KX3 temperature compensation experience

[Mooneer Salem]

Hi all,

I recently redid temperature compensation on my KX3 using a GPSDO that I
purchased to see what kind of results I could get. (I last did it a while
ago with the XG50 and didn't have any complaints about operation afterward,
but I also pretty much only used 20 meters and below.) Below is what I did
and what I observed.

*Device info:*

KX3 purchased February 2013 (revision D RF board, S/N in the 3000 range), 2
meter, battery charger and roofing filter options installed
VE7FMN heatsink installed

*Steps performed:*

1. Warmed up GPSDO (~24h beforehand).
2. Warmed up KX3 until OSC temperature stopped climbing (~1h to reach 34C).
3. Performed reference calibration at 50.000MHz (5th harmonic of 10MHz
square wave output) using the automatic method.
4. Placed KX3 in fridge for ~1-1.5 hours and set up heating equipment (100W
incandescent bulb and makeshift box using baking sheets to keep heat in).
5. Removed KX3, turned it on and began data collection/storage. I also
started logging using KX3 Utility. (OSC was ~14C at this point.)
6. After ~70min or so, stopped data collection at 52C and turned off bulb.
7. Reset reference calibration after allowing cooldown.
8. Started fldigi in analysis mode and recorded frequency data for ~15
minutes.

*Discussion:*

See attached. In short, frequency differences vs. temperature seem to be
linear up until close to the end of calibration, where the frequency starts
to go back up. The data collected from fldigi also seems to indicate an
average frequency shift of 0.04ppm at 10MHz, which IMO is perfectly
acceptable. (For reference, the specs for the Yaesu FT-817's TCXO claim
0.5ppm.)

That said, I did notice--as with others in previous posts to the various
Elecraft/KX3 lists--that the signal is significantly shifted on 6 meters
when performing the second reference calibration based on a HF signal (and
vice versa). However, there doesn't seem to be much of a difference
regardless of where one calibrates on HF. For me, since I don't do much
operating on 6 meters, I kept the 10MHz based reference calibration. It
would be nice if one could set a separate reference frequency for 6m/2m vs.
HF so that the radio can be right on frequency regardless of where it's
being used.

Also, I haven't tried transmitting with the new calibration yet, but given
that I didn't have any problems with the digital modes I commonly use in
the past, I don't expect any issues now. I did find
http://www.bigskyspaces.com/w7gj/KX3HeatSinks.htm before doing this, which
(a) indicated that there's a newer version of the RF board and (b) required
board modifications to get the oscillator to be stable enough for all but
JT65A on 6 meters. I'm not planning on modding my own KX3 in this manner,
but out of curiosity, is there a RF board newer than revision D (as
indicated in the schematics document on Elecraft's site)?

Anyway, let me know if you have any questions or if there's anything with
the methodology I could have done better. Offhand, calculating and setting
ppm offsets in fldigi might improve the frequency analysis a little bit,
though I don't know if it'd be by enough to matter much. Not to mention
doing a temperature compensation with the XG50 again to see how much a
GPSDO truly adds.

Thanks,

-Mooneer K6AQ

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[Mooneer Salem]

Hi all,

Here are some answers to questions that came up after I emailed my initial
post:

*1. How was transmit inhibited on the KX3?*
I set the power output to 0.0W as per the temperature compensation
instructions.

*2. Was any attenuation used during the procedure?*
Yes, a step attenuator set to approximately 40dB.

*3. Which GPSDO was used for the procedure and what is the power
specification of its output?*
https://www.tindie.com/products/nsayer/gps-disciplined-ocxo/. I used the
square wave output, which produced ~1.5Vpp per my oscilloscope prior to
performing the procedure.

Also, I performed some additional experimentation as to which frequency is
the best to perform the reference calibration from. I performed reference
calibration at 10/20MHz (both produced the same result for REF CAL), 30MHz
and 50Mhz and ended up with the following REF CAL numbers and their
frequency differentials:

*10/20MHz:*
*REF CAL:* 114.326.78
*50MHz:* 49.999.970 (-30hz => -0.6ppm)
*30MHz:* 29.999.993 (-07hz => -0.4ppm)
*20MHz:* 20.000.000 (+00hz => +0.0ppm)
*10MHz:* 10.000.000 (+00hz => +0.0ppm)

*30MHz:*
*REF CAL:* 114.326.81
*50MHz:* 49.999.986 (-14hz => -0.3ppm)
*30MHz:* 30.000.000 (+00hz => +0.0ppm)
*20MHz:* 20.000.004 (+04hz => +0.2ppm)
*10MHz:* 10.000.002 (+02hz => +0.2ppm)

*50Mhz:*
*REF CAL: *114.327.85
*50MHz:* 50.000.000 (+00hz => +0.0ppm)
*30MHz:* 30.000.010 (+10hz => +0.3ppm)
*20MHz:* 20.000.012 (+12hz => +0.6ppm)
*10MHz: *10.000.006 (+06hz => +0.6ppm)

Based on the above, performing reference calibration using a 30MHz
signal/harmonic seems to produce the lowest magnitude ppm differences in
frequency across the KX3's range. 50MHz is probably the next best
signal/harmonic to use, especially if one wants to avoid negative
differentials.

Anyway, let me know if you have any additional questions.

Thanks,

-Mooneer K6AQ

BTW, I also tried performing reference calibration using a 150MHz signal.
I'm not 100% confident of the values but that seemed to produce the worst
results for HF (bumping up pretty close to Elecraft's 1ppm spec). However,
for those who primarily use it as a transverter input, there may still be
some value in doing so.

On Wed, Jan 15, 2020 at 4:10 PM Mooneer Salem <[hidden email]> wrote:

> Hi all,
>
> I recently redid temperature compensation on my KX3 using a GPSDO that I
> purchased to see what kind of results I could get. (I last did it a while
> ago with the XG50 and didn't have any complaints about operation afterward,
> but I also pretty much only used 20 meters and below.) Below is what I did
> and what I observed.
>
> *Device info:*
>
> KX3 purchased February 2013 (revision D RF board, S/N in the 3000 range),
> 2 meter, battery charger and roofing filter options installed
> VE7FMN heatsink installed
>
> *Steps performed:*
>
> 1. Warmed up GPSDO (~24h beforehand).
> 2. Warmed up KX3 until OSC temperature stopped climbing (~1h to reach 34C).
> 3. Performed reference calibration at 50.000MHz (5th harmonic of 10MHz
> square wave output) using the automatic method.
> 4. Placed KX3 in fridge for ~1-1.5 hours and set up heating equipment
> (100W incandescent bulb and makeshift box using baking sheets to keep heat
> in).
> 5. Removed KX3, turned it on and began data collection/storage. I also
> started logging using KX3 Utility. (OSC was ~14C at this point.)
> 6. After ~70min or so, stopped data collection at 52C and turned off bulb.
> 7. Reset reference calibration after allowing cooldown.
> 8. Started fldigi in analysis mode and recorded frequency data for ~15
> minutes.
>
> *Discussion:*
>
> See attached. In short, frequency differences vs. temperature seem to be
> linear up until close to the end of calibration, where the frequency starts
> to go back up. The data collected from fldigi also seems to indicate an
> average frequency shift of 0.04ppm at 10MHz, which IMO is perfectly
> acceptable. (For reference, the specs for the Yaesu FT-817's TCXO claim
> 0.5ppm.)
>
> That said, I did notice--as with others in previous posts to the various
> Elecraft/KX3 lists--that the signal is significantly shifted on 6 meters
> when performing the second reference calibration based on a HF signal (and
> vice versa). However, there doesn't seem to be much of a difference
> regardless of where one calibrates on HF. For me, since I don't do much
> operating on 6 meters, I kept the 10MHz based reference calibration. It
> would be nice if one could set a separate reference frequency for 6m/2m vs.
> HF so that the radio can be right on frequency regardless of where it's
> being used.
>
> Also, I haven't tried transmitting with the new calibration yet, but given
> that I didn't have any problems with the digital modes I commonly use in
> the past, I don't expect any issues now. I did find
> http://www.bigskyspaces.com/w7gj/KX3HeatSinks.htm before doing this,
> which (a) indicated that there's a newer version of the RF board and (b)
> required board modifications to get the oscillator to be stable enough for
> all but JT65A on 6 meters. I'm not planning on modding my own KX3 in this
> manner, but out of curiosity, is there a RF board newer than revision D (as
> indicated in the schematics document on Elecraft's site)?
>
> Anyway, let me know if you have any questions or if there's anything with
> the methodology I could have done better. Offhand, calculating and setting
> ppm offsets in fldigi might improve the frequency analysis a little bit,
> though I don't know if it'd be by enough to matter much. Not to mention
> doing a temperature compensation with the XG50 again to see how much a
> GPSDO truly adds.
>
> Thanks,
>
> -Mooneer K6AQ
>
>
>

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