Dear Gary, LF group,
Since this kind of notch filter has other useful applications too, I had a
go at designing something useable from available bits. I used an IF
transformer from an ancient transistor radio as a parallel-tuned trap - see
the attachment.The circuit is essentially an RF bridge - the1:1 transformer
and preset resistor balance out the finite resistance of the trap at
resonance. This is a similar principle to Alan's bridged-T circuit. With
critical "safe-cracking" adjustments, rejection of 60dB can be achieved at a
spot frequency, although the notch is very narrow. But >30dB can be achieved
over a useful bandwidth. The 3n3 capacitors flatten out the passband near
the cut off frequency and make the response a squarer shape on the LF side
of the notch. The improved notch depth is traded off for higher insertion
loss, but this is still fairly reasonable at around 4dB. The input/output
impedances should be of the order of 50 ohms, but are not critical. You
could make a better filter if the Q was higher - the IF transformer had Q
around 90; an MW ferrite rod aerial can have a Q of 200 or so at this
frequency.
The performance of my prototype with 50R source and load looked like this:
Loss in passband - 4dB
Response down by 3dB at 348kHz, 511.5kHz, 540kHz
Bandwidth of rejection notch at -20dB: 2.2kHz, -30dB: 700Hz, -40dB: 200Hz
Looking at G4WGT's grabber, that should be adequate to more or less
eliminate the Navtex signal image. When I put it between my antenna and RX,
it certainly reduced the louder Navtex signals from S9 down into the noise.
Cheers, Jim moritz
73 de M0BMU
Navtex_notch.jpg
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