Hi Jim ah now that is nice, it nicely get round the loading effects on the
bridged-T in a 50 ohm environment. I guess a higher Q coil would be easy to
fabricate if necessary.
May I add that to the page on the filter on my site please?
Best Wishes
Alan G3NYK
----- Original Message -----
From: "James Moritz" <[email protected]>
To: <[email protected]>
Sent: Thursday, May 21, 2009 12:02 AM
Subject: LF: Re: 518khz - Notch Filter.
> 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
>
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