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| Subject: | LF: Re: 518khz - Notch Filter. |
| From: | "James Moritz" <[email protected]> |
| Date: | Wed, 20 May 2009 02:01:28 +0100 |
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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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