[DSP-10] 5khz birdies and noise

[Lee Scott]

After much thinking about the wave of noise, I've come to a conclusion. I 
may be so far off with my thoughts that I need someone like W7PUA to set me 
straight. - Anyway, here goes....

The audio window is just under 5khz wide and we are moving that window 5khz 
for a total of about 10khz.  We are asking the crystal filter to be flat 
for that 10khz.  If we adjust the filter coils L12 and L13 only for max 
signal, we may indeed be peaking that filter with a flat top width of about 
2khz.  If we start measuring the noise at a frequency just above the 5khz 
step and then slowly move up in frequency, you will see the peak of the 
noise at the high end of the audio window and it will look like a wave as 
we move up in frequency until this wave is at the low end of the audio 
window.  Once we reach that 5khz step point, the noise wave will then move 
back up to the top of the window.

Unless I am wrong, we really need a procedure for adjusting the crystal 
coils L12 and L13 so the crystal filter is as flat as possible for the full 
10khz and not just centered in the 10khz window with it's 3db points at +/- 
5khz.  What I tried was to set the frequency to the low end of the 5khz 
step and adjust L12, set the frequency to the high end of the 5khz step and 
adjust L13.  By doing so, I was able to see 2 noise waves which kind of 
backs up what I've been describing. At least in my mind.

If the local osc was continuously variable and we weren't moving an audio 
window, you would never see this phenomenon and we could happily set up the 
crystal filter for a 3khz bandwidth and be pretty much flat across it.

Anyone else have any thoughts on this???

As to the 5khz birdie I have, I have ran heavy grounding straps from the 
board to the chassis, from the board to the DSP box, I drilled 6 small 
holes in the top of the DSP box and soldered 14awg wire to tie the top and 
bottom foils together next to each screw. I have double checked all 
grounding wires and touched up any which might be less than perfect.  I 
think I've reduced this birdie about 1db so it is now only 4db in 
height.  I did manage to lower the overall noise of the receiver from 
-149dbm to -152dbm with a 50 ohm termination on the antenna so there was an 
improvement there.  The birdie still moves up in the audio frequency when 
you move up in receiver frequency.  Since there are no actual frequency 
changes within the 5khz step, I still feel this has to be internal to the 
DSP program.

One other ground point which eliminated my digital noise was to run a 
ground strap from pin 5 of the serial jack to its chassis mtg screw.  I had 
only grounded one end shields before at the DSP box.

Hope my ramblings aren't too far afield.

Lee Scott - AA1YN