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Message-ID: <5.1.0.14.0.20021113110754.00a927f8@gemini.herts.ac.uk>
Date: Wed, 13 Nov 2002 11:26:34 +0000
To: rsgb_lf_group@blacksheep.org
From: "James Moritz" <j.r.moritz@herts.ac.uk>
In-reply-to: <003401c28afd$99622420$f9997ad5@laptop>
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Subject: LF: Re: Audio filters
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Dear Kevin, LF group,

At 10:11 13/11/2002 +0000, you wrote:
>I'm currently trying to work out a way of getting more audio filtering
>into my latest 136kHz regenerative receiver without increasing the number of
>active devices

The small ferrite cored inductors that are fairly readily available can be 
used to make passive bandpass filters of several hundred Hz BW quite 
easily. If you don't mind winding some pot cores, and have some test 
equipment (audio generator & voltmeter), passive filters with bandwidth 
down to 50Hz are not too difficult. With limited dynamic range, you might 
be better to go for a passive preselector, which can also be made with 
several 100Hz bandwidth, if you can find suitable pot cores.

It is certainly possible to achieve adequate sensitivity with a small 
number of active devices - the electromechanical SAQ receiver I made a 
while back had none at all of course. Another project along these lines I 
once tried was a direct conversion RX for 80m, which was basically a diode 
mixer impedance matched to an antenna a pair of headphones, and a VFO. A 
signal of 2uV was audible with no gain at all; with a single transistor 
audio stage, you could hear down to the band noise level.

Cheers, Jim Moritz
73 de M0BMU