[Elecraft] L34 on K2 tuning question?

[David Woolley (E.L)]

Geoffrey Mackenzie-Kennedy wrote:

> for tuning L34 using the quote 'weak' 7 MHz birdie - more like a angry
> elephant. After doing some work to reduce the number of K2  receiver

It's not all that strong compared with sky noise with even a small antenna.

> birdies, it struck me that the 7 MHz birdie signal is entering the receiver
> chain at not just one place, or could be perhaps two separate signals from
> different sources.  If one 'birdie' signal had established itself in the
> receiver chain before L34 and another 'birdie' signal  entered the chain

I am pretty sure that the birdy is a harmonic of the main 
microcontroller instruction rate and is entering before the mixer.  It's 
very unlikely that a post mixer birdy would move in the same way with 
tuning setting.

> AFTER L34, then tuning L34 as suggested in the manual using the birdie 
> might
> not result in best overall receiver performance in terms of noise floor.

What I would have liked is an explanation of the theory behind the 
separate noise and signal peaks.  The possibilities I can think of are:

- some of the noise is coming in on the image frequency, but that
   would seem to be so far away as to be completely in the stop band
   for L34.
- there is a close in gap in the crystal filter stop band and the noise
   is coming in there - possible, but I wouldn't have thought a filter
   weakness would be enough for just white noise to be an issue.
- the combination of the L34 setting and reactance in one of the
   adjacent chips reduces the internal generated noise voltage at the
   the chip interface - I'm not sure why people should be saying that
   one should be specifically optimising for noise above the birdy.

-- 
David Woolley
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