[Elecraft] K3 diversity phase drift

[David Gilbert]

Hi, Geert.  Thanks for the extremely informative post!

I briefly mentioned it in my first post, but I am using the newer synths 
although I'm not sure if they are the B version or not.  Your post 
certainly seems to explain what I am seeing.

I originally bought the new synths not only for their lower noise and 
better low level in-band IMD, but also because they supposedly retained 
the same phase lock as the frequency was changed ... as long as you 
didn't change certain other things like which crystal filter was being 
used.  I believe that the old synths would change the phase of the phase 
lock when the frequency was changed.    At least that's what one of the 
Elecraft engineers said at the time. I've been thinking about this kind 
of phase measurement since before the new synths were available, so I 
jumped on the new synths when they came out.

If I leave the K3 running for several hours the drift seems to be less, 
but I haven't tested that rigorously.

In any case, the drift I see is secondary to the much greater impact of 
what is almost certainly highly varying multipath interference. I have 
almost completed the video I mentioned, and it pretty clearly 
illustrates the effect.  It has been disappointing in relation to my 
original goal of simply measuring and tracking arrival angle, but on the 
positive side it has given me new insight into what actually occurs.

Thanks again es 73,
Dave  AB7E



On 4/9/2026 3:27 AM, Geert Jan de Groot via Elecraft wrote:
> [Seems that including the KSYN3A schematics is causing my message to 
> get sniped by the mailing list software. Re-sending, w/o the 
> schematics; you will need to look them up on the Elecraft website 
> yourself]
>
> Hello,
>
> You may want to elaborate if the K3 you're using to measure, has the 
> old KSYN3 or the new KSYN3A synthesizers.
>
> The old KSYN3 is a PLL, *phase* locked loop and I expect both 
> synthesizers to keep the phase constant.
>
> The KSYN3A is a rather interesting design in this respect; Elecraft 
> calls it "frequency loop".
> I've included the KSYN3A schematics from the K3S schematics archive 
> for convenience [not anymore - download from elecraft.com yourself]
> Frequency generation is done using a SI570. It was a surprise to me 
> that the noise specs are so much better, but there you are.
>
> One problem though is that the Si570 can only use an internal 
> frequency reference so things like the stable TCXO or K3EXTREF won't 
> work.
> To that end, the Si570 signal is mixed with the signal from a Si531, 
> which *does* have an external frequency reference input, but 
> supposedly less good noise specs, and the results from mixing these 
> signals on the SA612 mixer is fed to a DSP PIC to create a control 
> loop that will compensate for a drifting Si570 on-chip clock generator.
> I'm not sure, but I think keeping exact phase reference is much more 
> difficult here. Not a problem for the classic KSYN3A use, but this is 
> what you may be measuring.
> It would be one of the technical reasons for the "both synthesizers 
> must be the same type, KSYN3A" as well as the additional cabling in 
> case of upgrading from KSYN3 to KSYN3A. I'll leave details on that for 
> you to explore.
>
> Wayne made some cryptic comments on this design on the list some time 
> ago. I spent an enjoyable evening studying these schematics alone and 
> it is stunts like this that make Elecraft so enjoyable for me.
>
> Perhaps also interesting, the KX3 uses the same SI570 signal generator 
> but does not have this frequency compensation loop which makes sense 
> given the low-power-usage requirements of the KX series.
> Rather, the KX3 sports a temperature sensor and a frequency-drift vs 
> temperature calibration to compensate for temperature frequency drift.
> (I'm not sure if this is calibrated off factory since I bought my KX3 
> second hand and the temp compensation thing is quite labor intensive, 
> but temperature drift of my second-hand KX3 got rather better when I 
> did/re-did the temperature calibration thing - details of the 
> procedure are on the Elecraft website.)
>
> The KX2 uses the simpler Si531 synthesizer, but does have the same 
> temperature probe hardware as the KX3. I am not aware though if the 
> KX2 software has frequency temperature compensation functionality - 
> never seen it.
> But at least this shows a performance difference between the KX3 
> (Si570, same as K3s, high performance) and the KX2 (Si531 simpler 
> generator)
>
> It is unfortunate that the K4 schematics are not made available. I can 
> understand the need to protect IP but I would happily sign an NDA to 
> get them (and not post K4 schematic details on the list).
>
> In any case, "K3 with KSYN3 is probably very different here than K3 
> with KSYN3A" for your diversity measurements. For the results of your 
> study, you may want to include that parameter.
>
> 73, Geert Jan PE1HZG
>
> PS: the schematics are the B version of the KSYN3A. A key difference 
> is the inclusion of the U12 reset circuit. Without, the KSYN3A can 
> possibly corrupt the programming of the DSP PIC it seems, and that 
> would warrant a trip to the factory since this device is not field 
> programmable. Elecraft sells retrofit mod boards to add U12 to 
> existing A versions of the KSYN3A and as long as the PIC isn't 
> corrupted you can fit the mod board yourself - I have.
> Back to the diversity phase shift discussion....
>
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