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Softrock Lite 6.2
Adventures in Electronics and Radio
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Elecraft K2 Frequency
Stability Measurements
This page collects several frequency stability
measurements I've made on my Elecraft K2 transceiver. The data covers normal
receive/transmit cycles at room temperature and one cold (20°F) to room
temperature warm-up.
My K2 (SN 5689) was purchased and built in 3Q 2006 and is
thus a relatively late production run, with Elecraft's temperature compensation
modifications supplied as original equipment. See Elecraft's "PLL Reference
Upgrade" document at http://www.elecraft.com/
for details on changes made in 2003 for improved temperature stability.
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Test Setup
All data was collected with the same automated setup, as
illustrated below.
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The K2 is tuned to 9999 KHz, USB mode to provide a nominal 1 KHz beat note when
a 10 MHz signal input is applied to the antenna port. The 10 MHz signal is
provided from the Racal 1992 counter's oscillator reference output port.
Comparision against WWV shows the oscillator to be as close to 10 MHz as I can
measure it and drift well less than 0.1 Hz / hour. The K2's audio output is
connected to the 1992 frequency counter's Chan A input and frequency data is
taken every 10 seconds and is captured via the 1992's GPIB output port using a
Proligix adapter card.
http://prologix.googlepages.com/.
This card looks like a serial COM port to the computer and
as a full bi-directional IEEE-488 port to the device it is attached to. If you
need an inexpensive ($120 or so) IEEE-488 adapter, I highly recommend it. It is
not fully compatible with LabView and most other software
written for a National Instruments card, so if that's your need, take a look at
Prologix's FAQ page for more detail. I use it with
PrintCapture to grab plotter and
print images from several instruments. It can be used with your favorite
language to write control programs via mimicking of a standard serial COM port.
In this case, I used Terminal
to capture the 1992's output data, which I then reformatted in Excel and plotted
with Origin 7.5.
I determined the K2's temperature with a Fluke 62 infrared
thermometer, measuring the K2's top surface heat sink temperature. The K2's
internal PCB temperature lags behind changes to the heat sink surface, as can be
seen in the delay between transmit stop and frequency change.
Test data involving transmit cycles was collected with the
40 dB attenuation comprised of a Bird 8323 100 watt, 30 dB attenuator and a
Minicircuits CAT-10 10 dB attenuator in series between the K2's antenna port and
the 1992's 10 MHz reference output port. Data collected without a transmit cycle
used two CAT-20 20 dB attenuators in series.
My K2 resides in my basement workshop, and the temperature
during the tests was 72°F ±2°F, relative humidity 32% during the tests. A
transmit cycle is made by placing the K2 into tune mode and operating at
approximately 50 watts output until the hottest temperature on the heat sink
surface equaled 145°F.
The data shows three tests:
- Room temperature stabilized, startup with one transmit
cycle after 14 hours of receive operation
- Continuation of the earlier data set, with a total of
three transmit cycles.
- Overnight cold soak in 20°F temperature, and then moved
into the shop for warm-up measurements.
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Run 1--Room Temperature Stabilized
The data below represents 14 hours of measurement,
starting around 5 PM and concluding around 7 AM. There's about a 60
Hz excursion from cold start to stabilized, with around a two hour stabilization
period, although the worst of the drift is over in less than one hour.
At the end of the test period, I disconnected the counter
and did a key-down transmit sufficient to warm the hottest spot on the heat sink
to 145F, as measured with a Fluke 62 Infra-red thermometer. The K2's internal
fan was running at maximum speed at this point. Upon return to receive, the
frequency had dropped about 30 Hz, and it started to recover when I terminated
the data set and started a second data set. The downward spike at the transmit
interval represents a partial counter reading and should be disregarded. Note
that the K2 continues to drift downward after returning to receive mode, as the
heat sink's energy is conducted to the rest of the chassis. |
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Run 2 --Room Temperature Stabilized with
three transmit cycles
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To see how long it takes the K2 to recover from the transmit
heat, I continued taking data with three additional transmit cycles. In each
case, I stopped transmitting when at 145F. The maximum excursion is around 30
Hz, and it takes the better part of two hours for the K2 to return to thermal
equilibrium, as measured by the frequency shift. |
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Elecraft's drift specification is "less than 100 Hz from cold
start at 25C," and my K2 more than meets that in these tests.
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Run 3 --Cold Soak to Room
Temperature
Since I don't have an environmental chamber, I took advantage of a
cold snap to see how my K2 reacts to cold temperatures. The K2 sat in my
attached garage overnight when the outdoor temperature started at 38°F and
dropped to approximately 22°F at 0600 local time. At 0630, the heat sink
temperature measured 40°F. I then moved the K2 to the back patio for about 90
minutes, at which time the heat sink temperature measured 20°F. At 0800 local
time, I moved the K2 inside and connected it to the test setup. It took about
five minutes to make the test connections and check the audio levels before
starting data collection. During this brief period, the measured heat sink
temperature moved up to 38°F. This rapid rise likely means that the K2 had not
fully stabilized at 20°F during the 90 minute outdoor exposure.
I noticed light condensation on the K2's outer surfaces
after bringing it into the workshop. In addition, the LCD was noticeably
sluggish when cold. Most liquid crystal displays are not rated for operation
below 0°C/32°F.
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After one hour of operation, the frequency error has reduced
to only 40 Hz. After two hours, the K2 has essentially made it to steady state,
thermal equilbrium. |
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