[ARC5] Selectivity improvement in ARC-5 receivers.

[Bruce Long via ARC5]

The rejection slope (dB/octave) of a coupled bandpass filter does not depend upon the filter center frequency in Hz but rather by the offset frequency ( distance from the filter center frequency) divided by the filter bandwidth.

Thus if you have a bandpass filter with a bandwidth of 10 kHz ( at any center frequency) and you are measuring the rejection at an offset frequency of 60 Khz ( ie 6 Octaves) you will be x dB down.



________________________________
 From: Kenneth G. Gordon <kgordon2006 at frontier.com>
To: Arc5 at mailman.qth.net 
Sent: Sunday, June 1, 2014 5:03 PM
Subject: [ARC5] Selectivity improvement in ARC-5 receivers.
 

This month's Electric Radio Magazine has Jeep Platt's article on the subject 
above in it. Jeep's article is very well written and very useful.

However, I have had a question concerning ultimate stop-band selectivity for 
some time now after a discussion with another of our list-members during our 
discussion on Jeep's final method.

It is my understanding that the ultimate stop-band selectivity is determined 
NOT by any physical characteristic of the IF transformers, but, rather, by the 
IF frequency itself.

I.e., at the 60 db down point, the true selectivity is determined more by the IF 
frequency than the separation of the two IF coils.

I vaguely remember from my EE course in filters that it was "so many db per 
octave", or something similar.

Can anyone here tell us what I am even talking about here, and fill in the 
details?

I'm at a loss, but still very confused.

Kenneth G. Gordon W7EKB

"Courage is being scared to death but saddling up anyway."--- John   Wayne

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