Softrock Lite 6.2
Adventures in Electronics and Radio
Elecraft K2 and K3 Transceivers
Jackson Harbor Press LF Converter
Written 27 August 2008
Last week, I purchased two
LF Converter kits
from Jackson Harbor Press,
run by Chuck Olson, WB9KZY.
Jackson Harbor describes the kit's function:
The LF converter takes the band of frequencies from
approximately 10 KHz to 300 KHz up to the 75, 30 or 20 meter ham bands (4 to
4.3, 10 to 10.3 or 14 to 14.3 MHz bands). The converter can be easily
changed to another ham band (or another 300 KHz range) by changing the
crystal. A socket is provided to make this easy to accomplish.
The kit is inexpensive at $14.00 plus shipping at $3.00.
(Chuck shipped both my kits for one $3.00 shipping charge.)
The kit includes a small printed circuit board and all
electronic parts. No printed documentation is provided; the purchaser downloads
the manual, schematic and parts layout from Jackson Harbor Press's web site.
I've prepared a block diagram of the converter below. The
kit includes both a 4 MHz and 10 MHz crystal so the user has the option of
up-converting 0-300 KHz to the 4-4.3 MHz band or 10-10.3 MHz band. (Actually, as
we'll see, the coverage extends beyond the 300 KHz filter cutoff.)
Since there is no output bandpass filter, the up-converted spectrum
is mirrored about the output frequency. If using the 10 MHz oscillator, for
example, a 100 KHz input signal appears at 10.1 MHz and also at 9.9 MHz. If you
have a ham-band only receiver, such as an Elecraft K2, you probably will wish to
use the 10 MHz crystal. You can then just mentally ignore the receiver's leading
"10" digits and read the input frequency directly. (The kit includes
a trimmer capacitor so you can net the converter's oscillator to be spot on.)
If your amateur band receiver doesn't have the 10 MHz
band, you could use the 4 MHz crystal and tune the mirror
spectrum 4.0 - 3.5 MHz, but you'll have to mentally invert the tuning, i.e., a
100 KHz input signal will appear at 3900 KHz, and tuning lower corresponds to
higher input frequencies. Or, of course, you could substitute another frequency
crystal in the kit.
The NE602 mixer (also known as the SA602, and its close relative, the
NE612/SA612) is popular in battery powered amateur radio receivers, as it
provides a balanced mixer, crystal or LC oscillator functionality, with about 15
dB net gain, with very low power consumption. As might be expected, "there ain't
no such thing as a free lunch" and the downside to this popular chip is that
it's intermodulation performance and gain compression figures are nothing to
write home about. Philips Semiconductor's SA602A data sheet specifies a
typical 3rd order intercept of -13 dBm. Since the converter may be exposed to
very strong AM broadcast signals in the 530-1700 KHz band, the input low pass
filter is very important for clean VLF/LF reception. I'll have more to say about
the low pass filter later in this review.
Each kit took about 30 minutes to assemble. The printed
circuit board is double sided, but does not have solder mask and silk screening.
The pad and trace spacing is sufficiently large that lack of solder mask is not
an issue. The kit is certainly suitable for a beginner; all through hole parts
and, if you build the stock low pass filter, even the two inductors are supplied
wound. No special tools are necessary.
The photo to the right shows the assembled board, but with
different low pass filter inductors than supplied with the kit. I also added the
0.1" spaced header pins to provide an easy way to make connections to the
I built one converter for Ron, K8AQC, to use in chasing low frequency
interference around his neighborhood, in conjunction with a battery powered
short wave receiver.
Since his converter must be
battery powered, I replaced the 78L08 8V linear regulator with a 78L06, a 6 V
part, as a 78L08 requires 10.5V input to be in regulation. In retrospect, even
the 78L06, with a minimum input voltage of 8.5V is a bit marginal operating with
a 9V battery that isn't fresh. A 5V regulator (7V minimum input) might have been a
better choice. The NE602 works down to 4.5V, but the lower the supply voltage
the worse the intermodulation performance.
I built this unit into a Radio Shack plastic case. (The
case has both a plastic and metal top, and I used the aluminum top.
It has a BNC input connector paralleled with an 8-32
threaded stud for a telescoping antenna. The output is also a BNC and the unit
has a power switch and power on LED.
I did not use Jackson Harbor's standard input low pass
filter in K8AQC's unit. The reason for that is the filter components supplied
with the kit are designed for 50 ohm input and filter output impedance.
At VLF and LF, a short whip antenna can be modeled as a
few pF capacitance in series with a full size antenna. At, say, 60 KHz,
therefore, the short whip
antenna has an output impedance in the neighborhood of 500KΩ capacitive
reactance. Operating into a 50 ohm impedance, a loss on the order of 80 dB can
be expected. One normally uses an JFET source follower to efficiently
match a short receiving whip at VLF/LF frequencies to a 50 ohm receiver or
converter, but in Ron's case such a front end would cause far more problems than
it would solve, as he lives in an area with AM broadcast signal strengths
exceeding 1 volt/meter, an exceptionally strong signal. Hence, no JFET source
follower front end.
As a compromise filter, however, I designed a 4.8KΩ low
pass filter of similar cutoff frequency to the filter supplied with the kit. Why
4.8KΩ? It turns out that is the input impedance of the NE602. There's still a
mismatch loss of 40 dB or so between the whip and the filter, but 40 dB is
better than 80 dB. And, of course, the filter's attenuation versus frequency
characteristics will vary wildly with over the VLF/LF range as the source
impedance presented to it by the whip antenna changes.
The second converter I built for my use around the shack and to use in working
on my active antenna project. I housed it in a small Bud utility box. Input and
output are BNC connectors and the DC power is brought in through a feed through
capacitor. The board is mounted with 4-40 aluminum standoffs, 3/8" tall.
Since my converter will be used with an external power supply, I built it with
the standard 78L08 8V regulator.
The filter inductors are wound on FairRite FT50-61 ferrite
How does it work? Considering the $14.00 price, it works very
well indeed. (It can be made to work a bit better if you are willing to do some
extra work, as we'll discuss below.)
The spectrum analyzer image below is captured with my
Advantest R3463 spectrum analyzer and shows the range 0-100 KHz as seen when
connected to my home brew active antenna. The double blip at 24-25 KHz are two
US Navy stations, Cutler ME at 24 KHz and one on North Dakota at 25.2 KHz. The
stations are a bit close to resolve into individual signals at this
resolution bandwidth. WWVB at 60 KHz can be seen, along with the Canadian Navy station CFH
at 73.6 KHz. Between 90 and 100 KHz are LORAN-C navigation signals, centered at
100 KHz so only the lower half of the signal and modulation sidebands are
The image below shows the same 0-100 KHz slice of the
spectrum, but this time shifted up to the range 10.0 - 10.1 MHz with the Jackson
Harbor converter. It is connected to the same active antenna used in the direct
spectrum analyzer sweep above. (The two images were captured 2 minutes apart in
time, so propagation, to the extent it changes from day to night at
these frequencies is not an issue.)
In general, the
direct and converter output spectrum plots look identical. A few comments should
be made, however:
- My R3463 spectrum analyzer has a minimum calibrated
frequency of 9 KHz, as below that the input DC blocking capacitor rolls off.
You can see this along with the local oscillator feedthrough in the first
half graticule division.
- My active antenna rolls off below 10 KHz as well, and
that rolloff is quite visible in the converter output, although it is not
visible in the direct connection.
- Signals through the converter are about 10 dB
stronger than for the direct connection case, representing the NE602's gain.
- I live in an area thankfully free of strong AM
broadcast band signals and I found no trace of AM broadcast
intermodulation interference in the converter's output.
I also spent some time tuning around using an Elecraft K2
transceiver in the 10 MHz band to listen to the converter output. With my
prototype active antenna, WWVB was quite audible at my northern Virginia
location, with both the high power and low power (-17 dB) portions being easily
heard. I did not hear AM broadcast band intermodulation products, but my
location fortunately has no extraordinarily strong AM broadcast signals. (For
testing in areas of high AM broadcast signals, I rely upon Ron, K8AQC, who lives
within the 1V/meter contour of a couple of AM stations.
Low Pass Filters
mentioned the converter's low pass filter input several times for a reason.
Before going further, however, I wish to clearly state that the low pass filter
supplied with the kit is perfectly serviceable and there's no reason for the
average builder to make changes in it. I can't resist, however, a chance to
tweak the filter and make things slightly better. And, the Converter serves as a
example to discuss filter design and component selection.
The figure below illustrates the low pass filter section
of the Converter. Although L1 and L2 are not specified by inductance, 8
turns on an FT37-43 core corresponds to 22.4 µH.
Chuck Olsen, in an exchange of E-mail messages says the
input low pass filter is a half-wave design, with a 300 KHz cutoff frequency and
50 ohm input/output impedance. Chuck used a half-wave design as it is said to be
more forgiving of component tolerances and impedance mismatches than more exotic
Whether a filter works as designed depends on several things.
Assuming the components are within a percent or two of the design value, the
most important remaining factors are (a) the impedances presented to the filter
on both the input and output side and (b) losses in the inductors, i.e., their Q
factors. Other considerations include parasitic capacitance in the inductors and
parasitic inductance in the capacitor as well as unwanted coupling due to the
filter's physical construction and layout.
From what I can determine, all NE602/SA602
data sheets say the device's input impedance is 1.5 KΩ. That figure is repeated
in more amateur radio articles than I cared to read through.
The problem is, it's not correct.
Although the data sheet is not completely clear, it seems
that the input impedance quoted is for balanced input, i.e., the signal
is applied differentially between pins 1 and 2. The Converter grounds pin 2 and
applies a single ended signal at Pin 1. (This also throws away 6 dB of gain and
also likely degrades the 3rd order intermodulation intercept, but adding a
balanced input to the Converter that operates properly at several kiloohms
impedance over a frequency range 10 KHz - 500 KHz is not trivial.) To better
understand the NE602's input impedance, I built one kit without an input low
pass filter and measured the input impedance using an HP 87510A vector network
The figure below shows a measured input impedance of 4.8
KΩ essentially pure resistive over the range 25 KHz to 500 KHz. The fluctuations
below 25 KHz are in the test setup, as the 87510A is operating outside its
recommended frequency range for this particular setup.
Looking around the internet to find confirmation for my measurement, I found an
incidental comment by Wes, W7ZOI, at
Regarding the NE602 input impedance:
Often we see the NE602 input modeled as ideal transistors
(which means infinite beta) with 1.5K resistors from each input to �ground.�
From what I�ve been able to tell, the circuit is really just a differential
pair with a total bias current of about 2 mA (1 mA per transistor.) The
equivalent emitter résistance is then re=26/Ie(mA), or 26 Ohms. If the beta
is 100, this would put the resistance looking to the base at 2.6 K. If we
consider frequencies near or just above the beta cutoff, the net impedance
drops down to 1.5K, justifying the traditional model.
But there is one more detail: The base-to-base input
impedance of a bipolar differential pair is twice that of the single
transistor. This is the result of the common emitter connection that is
between the two devices. This point would be a virtual ground if the bases
are driven differentially. It is a floating point with half the input
voltage if one of the bases is bypass to ground, which is the usual
situation with a Gilbert Cell. Hence, the input impedance is around 3K
paralleled with a couple of pF of capacitance. This is just what I measured
several years ago when I had daily access to a wideband network analyzer.
This prompted an exchange of E-mail messages with Wes, in
which he noted:
The 3 K or more value makes sense with regard to the
part having a diff amp input stage and standing a current of 2 mA. Glad
to see you came to the same conclusion.
It's difficult to know what the driving impedance of the
converter's input will be. My prototype active antenna, for example, has an
output impedance very close to 50 ohms over the frequency range 10 KHz - 30 MHz.
A random wire antenna will likely to range between tens of ohms and hundreds of
thousands of ohms reactive impedance over the range 10 KHz - 30 MHz. (It's
important to consider the filter's response over the full frequency range,
not just the 10 KHz - 300 KHz range as the filter's job is to reject signals
above 300 KHz or so, including those at 4 or 10 MHz that otherwise might
leak through into the converter output. To some degree, these leakage signals
are suppressed by the NE602's mixer balance, but most of the suppression must
come from the input low pass filter.)
As a sweeping generality, filters that are not driven by a
source and terminated in a load matching the design impedances have "lumpy"
responses, with peaks and dips not seen when the filter is properly terminated.
To obtain a quick look at the converter's response, I made
a series of spot attenuation measurements with an HP 8657A signal generator
source (50 ohms). The filter output is based upon reading the converter output
signal level with an Advantest R3463 spectrum analyzer. When a 50 ohm source,
such as the 8657A signal generator, is operated into a different impedance
than 50 ohms, the delivered signal level will not match the generator level
setting. Into an open circuit, for example, the delivered voltage will be twice
the indicated value. Since the filter input impedance varies, I've normalized
the plot to 0 dB at 100 KHz to provide a better comparison.
The red plot is the output when the converter is built
according to the instructions, with the supplied parts. The green plot adds a
49.9 ohm, 1% resistor across the input filter's output (C6 in the schematic)
thereby providing both a 50 ohm source and termination.
Several things are immediately apparent from this plot:
- When terminated into the 4800 ohm impedance of the
NE602, there's a clear peak in the filter response, broadly centered around
- Without the correct termination load, the filter
gives up 5 dB or more rejection in the lower end of the AM broadcast
- When terminated with 50 ohms, the filter shows
passband tilt with no clear transition between passband and stopband. The
passband tilt is generally evidence of low inductor Q.
The kit is supplied with two wound inductors, each consisting
of a few turns of no. 30 Kynar "wirewrap" type wire on a small binocular core of
FairRite type 43 material.
I've written at
Type 43 material and using it in tuned circuits. In general, Type 43
material has quite low Q and has an inductance that varies with frequency. For
most filter designs, even one of these problems is bad news and both together
are even worse. It's tempting to use Type 43 material in filters and tuned
circuits because it has a high effective permeability and thus permits
relatively high inductance in a compact form factor. However, one must be fully
cognizant of the problems as well as the benefits. (Note that the newest Type 43
material formulation has significantly reduced losses [higher Q] and may prove
more suitable for filters and tuned circuits. Care must be exercised, however,
before designing it into a circuit.)
Using the HP87510A vector network analyzer, I looked at
the inductance and Q of the filter inductors provided with the Converter kit. A
few spot values show the problem.
The inductance varies by 2:1 over the frequency range of
interest and the Q never breaks 5, an exceptionally low value for a low loss
filter. Looked at over the full frequency range of interest, up to 30 MHz, an
even greater variation in inductance and lower Q will be seen.
Considering these problems, why does the stock filter work
as well as it does? First, this is a fairly forgiving application, with only
five frequency selective components. Second, to some degree the low Q benefits
the filter. The inductors can be thought of (and modeled as) as a high Q
inductor in series with a resistor, with the resistor value increasing with
frequency and the inductance decreasing with frequency. This transforms the
filter into a mixed LC and RC low pass filter. The main drawback is that the RC
aspects of the filter cause increased loss everyplace, including the passband�hence
the tilted passband region seen in the plot above.
One easy modification is to replace the two supplied
inductors with higher Q parts. For low frequency work, I like FairRite Type 61
material. It has a moderate relative permeability (125) that is stable with
frequency over a reasonable frequency range. Equally useful, up to 10 MHz or so
it has quite low loss compared with Type 43 material.
I wound two 22.8 µH (nominal) inductors (18 turns, No. 22
AWG solid magnet wire) on FT50-61 cores, and measured their inductance and Q at
2.5 MHz using an HP4342A Q-meter. At 2.5 MHz, the measured Q values were 185 and
At 1 KHz, the measured inductance was within 5% of the 2.5
Using two FT50-61 based inductors makes the frequency
response look more like a filter should, as seen in the plot below. Both data
sets are taken with 49.9 ohm terminating resistor across C6 so that the filter
is correctly terminated. Note the passband is flat, within a half dB or so and
there's a clear transition from passband to stopband.
The two filter's stopband differences above 600 KHz are
negligible. However, the increased loss in the stock inductors does provide
extra attenuation in the range 400 - 600 KHz, albeit at the price of increased
It's possible to squeeze improved stopband performance from
the filter if one
uses a different set of design criteria. To see the possible difference, I
designed and built a Chebychev low pass filter. In order to provide a "head to
head' comparison of the filter types, I built three low pass filter prototypes
on a breadboard:
- Stock filter with the binocular 43 material cores
- Stock filter with the high Q FT50-61 cores
- Chebychev filter, 400 KHz cutoff, 3 dB passband
ripple. It also uses FT50-61 inductors wound with Litz wire.
Pictured to the right is the stock filter design with
FT50-61 cores. As an experiment, I wound these inductors with Litz wire to
improve the low frequency Q. The improvement over solid No. 22 AWG magnet wire
is small, however.
I used the excellent filter software AADE Filter Design,
http://www.aade.com/filter.htm to design the Chebychev prototype.
Using the HP87510 vector network analyzer, I swept all three
filters over the range 10 KHz - 10 MHz. The horizontal axis is log scale in
these plots, and I set markers for three passband frequencies and the lower and
upper AM broadcast band limits. Building the filters on the prototype board
generally yields better results than a typical printed circuit board design
because the ground plane is larger. In addition, the HP87510A provides better
source and load terminations.
First, the stock
filter with stock components.
Next, the stock filter design, but with the low Q
inductors replaced with high Q parts, FT50-61 core, wound with Litz wire. Note
the clean transition from passband to stopband, as well as how flat the passband
is up to the 300 KHz 3 dB down cutoff frequency.
Finally, the Chebyshev filter.
Superimposing all three filters onto a single plot:
If 3 db ripple in the passband is acceptable, the
Chebychev filter pays significant benefits in the stopband. If a flat passband
is more important, then the stock filter design with FT50-61 inductors is a good
To repeat my comment at the outset, in
almost all applications for which the Jackson Harbor Press converter is
used, the stock filter, built with the supplied components will prove
satisfactory. However, some filter performance improvement is possible with
either (a) better quality inductors and (b) a Chebychev style filter design.
To show the effectiveness of the converter low pass filter (with high Q FT50-61
inductors but otherwise stock filter design), I've made two spectrum analyzer
captures. Both use my prototype active antenna with a 50 ohm output impedance,
so your results with a random wire antenna, for example, may differ. I've also
modified my converter by adding a 49.9 ohm, 1% terminating resistor across C6.
The first spectrum analyzer capture is made with my Advantest
R3463 connected directly to the active antenna. Note the many strong AM
broadcast signals between 530 and 1700 KHz.
The capture below shows the converter output over the same
input range 0 - 2 MHz. The lower end of the broadcast band (the blue dot marker
is at WMAL, 630 KHz) is not suppressed as well as the higher frequencies, of
course. Still, the difference is quite noticeable, particularly when you
consider that the converter has about 10 dB gain.
I also designed and built a Chebychev band-reject filter for
the AM broadcast band, to be discussed in a future web page. Its effectiveness
in knocking down AM broadcast signals can be assessed with the spectrum analyzer
image below, showing the results of connecting my active antenna to the spectrum
analyzer through the band-reject filter.