[Elecraft] Chirp on K3

[Bill W4ZV]

Poul Erik Karlshøj (PKA) wrote:
> 
> This is only interesting for users of CW on the K3.
> 
> You might want to check and see if your K3 - like mine #3674 with 3.68
> firmware - produces a badly chirping signal when the VFO is set to a
> frequency in a very narrow range of 3525.970 to 3526.030 kHz while the
> rcvr is using 400 Hz x-tal filter (in my case FL3). Be aware that TX must
> be set to use FL1. You need to check for chirp on a monitoring rcvr.
> 
> It was found by pure coincidence during a QSO and it was quite embarassing
> to get that kind of report when using a rig like K3. It has nothing to do
> with RF level, RFI or poor PSU regulation.
> 
> Several others have reported to me, that they don't observe this problem
> on their K3s. So maybe you are just as lucky - if it is a question of luck
> ;-)
> 
> Elecraft have (quickly!) responded that they discovered it "not long ago"
> and suggest thast I solder in a 51-55 ohm resistor across R20 on the main
> Synth board. I haven't done this yet, but will report here when I have
> done so.
> 
> I don't know if the chirp only applies to this particular combination of
> VFO frequency and x-tal filter. Anyone else having relevant info please
> post here.
> 
> 
> 73/OZ4UN
> Paul
> 

I had this problem on my #1361 during the Stew Perry contest in December
when a friend forwarded an embarrassing recording.  Here's a comment from
the VCO designer KI6WX:

"The problem you are seeing is probably due to ringing in the DDS crystal
filter when it shifts from one edge of its passband to the other.  You can
confirm this by looking at your DDS frequency in your menu at these two
frequencies.  You will probably see that the DDS frequencies are separated
by about 2 kHz.  This problem only occurs rarely at specific frequencies.  I
had developed a more complicated fix and Wayne figured out the simpler one. 
I would recommend making this one change of adding the 51 ohm resistor in
parallel (de W4ZV...across R20 on the Main K3SYN board...not necessary on
Sub since TX only uses Main).  It is independent of any other synthesizer
changes that may occur in the future and will fix this one specific
problem."

73,  Bill


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