[Elecraft] Birdies

[Vic K2VCO]

Hans-Christoph Scheiblberger wrote:

> My rig seems to have quite a few birdies

I have been struggling with them for some time, and have had the able 
help of Geoff, GM4ESD, who has done various experiments and analyses.

The worst ones seem to be related to the bias oscillator in the KPA100 
(note that this oscillator runs at all times when the KPA100 is 
installed, whether or not the power is over 10 watts).  The oscillator 
crystal was originally 16.xxx MHz and in newer KPA100s or those with the 
mod kit installed, 18.xxx MHz.  The birdies are the result of harmonics 
of the oscillator mixing with harmonics of other oscillators in the K2. 
  For example, I have one at 28.018 Mhz which results from (5 x 23.105) 
- (6 x 18.434) = 4.9 Mhz. approximately.   The 18.434 is the bias 
oscillator and the 23.105 Mhz is the VCO when the K2 is tuned to 28.018.

Note that the mixing takes place at VHF, so it's quite hard to get a 
good ground point for bypassing and shielding purposes.  Some of the 
following may help a little with some birdies:

1) Adding additional filtering to the output of the bias oscillator, 
where it feeds the pin diode switch and where it goes to the RS232 chip. 
   I used 10 uh chokes and 0.001 uf capacitors.

2) Putting a the ribbon cable from the amplifier through a ferrite toroid.

3) Replacing the traces that carry the speaker lines across the KPA 
board with a length of RG/174 coax that is grounded only at the K2 RF board.

I was able to reduce my 28 MHz birdie from S7/S8 to S2/S3.  Still loud 
enough to hear quite well with the antenna on.

I went so far as to install a shield over the top of the K2 RF board. 
It didn't seem to help with the 28 MHz birdie.  You can see it as well 
as the toroid on the ribbon cable and the coax to the speaker at
<http://www.qsl.net/k2vco/k2shield.jpg>.  I left it in place because who 
knows what other spurs it may reduce!

-- 
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco