[Elecraft] Elecraft] K3 K144XV spurious signal on 144,414.1

[Sverre Holm (LA3ZA)]

Thanks, Ian, for your very thorough detective work. This is very interesting.
The odd thing is that the frequency doesn't seem to compute easily from any
of the known frequencies inside the K3:

- 14 645.6 kHz DSP clock
- 16 000 kHz PIC clock
-   8 230 kHz 2nd mixer crystal 
- 49 380 kHz Reference frequency

but there may be more


gm3sek wrote
> Probing around using a very small insulated pickup loop and a completely
> separate 2m receiver, the 144.416MHz signal is definitely not a spurious
> response of the K3's receiver. It is a real carrier and it's coming from
> the region of the DSP board(s) behind the front panel. 
> 
> The frequency seems to be completely independent of the K3's main
> tuning, band or mode,  and on my particular K3 the frequency is
> 144.4162MHz (zero-beating on a GPS-locked receiver).
> 
> The signal is quite loud with the probe loop held anywhere close to the
> DSP board along its whole length and on either side. The signal is
> louder when the loop is anywhere close to the main voltage regulator,
> the board interconnects or one of the mounting pillars; and when the
> probe is directly above or below U17 (EPM240TQFP100) it's S9+. All this
> suggests quite a strong discrete signal source that doesn't have
> sufficient local decoupling, so it is escaping via the supply rails and
> probably creating circulating currents in the groundplane and metalwork.
> 
> The frequency is somehow being generated from the 14.7456MHz DSP clock
> crystal Y1 (as can be heard by probing very close to the board, which
> detunes the frequency). That isn't a precision oscillator, which
> probably accounts for any small differences in the reported frequency. 





-----
Sverre, LA3ZA

K2 #2198, K3 #3391,
LA3ZA Blog: http://la3za.blogspot.com,
LA3ZA Unofficial Guide to K2 modifications: http://la3za.blogspot.com/p/la3za-unofficial-guide-to-elecraft-k2.html
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