LF,
Thanks for all the replies, comments & suggestions.
I thought I could do the filtering within the SpecLab software but all my
attempts have had no effect at all, I am not really aware how to manipulate
the filter controls to do this, if anyone can help with that it could be the
answer. I will post a query to the SpecLab group.
Jim, that is correct regarding the LO=512KHz & I do have a couple of
alternatives but unsure until I check if they will resolve the problem.
Rik, I guessed that the Q of the circuit needs to be improved but unsure how
to go about it in that sort of arrangement, I will look at your ideas.
Andy, I will look at the link you provided soon.
Alan, thanks for the link & your comments.
Graham, the 518KHz signal is so strong at times I think attenuator would be
so large it would degrade the rest of the band, thanks for your comment.
73
Gary - G4WGT.
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of James Moritz
Sent: 20 May 2009 02:01
To: [email protected]
Subject: LF: Re: 518khz - Notch Filter.
Dear Gary, LF Group,
An image of 518kHz at 506kHz implies LO = 512kHz? You could change the LO
frequency to 500kHz, although then you would not be able to receive signals
on that spot frequency. You could also try optimising the amplitude/phase
"tweaks" in the software to give maximum rejection of the unwanted sideband
at 518kHz.
A simple notch filter with high rejection will inevitably have significant
loss at nearby frequencies - there is a trade-off between Q, depth of notch,
loss of wanted signal and frequency difference between wanted signal and
notch frequencies. This can be improved by adding aditional components, e.g.
adding a series capacitor to a parallel-tuned trap can "tune out" the loss
at a frequency below the notch frequency. It is feasible to get something
like a few dB loss at 504kHz with 20 - 25dB at 518kHz. More complicated
designs like Andy's have a much nicer response shape, but may require
unachieveably high inductor Q to get the design response.
Cheers, Jim Moritz
73 de M0BMU
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