LF: Re: 518khz - Notch Filter.

["James Moritz" <james.moritz@btopenworld.com>]

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