mistakes with the first receiver, installing one
resistor lead in the wrong hole and not making a good solder joint on one IC. The resistor body is indicated by a silk
screened circle around the pad and the associated hole for the free lead is
shown by a short line between the two pads. In several places the lines are not
easy to see, and there are other pads spaced the same distance as the
correct one. I wound up inserting the free end in the wrong hole, and caught the
mistake when I found the pad already used when it came time to install the
correct part. The soldering error was U5, the TLV2462 op-amp, pin 3 or 5.
Although I examined the solder joints with a 10X stereo microscope, I did not
find the bad connection until troubleshooting the receiver. Since I re-soldered
both pins 3 and 5 at the same time, I don't know which one was bad.
The builder has to wind two toroidal cores. One is an
inductor used in a low pass filter on the antenna input and the second is a
three-winding transformer to connect the antenna to the mixer IC. In the two
kits I built, both are wound on T25 (0.25 inch diameter) cores, with no. 30 AWG
wire. (Receivers for some bands use larger cores.) The receivers I built required 36 (30 meter receiver, 36T on T25-6 core)
and 38 turns (40 meter receiver, 38T on T25-2 core). This is tedious to wind on
a small core and in fact significantly exceeds Micrometals' single layer
specification of 24 turns for No. 30 wire on a T25 core. For 38 turns,
Micrometals' reference book calls for No. 34 AWG wire, which allows up to 41
turns in a single layer. (Micrometals is the main manufacturer of powdered iron
cores such as used in this kit.) I found not possible to wind the specified
number of turns in a single layer, so some turns are in a second layer.
Multi-layer toroidal coils have increased distributed capacitance compared with
a single layer coil of identical inductance, which in this application would
show up as degraded high frequency rejection in the low pass filter.
The transformer is easier to wind as it has fewer turns.
When first powered up, the 30 meter receiver would not
operate. Looking at the waveforms with an oscilloscope showed the 40.5 MHz
oscillator to be running and the 2N3906 oscillator buffer (Q2) to be running,
but with lower peak-to-peak voltage than the 40 meter receiver showed. The
74HC74 divider IC to which Q2 is connected was not toggling. When I substituted
a lower frequency crystal (30 MHz), however, Q2's output swing increased and the
74HC74 divider showed correct output. This lead me to suspect Q2 did not have
enough gain at 40 MHz.
The 30 meter oscillator runs at 40.5
MHz (remember, it is divided by four to get two 90° phase shifted waveforms to
drive the mixer in quadrature) , whilst the 40 meter oscillator runs at 28.2
MHz. The 2N3906 did not provide enough voltage swing to toggle the 74HC74 divider, so I
replaced it with an MPSH81 transistor. The MPSH81 part has Ft = 600 MHz, while
the 2N3906 Ft is 250 MHz. (The MPSH81 has a different pin-out, so it's not a
direct swap—you have to exchange the base and emitter pins.) Sure enough, the MPSH81
improved drive to the 74HC74 divider and it started toggling as it should.
From comments on the Softrock Yahoo group, I've noted
other builders have experienced the same problem, leading me to believe it is a
design error, with some receivers working and others failing, depending on the
parameters of the particular 2N3906 supplied.
30 meter receiver Q2 output with 2N3906 transistor. The
output waveform is 2.7V PP.
30 meter receiver Q2 output with MPSH81 transistor. Output
voltage is now 4.47V PP.
My initial work with the 40 meter Softrock receiver and the
Dell SX260 was troublesome in several aspects:
- Very high CPU resource loading, particularly with Rocky
- Large "zero IF" spike at the center frequency with both
Rocky and PowerSDR
Based on recommendations from Joe, K9HDE, I made several
changes in my setup:
- Purchased a new sound card. Since the SX260 limits me
to external USB cards, my research showed the E-MU 0202 to have decent
specifications for noise and sample rate.
- Packaged the receiver in an enclosure with isolated
antenna input and isolated audio output.
- Investigated and discovered why CPU resource
consumption is a problem with the SX260 computer.
Since the E-MU 0202 sound card has optional balanced audio
input, I set up the audio isolation transformers for balanced output.
Receiver packaging. The black blob near the antenna input
is a 1:1 isolation transformer wound on a binocular core, Type 61 material.
The two transformers are 600 ohm : 600 ohm audio
transformers. The outputs connect to shielded twisted pair cable, with 1/4"
tip/ring/sleeve phone jack connectors.
The enclosure is reused from an earlier project, so
there are some extra holes here and there.
E-MU 0202 USB sound card. The input levels are adjustable
via front panel knobs. In addition, LEDs show overload level conditions.
The rear panel has a variety of input connectors. I use
the two line level 1/4" tip/ring/sleeve phone jacks in balanced mode. One side
of the audio is applied to the tip and the other to the ring. The sleeve has
ground connection, with a slide switch on the bottom of the E-MU 0202 to lift
the ground connection if necessary for hum reduction.
The E-MU 0202 supports sampling speeds through 192
Note that the E-MU 0202 does not have drivers for Windows
2000, nor are Windows Vista drivers released yet. W2K will not be supported
according to comments I've found.
There is an
"official" work around to use E-MU 0202's Windows XP drivers with Vista at
Real Vista drivers for the E-MU 0202 are scheduled for 3Q
The Triad 5-58X 600Ω to 600Ω audio transformers I use to
isolate the Softrock's audio output have quite decent audio response, with a 3
dB bandwidth from 158 Hz to 74.83 KHz when measured with 600Ω source and
termination. Unfortunately, these transformers are no longer in production, I
The data is taken with an HP3562A Dynamic Signal Analyzer,
with tracking generator.
In the first plot, below, I've added a 560 ohm series
resistor to the tracking generator's 50Ω output to bring it up to approximately
600 ohms. Likewise, a 620 ohm resistor is applied across the transformer's
output, in shunt across the HP 3562A's high impedance input, so the transformer
sees approximately 600Ω on both the source and termination.
This measurement protocol duplicates how a 600Ω to 600Ω
audio transformer is normally used; both the driving source and destination
receiver have 600Ω impedances. But, is this the case when used to isolate a
Softrock receiver? And, if it isn't how does that alter the transformer's
Let's first look at the Softrock Lite 6.2's audio output
(other Softrock designs are similar, so this analysis is not limited to the 6.2
Lite). The audio output is taken from an op-amp, through a blocking capacitor.
The output impedance of an op-amp when running in a feedback configuration, as
is the case here, can be taken as close to zero. Hence, the receiver's output
impedance is that of the 0.1 μF capacitor. At 1 KHz, its impedance is 1.6 KΩ,
dropping to 160 Ω at 10 KHz and 16 Ω at 100 KHz.
Hence, we expect C16 and C17 to roll off low frequency
response when a 600 Ω isolation transformer is used. C16 and C17's size is not as
important when the Softrock works into a high impedance termination, such as
directly into a sound card. In this case, the 0.1μF capacitor's reactance is
small with respect to the sound card's input impedance, even below 1 KHz, and
hence will not cause appreciable low frequency roll-off until we get well below
So, what impedance does the transformer present to the
Softrock's I and Q output? I don't have time to measure the impedance today, but
in general, at low frequencies, a transformer operating into a high input
impedance load has its input impedance dominated by magnetization inductance.
This assumes the transformer has a relatively high coupling coefficient and the
leakage reactance is negligible. At mid-range and upper frequencies, the leakage
reactance dominates, combined with distributed capacitance, effectively turning
the transformer into a low-pass filter.
If you are
interested in how to analyze and model a real transformer, read Midcom's Tech
Note 82—Tips for Transformer Modeling, written by Dave LeVasseur and available
http://www.midcom-inc.com/Tech/pdf/tn82.pdf. Dave's technical note explains,
in simple language,
leakage reactance, magnetization reactance and other nuts and bolts of how a
transformer looks to the outside world. Of course, even more complex models of
transformer behavior are possible, and necessary in certain circumstances, but
you can go a long way with the simple model developed in Tech Note 82.
To simulate the Softrock, I ran a second frequency sweep
with the same transformer, but fed with a 0.1μF series capacitor, with the
50:600 Ω matching resistor removed. Hence, the driving impedance in the
following plot is 50 Ω in series with a 0.1μF capacitor. Although not the nearly
zero ohms of the Softrock's op-amp stage, it's close enough for the purpose. I
also removed the 620 Ω termination resistance, so the transformer's secondary
is loaded only with the 3562A's 1MΩ input impedance, plus, of course, its input
capacitance and the capacitance of a 3' length of coaxial cable test lead.
The resulting plot, below, certainly looks different than
the first plot. As expected, the 0.1μF series capacitor rolls off the low
frequency response quite a bit below about 700 Hz. But more objectionable is the
resonance around 850 Hz where the 0.1μF resonates with the transformer's
inductance causing a 10 dB response peak, up through three or four KHz. (If you
read Tech Note 82, you should understand why this peak occurs.)
There's also a bit of resonance showing at the upper
Whether your particular isolation transformer will have a
similar resonance with the Softrock's output coupling capacitors depends on the
transformer, and your sound card's input impedance. My E-MU 0202 card has either
a 1.5KΩ or 1MΩ input impedance, depending on which amplifier is selected.
Presumably, operating my E-MU 0202 with the 1MΩ amplifier risks a peaked
frequency response, although the exact location and range of the peak will not
necessarily match the plot below.
Response peaks are quite sensitive to load, and if we look at
the same setup, but with 1500 ohms (representing the E-MU 0202 card's low-Z
input impedance) terminating the transformer, the picture is
The low frequency roll-off is still
present, of course, but the peak has vanished at both the high and low end.
In fact, as the expanded plot below reveals, the response is
within 3 dB from below 1 KHz to 100 KHz.
The point to be made from this exercise is that the frequency
response of a transformer cannot be considered in isolation, as it sensitive to
both the driving impedance and the terminating impedance. A systems approach
must be used and the measurements should duplicate the actual application to the
greatest extent possible.
For example, if the
objective is simply to characterize your particular installation, I would use an
RF signal generator with your normal operating software and see how the signal
amplitude varies as you move in frequency. Or, if you use Rocky, the built-in
amplitude versus frequency plot can give you an idea of the overall frequency
response of your setup.
Using test equipment as I've done, lets me understand
better why things behave the way they do, and, when combined with system
measurements in situ, allows fixes or adjustments to be made on a logical
The transformers I use. Obtained surplus, they are mounted
on plug-in bases.
Kees, K5BCQ, loaned me a Triad MIL-T-27E-SP68 audio
transformer to measure, as a possible audio isolation transformer to use with a
Softrock receiver. These transformers are available in a variety of impedances,
identified by the SPxx suffix. The SP68 transformer supplied by Kees has a
10KΩ design impedance primary and dual 2.5KΩ secondaries, or 10KΩ : 10KΩ when
the secondary windings are in series. Allied Radio stocks some MIL-T-27E parts,
at $18.70 each.
The transformer has, when properly terminated, a good
frequency response, up to 100 KHz, but you must ensure it is terminated
with the 10K design impedance to avoid a nasty high frequency resonance. Some
sound cards will present the required 10KΩ impedance to the transformer, whilst
others, such as the E-MU 0202 unit I use, would require auxiliary terminating
resistors, as the card's input impedance is 1 MΩ. Driving this transformer from
a low impedance source, such as the 50 Ω signal generator in the 3562A, or from
a Soundcard's line out port, is not a problem and impedance matching is not
required at that end of the circuit. And, it should not be matched at the source
end via series resistance, for example, as the result will be increased
end-to-end voltage loss.
First, the transformer's key parameters:
secondaries, or 10KΩ secondaries in series.
||From specification sheet.
||±2 dB 300 Hz to 100 KHz
||Specification sheet data. Measured data
agrees when tested with 10 KΩ source and load; different source and load
impedances yield different frequency responses.
||With both secondary windings short
circuited. Measured @1KHz with General Radio 1658 DigiBridge.
||Secondary windings open.
||One secondary winding with all other
windings open circuit. (Transformer has two secondary windings, 2500Ω each,
10KΩ in series.)
||Mean of measurements by two methods.
Mutual inductance method with windings series aiding/series opposing:
k=0.9966; based on secondary open/short data, k=0.9981.
||Modeled as across secondary windings.
This value is not as accurate as other data presented.
||From spec sheet
||From spec sheet
SPICE Model. The diagram below provides a model of
the MIL-T-27E-SP68 transformer, in a representation of the HP3562A Dynamic
Signal Analyzer test circuit (50 Ω generator output source; 1 MΩ input
The following three plots show the transformer's frequency
response under differing conditions of drive impedance and termination
impedance. The absolute levels are arbitrary and do not show the transformer's
actual insertion loss.
All data is taken over the range 10 Hz - 100 KHz, log
horizontal axis. The vertical axis varies from plot to plot, with the specific
value shown on the plot. (The plot below is 2.0 dB/division.)
50Ω generator source / 1 MΩ termination.
Note the peaking response, with about a 10 dB peak
at 100 KHz. This is due to a combination of the transformer's distributed
capacitance, stray capacitance in the test setup and the 3562A's input and
source capacitance. The frequency and magnitude of the resonance will differ
from setup to setup, as can be seen below. This transformer performs best when
terminated with the 10KΩ design value.
50Ω generator source / 10 KΩ termination
Terminating the transformer with its 10 KΩ design value
knocks the resonance peak down to negligible levels. At 100 KHz, under these
test conditions, the response is down about 1 dB from mid-band values.
10 KΩ generator source / 10 KΩ termination
Terminating the transformer with its 10 KΩ design value for
both source and load, should produce the response the designer intended. Indeed,
the measured response easily meets the specified ±2 dB from 300 Hz to 100 KHz. The measured
data shows the ±2 dB range as about 100 Hz to > 100 KHz.
demonstrate the usefulness of SPICE modeling, the plot below shows the predicted
response for the transformer under the test condition 50 Ω source / 1 MΩ
termination, as in the first measured plot shown above. The simulation shows
generally good agreement with measured data, except that the actual low
frequency response is a bit better than predicted by the model.
In February 2010, a controversy flared up on the Softrock
mailing list over the bad effects of isolating audio transformers. Claims were
made that transformers have too much phase shift and amplitude shift.
Among the best transformers for the purpose of breaking
ground loops, i.e., isolation, of the Softrock are Triad's SP-70 parts. These
are the 600:600 ohm versions of the SP-68 parts examined above. I have provided
distortion and frequency response measurements for the SP-70 at
Non-Linear Transformer Behavior.
I have four SP-70 transformers in my junk box and measured
the phase and amplitude response of all four. The test setup is an HP 3562A
Dynamic Signal Analyzer driving the transformer under test with its internal
signal source, set at 5 mV PP, over the frequency range 100 Hz - 100 KHz. This
low signal level is typical of the output of a Softrock when exposed to normal
radio signal levels. The 3562A is run with leveling enabled to hold a constant
drive level. The source drive impedance is 50 ohms, and the SP-70 was terminated
with a 10K 5% carbon film resistor. This simulates a typical sound card
impedance. The 3562A has a 1 Meg input impedance which is negligible compared
with the 10K load resistor. Both the 3562A's reference channel (used for
leveling) and the transformer under test are connected to a Mini-Circuits
resistive power splitter, model ZFRSC-2050, usable from DC to 2 GHz, through
identical length RG-223 double shielded coaxial cables.
Before measuring the phase shift associated with the SP-70
transformers, I established the residual error in the instrumentation, bypassing
the transformer under test. The plot below demonstrates the typical phase error
is within ±80 milli-degrees of 0 over almost the full range, with the residual
increasing to around -120 milli-degrees at 100 KHz. Note the horizontal axis is
I then measured the phase shift of four SP-70 transformers,
with the results below.
Above 1 KHz, the total
differential phase shift is on the order of 0.4 degrees. And above 2 KHz, the
difference is, for all practical purposes, zero. (Vertical scale is 2.5 degrees
Although there is a total phase shift between 1 kHz
and 100 KHz of about 12 degrees, as I understand it, the current crop of
software used with Softrock receivers is capable of correcting for overall phase
shift via a curve fitting algorithm, originally (to the best of my knowledge)
introduced in Rocky software.
I also looked at the amplitude of the four SP-70
transformers. The vertical scale is 0.156 dB/division (the odd dB/division
results from the choice of +1.00 dB to -0.25 dB as the scale range).
The data shows that the differential amplitude above 200
Hz is very small and is essentially immeasurable above 400 Hz. Again, this
degree of shift can easily be compensated for by the receiving software.
One final observation on the suitability of the SP-70
transformer for use with a Softrock. Telepostinc's LP-PAN panadapter
architecture is very similar to a Softrock. It is used with the same software as
are Softrock receivers.
The LP-PAN uses a pair of SP-70 transformers to isolate
the audio output, with hundreds of units in the field. My understanding is that
there are no complaints of inadequate image rejection.
Crispino, I5XWW, has supplied me with some Bourns
LM-NP-1001-B1 600Ω : 600Ω transformers to evaluate. Data sheet at
http://www.bourns.com/PDFs/LMNPLP.pdf My conclusion is that these
transformers will work if you have the correct terminating impedance—not 600
ohms, but something closer to 10K. Their high sensitivity to terminating
impedance, however, requires considerable experimentation with your particular
sound card before providing acceptable bandwidth. And, depending on your sound
card's input impedance, they may not be acceptable at all. Hence, I do recommend
these transformers as "plug and play."
From the data sheet, the transformers in the LM-NP line
are intended for interfacing to the public switched telephone network, or
similar low-fidelity applications. The transformers are small, intended for PCB
mount, although the leads are long enough to permit soldering wire extensions.
There are 10 transformers in this series, and the data
presented is only for the LM-NP-1001-B1 model.
Top and bottom view of LM-NP-1001-B1 transformer
The following is a combination of data sheet specifications and my measurements.
||600 Ω primary and
secondary. Windings are not center tapped.
||From specification sheet.
||+0/-2.5 dB 200 Hz to 10 KHz
||Specification sheet data. Measured data
agrees when tested with 600 Ω source and load; different source and load
impedances yield different frequency responses.
||Within specification. Measured 1.014 @ 1
||With both secondary windings short
circuited. Measured @1KHz with General Radio 1658 DigiBridge, parallel mode.
This is a measure of leakage inductance. Specification is "typically 14.0 mH."
||Secondary windings open. Specification
is 2.8H, but at 200 Hz, and measurement (series or parallel not provided.
||One secondary winding with all other
windings open circuit. (Transformer has two secondary windings, 2500Ω each,
10KΩ in series.)
||Computed with using open/short data.
When measured with series inductance model at 1 KHz, coupling coefficient
computed as 0.9965. Ls open = 1.627H, Ls shorted = 11.31 mH.
||From spec sheet. Measured within
The following plots show the transformer's frequency
response under differing conditions of drive impedance and termination
impedance. The absolute levels are arbitrary and do not show the transformer's
actual insertion loss.
The vertical axis varies from plot to plot, with the specific
value shown on the plot. (The plot below is 3.0 dB/division.) The horizontal
axis shows the sweep range, with a log frequency basis.
600 Ω source and 600 Ω load
With this test condition, the specification sheet
environment should be duplicated. The performance meets the data sheet
specification of +0/-2.5 dB, 200 Hz to 10 KHz. In fact, the -2.5 dB point is
around 15-16 KHz.
As discussed in connection with the Triad MIL-T-27E-SP68
transformer measurements, however, when used as an audio isolation transformer
for a SoftRock receiver, the source driving impedance is low, but with a 0.1 μF
series capacitor, and the terminating impedance is determined by the sound card
and may be anywhere from 1.5 KΩ to 1MΩ. To simulate this environment, the
following measurement is with a 50 Ω source impedance and 0.1 μF
capacitor. The terminating impedance is 1600 Ω, to approximately match my E-MU
0202 sound card's low impedance option, 1500 Ω
Ω & 0.1 μF Series Capacitor Source, 1600 Ω Termination
This combination shows roll off at both the low and high
frequency end of the spectrum, showing a 3 dB response from about 700 Hz to 30
purpose of the analysis is to determine the suitability of the transformer to
isolate the tip and ring audio outputs of a SoftRock receiver, I made
measurements with the transformer connected to a SoftRock Lite 6.2 receiver for
the 30-meter band (center frequency 10.125 MHz.). The test setup is shown in the
block diagram below. The custom software steps the SG-100 function generator in
frequency and records the audio output level. Resistive terminations were added
on the transformer output for the various test conditions studied. The SG-100 is
set at an output level of 10 mV PP.
The first test swept the
receiver over the range ±125 KHz from center, with a direct connection from the
receiver's tip and ring outputs to the HP3456A voltmeter. The purpose of this
test is to obtain a baseline performance picture of the receiver, operating
without a transformer.
As the data shows, the receiver's 3 dB bandwidth is
approximately 100 KHz on either side of the center frequency. There is some
asymmetry in the response, and difference in response level between the tip and
ring audio channels. These imperfections would be calibrated out in the
associated software in normal use. The spike/dip at zero offset corresponds to
the normal "DC" spike and associated 60/120 Hz noise around zero offset.
Receiver Response with LM-NP-1001-B1 Transformer
The plot below shows the ring audio output when coupled
through an LM-NP-1001-B1 transformer, with various terminating resistances.
For reference purposes, the plot also depicts the
receiver's response with no transformer.
The data shows the transformer's response beyond 50
KHz or so is highly sensitive to terminating impedance, with relatively good
performance provided only around 10 KΩ. The data taken at 1 MΩ, which is the
impedance of my E-MU 0202 sound card, shows severe resonance effects around 110
KHz from center. The lower values of terminating resistors dampen the resonance,
but result in high frequency attenuation. Only at around 10 KΩ is the response
The data also shows that for a restricted frequency range,
up to 50 KHz or so, the transformer is much less sensitive to termination
impedance, so long as it exceeds about 5 KΩ.
Close In Response
final plot shows the response with the LM-NP-1001-B1 transformer,
terminated with 1500 ohms, expanded to ±5 KHz from center. The data clearly
shows the low frequency rolloff caused by the transformer and the SoftRock's
series 0.1 μF blocking capacitor. Fortunately, less than 1 KHz is severely (more
than 10 dB) attenuated near zero Hz.
The Bourns LM-NP-1001-B1 transformer can be used with a SoftRock receiver, with
reasonable success over the 100 KHz bandwidth range if, and this is a
major if, the sound card termination impedance is, or can be made to appear to
be, around 10 KΩ. The exact value of termination required for optimum
performance will likely vary somewhat from this value, depending on stray
capacitance and wiring shunt capacitance, and should be determined by
measurements in your particular installation.
If your requirement is only 24 KHz bandwidth, then the
terminating impedance is far less critical and almost any reasonable value > 5.6
KΩ will work. Even with 48 KHz bandwidth, the terminating impedance is not
overly sensitive. However, to obtain acceptable results over a 96 KHz bandwidth,
the terminating impedance must be carefully selected.
If the sound card's impedance is higher than 10 KΩ, then
adding a parallel resistor of suitable value should provide the necessary 10 KΩ
impedance. For example, the E-MU 0202 sound card, when in high impedance mode,
has an input impedance of 1 MΩ. For this card, in this mode, a 10 KΩ resistor on
the LM-NP-1001-B1's output winding should prove satisifactory. On the other
hand, if the E-MU 0202 card is operated in low Z mode, the input impedance is
1500 Ω. In order to let the LM-NP-1001-B1 transformer "see" 10 KΩ in this case,
a series resistor of 8.5 KΩ is required. Although providing the desired
terminating impedance to the LM-NP-1001-B1 transformer, the voltage divider
effect will reduce the audio into the E-MU 0202 sound card by about16 dB, a
major loss in sensitivity, which renders the LM-NP-1001-B1 transformer
unacceptable in this case.
Whether the LM-NP-1001-B1 transformer will work for you
depends on the sound card impedance. If greater than 10 KΩ, it should be
relatively easy to make it work. If much less than 10 KΩ, the extra loss
resulting from resistive matching likely will produce poor results. I can
recommend the LM-NP-1001-B1 transformer only if you are comfortable making
performance measurements verifying its operation, and, of course, if you have
the necessary test equipment. You should be able to get a good view of the
transformer's performance with the band scope feature in the standard software,
plus either an RF signal generator or a broadband noise generator.
Radio Shack Audio
Isolation Transformer P/N 273-1374
Pete, N4ZR, supplied a Radio Shack 600:600 ohm audio
isolation transformer to evaluate its suitability to isolate a Softrock
The transformer is small, about the size of a sugar cube,
or, for that matter, the Triad SP-xx series of military spec audio transformers.
It is encased within green heat shrink tubing, with the windings brought out
through short (3" or 75 mm) wire leads. As is the norm with Radio Shack
components, specifications are on the minimal side:
- 600-900 ohms
- 300Hz to 5kHz response
- 100 megohms insulation resistance at 250VDC
The frequency response specifications are not encouraging,
but measured data shows the transformer is much better than these specifications
The 273-1374 transformer has a list price of $3.99 and
should be available in most Radio Shack stores, or via Radio Shack's web
ordering service at www.radioshack.com.
Rather than run tests on the 273-1374 transformer as
a stand-alone device, I decided to look at its performance with an updated
version of the test setup used for some of the 2006 tests. The signal generator
steps, in 1 KHz increments, from -125 KHz to +125 KHz from the Softrock's center
frequency of 8192 KHz. The tip audio output is read by an Agilent 34410A digital
voltmeter. Both the VP8191A and 34410A are controlled via a GPIB bus with
a Prologix controller.
Over the range ±125 KHz, the plot below shows three connection arrangements.
Direct connection to the voltmeter, connection through the Radio Shack 273-1374
transformer and through a Triad SP70 transformer. (The SP70 is the 600:600 ohm
version of the Triad SP-21 analyzed above.)
points of interest emerge from the plot.
- The zero reference point is the direct connection at
8191 KHz, representing a 1 KHz offset audio tone. Because the transformers
do not have exactly a 1:1 winding ratio, the output of the two transformers
is slightly greater than when directly connected to the voltmeter. This
difference is negligible for the intended purpose.
- All connection methods have artifacts around zero,
discussed in more detail later.
- Although the SP70 transformer is better performing
than the 273-1374, the difference is modest at best, amounting to about 0.5
dB at 125 KHz offset.
- The Softrock receiver has a 3 dB bandwidth of ±100
KHz. This results from the low pass filter implemented in the op-amp output
We expect transformers to exhibit some artifacts about zero
Hz, if for no other reason than transformers have a low frequency response limit
of a few hundred Hz for the two transformers used in this test. This is
the reason for the deep dip around zero offset.
note, moreover, some artifacts in the direct connection as well. This could
result from hum and noise on the VP8191A signal generator.
The peaked response for both transformer connection likely
is a product of series resonance between the transformer's winding inductance
and the series blocking capacitor in the Softrock. (I've modified this
particular Softrock receiver by replacing the stock 0.1 µF output capacitor with
1.0 µF units for better low frequency response.) The resonance is negligible
more than 500 Hz from center.
The bottom line result is that the Radio Shack 273-1374
transformer has quite decent high frequency response when driven from the
Softrock receiver and working into a high impedance load, in this case 10
KΩ. I also looked at the response with a 1 MΩ termination and found the results
similar to the 10KΩ case.
Considering the 273-1372's
modest cost, it's worth considering as an isolation transformer.
I found it also moderately helpful to double-isolate the
antenna from the Softrock receiver with an RF isolation transformer. The
Softrock's design provides isolation as the input transformer's primary floats
with respect to ground. Hence the antenna isolation transformer described here
provides a second isolation stage. I can't say it made a huge difference, but it
may have helped a bit. I made multiple changes in one test (not a good idea but
I didn't want to spend all day tinkering), with one of the changes being the
antenna transformer. I saw improvement, but can't accurately apportion the
benefit to the various simultaneous changes.
An isolation transformer can take many forms, and the one
I used is based on the parts I happen to have at hand. Still, it works
reasonably well, and should be easily duplicated if desired.
The transformer consists of 6 bifilar turns wound on a 61
material binocular core, Fair-Rite part number 2861000202. (Fair-Rite calls
these "multi-aperture cores.") This core is available from many sources,
including Ocean States Electronics, part number BN61-202.
http://www.oselectronics.com/. As of
late July 2007, the core costs $0.65 from Ocean State.
To wind the transformer, cut two lengths of wire, 11.5
inches long. I used #30 AWG "wire wrap" wire, one length with red insulation and
one with blue, to help distinguish the two windings. You can use almost any wire
that will physically fit. No. 28 magnet wire would be a good substitute.
After cutting the two wires, twist them together, at about
two twists per inch. This is not a critical value, so don't worry if it's not
exactly 2 twists per inch.
Wind the twisted wire through the core, six turns total.
Remember—one turn requires the wire to go through both holes. Leave about 1"
excess wire at the start. When finished winding, trim any excess wire to the
length needed in your installation.
If you have test equipment, each winding should measure
approximately 12.5 μH at 2.5 MHz. (Q measured at 220, HP 4342A Q-meter.)
I measured the transformer's performance over the range 1 MHz
- 100 MHz for return loss and insertion loss. The data is taken with an HP8752B
vector network analyzer.
The plot below shows the
transformer's return loss when terminated with a precision 50 ohm load. It is
less than 10 dB over the range 1.8 MHz - 29.7 MHz.
Return loss may be more familiar to hams when recast into SWR. The plot below
shows the transformer's input SWR when terminated by a precision 50 ohm load.
Over the range 1.8 - 29.7 MHz, the SWR is 1.8:1 or better.
Finally, we look at the transformer's insertion
loss. Over the range 1.8 - 29.7 MHz, it is less than 0.5 dB, and it's closer to
0.25 dB between 3.5 MHz - 10 MHz.
The Softrock is a direct conversion receiver, but with I
and Q outputs, which permits image rejection when used with the correct hardware
or software audio combining.
Most Softrock users connect their receiver to a PC's
soundcard and use one of several software programs available to provide standard
receiver functions, such as tuning, filter bandwidth, spectral display and the
like. I've started with Rocky, written by Alex, VE3NEA, and available without
charge at http://www.dxatlas.com/rocky/.
I like Rocky. It has, in my view, a near perfect balance
of features and user interface. However, it has a few rough edges compared with
I won't duplicate Alex's instructions, but will point out
the main features. The software's spectrum analysis tuning mode is shown below.
(There's also a great waterfall display option--see below.). The pip at the
center is the receiver's DC output, and there is a band a couple of KHz on
either side of it with increased noise as seen below. The 40 meter receiver
center frequency is 7056 KHz, and with 48 KHz sampling rate, you can tune 24 KHz
either side of center. (Since the receiver has I & Q samples, the sample rate
equals the bandwidth.) The image below shows two SSB signals above the center
and several CW signals below center.
To tune, you can place the mouse on the display and click.
Or you can use the keyboard's arrow keys to tune up or down, or if your mouse
has a wheel, use it to tune.
The main Rocky screen
is shown below. It's a panadpter view.
Waterfall image. I've reduced the size, but
in the original you can easily read the Morse. The large waterfall is fast
moving, and the smaller waterfall to the right is slower, to provide a better
view of activity over the space of several minutes.
In an I-Q receiver, the image rejection is determined by the phase and amplitude
error between the two channels. Theoretically, the channels should be equal in
amplitude and exactly 90° phase shifted. Of course, component variation makes
achieving those goals impossible, so the receiving software has a calibration
process to offset hardware errors. Rocky's calibration process is particularly
elegant, as it happens automatically, without user intervention. The software
measures off-the-air signals and computes the phase and amplitude error and then
fits an equation to the phase and amplitude correction. As I say, this is all
done automatically in the background, using off-the-air signals.
Correction screen showing phase upper) and amplitude
The phase difference over the ±24 KHz range is about
1.25 degrees and the amplitude correction runs from 0.94 to 1.00 over the same
After Rocky ran for a while, I measured as much as
70 dB unwanted signal suppression. And because the phase and amplitude
corrections are applied on a frequency-related basis, this level can be achieved
across the full band.
I found two major issues with Rocky. Some
(perhaps all) are unique to my particular combination of computer and sound
card. Still, they are real for me.
CPU Resource consumption
Rocky has two major activities that are computationally intensive. First is the
DSP-related code, filtering, detecting and the like. The second activity is
graphic-related; to update the panadapter display many times a second is a
graphically intensive process.
To make a long story short, Rocky's DSP code component
runs relatively efficient on the SX260, consuming less than 10% of the CPU
resources. However, the way Dell designed the SX260 makes graphics rather
inefficient. Rather than install dedicated memory for the graphics processor,
Dell instead reserves 32 MB of main memory for the graphic processor. This
decision saves some money, but it is a major bottleneck when a program—such as
Rocky—frequently updates the screen. When using the built-in sound card, running
Rocky in anything other than a small window causes severe conflicts with other
programs need for CPU cycles. The memory bottleneck problem is aggravated by
running the SX260 in 1600 x 1200 resolution, its highest resolution mode.
As the data below shows, using an external USB card adds
to the CPU requirements. (These figures are unlikely to apply to your computer
if it has a separate graphics card.)
||CPU Requirements with Internal
||With E-MU 0202 USB 2.0 Sound Card
|Full size, 1600x1200
|Half size, 800 x 1200
|Quarter size, 400 x 600
|About 1/6th, 600 x 240
Testing Rocky on my Gateway laptop with separate graphics memory and internal
sound card shows about 7% CPU usage even running at full screen. Hence, it's
clear that the SX260's unique architecture is ill suited for Rocky.
Gateway laptop with Rocky running full screen shows 7% CPU
Sound Card Initialization
The second problem with Rocky surfaced when I installed the E-MU 0202 sound
card. Although I set Rocky's settings for 48 Ks/s, the E-MU 0202 card was not
reset to this value. Rather it retained the last setting, 192 Ks/s in this case,
from the PowerSDR software. The workaround for this problem is to use the E-MU
0202 configuration utility to manually set the sample rate.
Rocky supports 96 Ks/s with some sound cards (Delta 44),
but this mode does not work with my E-MU 0202, even when I manually set the card
to match Rocky's setup parameter. I understand this is a known issue, in that
not all 96 Ks/s cards are supported with Rocky.
Although initially written for its transceivers, the folks at FlexRadio have
graciously modified their PowerSDR
software to work with Softrock equipment, and made it available for free
download. (For that matter, the source code is available as well.)
The image below shows PowerSDR running on my SX-260
with the E-MU 0202 sound card, in 192 Ks/s mode. At 192 Ks/s, PowerSDR
displays (and will tune) over a 192 KHz range, ±96 KHz from the 7056 KHz center
As you might judge from the screen image above, PowerSDR has
many more controls and options than Rocky. Although many of the options and
controls are tied to FlexRadio's equipment, a surprising number are applicable
to the Softrock. This makes the PowerSDR more complex to set up and calibrate
For example, to adjust the I & Q channels for phase and
level balance to null the image requires a two-step process. With a signal
generator input, you run an automatic nulling process. Then, you manually fine
tune the level and phase settings.
Manual fine tune for I & Q balance in PowerSDR software
I found this process to work well for a single frequency,
with 70 dB null or more achievable. But, the null is valid only for a single
input frequency and even a slight change in frequency causes a major change in
null depth. Rocky's automatic calibration and, more importantly, a
frequency-sensitive level and phase corrections permits much better image
The screen capture below shows the depth of null possible at the calibration
frequency. In this case, the signal at 7085 KHz is the desired frequency. Its
image frequency is at 7027 KHz and is in the noise level, some 90 dB down.
However, if we move the test signal 15 KHz, to 7100 KHz, the image is suppressed
only about 40 dB, as seen below.
I may well be
missing something in the calibration process, such as repeating the calibration
every 10 KHz. As I say, the program and set up is far from simple and the
documentation only discusses image calibration at a single frequency.
M0KGK's software looks interesting, but has serious
compatibility problems with my Dell SX260.
Even resized to as small as is reasonably feasible, it
still consumes a lot of CPU resources.
|Sound Card Sample Speed
48 Ks/s is on the edge of being usable, so long as no
other programs are running. Any program that runs in addition to M0KGK grabs too
many CPU cycles, causing the audio to break up and tuning to become jerky. The
two higher speeds are unworkable, even with no other programs running.
I don't believe this is totally a graphics issue, as
I've made the window as small as the size I use for Rocky and smaller than
PowerSDR runs in. (The CPU data is for this small window size.) In the process
of resizing KGKSDR to make the panadapter window disappear, I got into a
cascading error message problem which required closing the program with Task
Like Rocky, KGKSDR does not change the E-MU 0202 card's
sample rate. I have to manually adjust it using the E-MU 0202 control
application. On the plus side, KGKSDR supports sample rates through
Like Rocky, KGKSDR has automatic I & Q amplitude and phase
adjustment capability, a very nice addition. It also has AM and FM demodulation,
also useful features.
KGKSDR's calibration window is shown below. Three display
screens are used to see the state of calibration, amplitude, phase and number of
samples versus frequency offset.
I like KGKSDR's user interface, which is more complex than Rocky, but not nearly
as baroque as PowerSDR. Unfortunately, KGKSDR is not compatible with my computer
and sound card.
At Aldo's ( IW2DZX) suggestion, I tried two SDR programs
from Alberto, I2PHD. Alberto has written many useful program for amateur radio,
with Winrad and SDRadio his two SDR projects. The programs are available for
download at http://www.winrad.org/ along
with Alberto's other software.
Winrad's main screen is shown below. It has about every
display known to man, all running simultaneously. A panadapter-type display,
waterfall display and graphical displays of the receiver section bandwidth (you
change bandwidth by grabbing the selectivity curve with the mouse and moving it)
and a zoomed waterfall view of the signal within the receive section bandpass.
Winrad works with my E-MU 0202 sound card at 48, 86 and
My take on Winrad, after a brief exposure to it is:
- The screen is way too busy with graphic displays and
consequently consumes a great deal of CPU resources from my Dell SX260. With
the panadapter and waterfall display running in the slowest refresh mode, the
total CPU resource consumption was 75%, with Winrad grabbing 65%. I also
found a discrepancy between CPU resources reported by Winrad's status
indicator and those reported by Windows in Task Manager. For example, Winrad
shows 48% CPU resources whilst Task Manager shows Winrad consuming 70% CPU
- I would find Winrad easier to use if "clutter control"
were available, so that unwanted displays could be disabled and hidden. This
would simplify the display and also reduce graphics and CPU loading.
- I like the grab and move bandwidth adjustment, but
there's no need to show it all the time.
- I & Q balance (amplitude and phase) requires manual
adjustment. The adjustment is made only for a single frequency and
consequently although I could achieve 80 dB+ image rejection at one frequency,
this could not be maintained over the full frequency range.
- Support for ASIO drivers and the E-MU 0202 at all
sample speeds is good and smooth.
Alberto's second program SDRadio is still in beta test state.
SDRadio may be more intended for SWL listening than amateur radio, as, for
example, it omits CW mode, but includes FM.
SDRadio's interface is much simpler than Winrad and consequently presents a less
cluttered appearance and consumes fewer CPU cycles. At 96 ks/s, SDRadio grabbed
about 50-55% CPU resources on my SX260. That's significantly less load than
Winrad, a fact I attribute to the less graphic intensive nature of SDRadio.
Like Winrad, SDRadio has a single frequency manual I&Q
If I were writing the specifications for the ideal SDR
software package, it would start with Winrad, but it would give the user
significantly more control over which displays were active. Turning off
un-needed graphics will reduce CPU loading and, more importantly, reduce screen
clutter. Frankly, I found all the windows and activity in Winrad fatiguing to
watch, compared with Rocky. The user should have a choice of what to view and
what not to view. It would also include Rocky's automatic, curve-fitted I&Q
amplitude/phase balance feature.
I should add that both Winrad and
SDRadio work well in terms of audio quality and the like. My issues relate
mostly to user interface concerns. To a large extent this is a matter of
personal preference and my desire for a clean, uncluttered display may be
considered foolish by those who like to see all the possible options visible at
Radiation from the Softrock Receiver
One issue with all direct conversion receivers is local
oscillator leakage out the antenna port. This can be a particular problem if
one uses a DC receiver as a panadapter and the receiver has inadequate reverse
gain in the IF pickoff circuit, as the DC's local oscillator will be injected
back into your receiver's IF chain, with generally unpleasant consequences.
One partial solution to this problem is to offset the DC
receiver's frequency, so that its LO falls outside the area of concern.
Frankly, in my view, this is a poor answer as the last thing you want in a
carefully designed receiver is a strong signal pumped into the IF chain at a
frequency where you "think" it will not be a problem. Offsetting the DC
receiver's LO has another advantage, in that it permits you to view the target
receiver's IF output at zero offset. As the images show, there's a dead band
centered around the DC receiver's LO frequency, so moving the LO outside the
target receiver's IF is a good thing. However, this means the span is now not
I connected the 40 meter Softrock's antenna to an
Advantest R3463 spectrum analyzer and measured the spurious outputs. The data
below likely represents a worst-case measurement, as the receiver board is
laying on the bench, with no enclosure.
The main spurious is at the Softrock's local
oscillator frequency, 7.056 KHz. It's level is -39 dBm, a hefty signal. A second
weaker spurious can be seen at 8.015 MHz.
The crystal oscillator at 28 MHZ also leaks through
the antenna port, along with the second and third harmonic of the divide-by-four
I've made IP3 and MDS measurements on the 10 MHz Softrock
Lite 6.2 receiver. To avoid sound card issues, I've made the measurements with
other test equipment here at Clifton Laboratories.
The signal generation part of the setup is conventional
and I've used it to make many IP3 measurements. The signal generator, pad and
hybrid combiner setup is capable of measuring IP3 figures in the +30 dBm or so
The Softrock's output feeds an HP 3562A Dynamic Signal
Analyzer. A DSA is a combined 0-100 KHz spectrum analyzer and tracking
generator, in addition to many other things. It's a digital box, with A/D
converter and dedicated hardware for signal processing and the spectrum analysis
portion is done via fast Fourier transform technology, similar to the way Rocky,
PowerSDR and other programs generate their panadapter display screens.
The advantage of using the 3562A instead of a computer and
sound card is that it avoids sound card and computer noise issues. By the
standards of sound card and computer ratings, the 3562A seems distinctly under
powered, but it has excellent noise performance, with a noise floor over the
range 1 KHz - 100 KHz of -116 dBv, or 1.6 μV. That's for the entire band, and
for a 1 KHz span within the band, the noise level is in the -130 dBV range, or
about 0.3 uV. The instrument has a spurious (IMD and harmonics) range > 80 dB.
Considering the 3562A is an early 1980's product, it specifications are
even more impressive.
HP3562A. The green clip lead grounds the input connector,
as the instrument has a high impedance balanced input, with the BNC's shell
floating. The signal displayed is the IP3 test. (I later discovered an option
setting to ground the input BNC shell, so a clip lead is unnecessary.)
The limiting factor in the Softrock Lite 6.2's intermodulation performance is
TLV2462 op-amps. The amplifier is used as a low pass filter following the
FST3253 QSD mixer/detector and has a passband voltage gain of 20, or 26 dB. The
TLV2462 is powered from a +5 V regulated supply bus and, although it is a
rail-to-rail input/output amplifier, measurements show that it begins clipping
at about 4.5 volts output. The data is taken with a single signal generator at
10.160 MHz, producing a 35 KHz output signal.
With RF input to the receiver of -4 dBm, the output sine
wave looks clean on the oscilloscope. It also appears clean on the 3562A when
studied for harmonics.
The peak-to-peak output is
Increasing the signal input 1 dB, to -3 dBm, shows
unmistakable positive and negative clipping.
The peak-to-peak output is 4.77 V.
When summing two signal generators with a combiner, the peak voltage is twice
the output of either generator (assuming, of course, both generators are set to
identical output voltages), or 6 dB. Hence, IMD measurements to avoid op-amp
clipping should restrict the input signal level to -4 dBm or a bit less.
The image below shows the Softrock's output spectrum over
the range 10 KHz - 50 KHz. The two signal generator tones at 10150 and 10160 KHz
(shifted down to 25 and 35 KHz, respectively, by the receiver's 10125 KHz local
oscillator) are centered. The stronger intermodulation product is -57.36 dB with
respect to the two test signals. This capture is with the single tone
input to the receiver of -11 dBm, or two tone input of -5 dBm, safely below the
op-amp clipping point.
The resulting IP3, with respect to the single tone input
IP3 = -11 dBm + 57/36 dB/2 = +17.7 dBm.
Although I've shown one decimal place in the IP3 figure, I
certainly don't imply the numbers are accurate to 0.1 dBm! I don't have a good
way to quantify the error, but I suspect the IP3 measurement error in in the
order of ±1 to2 dB.
To see how low a signal level might be detected out of the
Softrock, I connected one generator directly to the receiver, set for 0.3 uV
output. I ran the 3562A with 10 sweep average to reduce the noise. As the figure
below shows, under these conditions, the 0.3 uV signal is clearly seen, at about
10 dB above the average noise level.
One problem I've experienced with both the built-in SoundMAX
card and the external E-MU 0202 card is noise around 0 Hz. Although the E-MU
0202 card is much better than the SoundMAX system, I've seen screen captures
from other Softrock owners displaying essentially zero noise rise around the the
receiver center frequency.
To see whether this
is a problem with the Softrock receiver or my sound card / computer / power
supply / etc., I looked at the Softrock's output over the range 0 - 1 KHz with
the 3562A. As the figure below shows, there are five distinct spectral lines
seen, 60 Hz and its harmonics through 300 Hz.
I'm seeing much more low frequency crud using the E-MU
0202 sound card than found when using the 3562A, even though I've added isolation transformers to
the I and Q output, and an antenna isolation transformer. The 3562A data shows
the level to which I should aspire, should I have sufficient time to devote to
Over the range 10100-10150 KHz, the Softrock's input
return loss is around -17 dB, corresponding to a VSWR of 1.3. This value is
The data is taken with an HP 8752B Vector Network
I recently (as of early November 2009) used a 7 MHz
Softrock Lite I receiver to look at noise and hum sidebands on a signal
generator and found that with long averaging times on the audio spectrum
analyzer connected to the Softrock's audio output, some evidence of frequency
drift was present.
The amount of drift was small, on the order of 1 Hz over
the space of 15 or 20 minutes, but in order to determine whether the drift
originated in the signal generator or in the Softrock, I measured the Softrock's
oscillator stability with the results below.
My measurement technique is to use a 30 dB gain amplifier
connected to the Softrock's antenna port to amplify the local oscillator leakage
signal. The amplifier's output is connected to a Racal 1992 frequency counter
set to measure the frequency with a resolution of 0.01 Hz. The 1992 counter is
connected to my shop master frequency standard, a Trimble Thunderbolt 10 MHz
GPS-disciplined oscillator. Data is collected once per second over a GPIB
interface (Prologix interface adapter) using a simple program I wrote running in
The Softrock board is in a small aluminum minibox, with
the upper section removed so the board is open to random air currents in my
basement shop. The data shows the effects of temperature variation in my
basement as the furnace cycles off and on. With my particular Softrock,
increasing temperature causes a drop in oscillator frequency, and as
the plot shows, when the furnace runs, there's a rather fast drop in oscillator
frequency of between 3 and 4 Hz. When the furnace shuts off, the ambient
temperature slowly drops and the Softrock's frequency climbs until the
thermostat kicks the furnace back on.
I didn't measure the temperature excursion in the shop,
but my sense is that it's around 3 to 4 degrees F. Call it 2 degrees C for a
rough estimate. The Softrock oscillator changes about 4 Hz over the
temperature cycle, or 2 Hz/°C. At 7 MHz, therefore, the temperature coefficient
is around 2 Hz/7 MHz, or 0.3 PPM/°C. For an inexpensive microprocessor crystal
with no temperature compensation, this isn't terribly bad.
It is interesting that with a high quality time base, it's
possible to measure the basement temperature to a fraction of a degree
through the Softrock oscillator frequency. In fact, Hewlett Packard used a
similar approach many years ago in a precision thermometer, where the sensing
element was a crystal oscillator with known frequency versus temperature
It would, of course, be possible to improve the Softrock's
crystal oscillator should higher stability be needed. For example, by either a
constant temperature oven or a thermistor-based compensating network, or a
combination of the two. Or, a suitable external TCXO might be substituted for
the Softrock's oscillator. On a long term basis, of course, inexpensive
microprocessor crystals will drift downward due to contamination and outgassing
which shifts the crystal's resonant frequency lower as the contaminants add mass
to the quartz plate. This process is accelerated when the temperature is
increased, so while adding an oven arrangement to an inexpensive crystal
oscillator may improve short term stability, it harms the long term stability by
increasing downward drift. High quality timebase crystals are in hermetically
sealed holders constructed from material with minimum outgassing problems.
There's no doubt that a Softrock Lite 6.2 receiver wins
the performance per dollar contest by a long measure. For $10 (including
postage!) you get a single band receiver that, when used with a PC and suitable
software, has all the bells and whistles that one could want. And, the software
is free, thanks to the dedicated efforts of some very talented hams. The
Softrock's IP3 and minimum discernable signal performance is more than
However, many built-in sound cards will require
replacement to obtain acceptable performance from the Softrock. And, an
enclosure, isolation transformers and other bits and pieces will be necessary to
obtain better performance. Depending on how well your computer is equipped and
your junk box is stocked, these add-ons can add anywhere from $50 to $200 to the
$10 Softrock receiver. I paid $125, for example, for the E-MU 0202 USB sound
card. The parts required for the enclosure and transformers were on hand, but
probably would run another $30 if purchased new.
For my particular computer and sound card arrangement,
significant CPU resources are required to run the necessary SDR software. As I've
said earlier, this is a function of how Dell designed the SX260's video memory,
and how it bottlenecks graphic intensive programs such as those used for SDR
A computer with a normal graphics card should do much better.
Of the programs I've tried, PowerSDR offers the best support for sound cards
operating at 96 or 192 Ks/s.
If your taste runs to a graphically "fully featured"
program with simultaneous displays of about every possible signal, Winrad may be
your desired program. I find it cluttered but that's personal opinion. It does
well with my E-MU 0202 sound card at 192 Ks/s.
I would not recommend Softrock as a beginner project, nor
would I recommend it to anyone without at least an above average understanding
of computers. It is nowhere near the level of "open the box, plug it in and
start copying signals."
Antenna leakage makes questionable using a Softrock 6.2 as
a panadapter without better shielding and a buffer amplifier permitting at least
60 to 70 dB net isolation between the receiver's IF and the Softrock's antenna
port. The AD8007-based buffer amplifier I designed (the Z10000) provides this
level of isolation, at least up through 5 MHz.