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
"The Elecraft list is a forum for the discussion of topics related to
Elecraft products and more general topics related ham radio"
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