Message-ID: <006c01bf0e66$124f4060$7296b38f@w8k3f0>
From: "Dick Rollema" <d.w.rollema@gironet.nl>
To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: Re: Receivers
Date: Mon, 4 Oct 1999 14:17:26 +0200
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To All from PA0SE

Bob, ZL2CA wrote:

>I am not aware of a
>standard test method to produce measured results of how a receiver
>performs in noise to signal ratio situations (especially with impulse
>QRN on LF), but that particular aspect would be of great interest to
>this discussion group.


What an excellent treatise by Bob. Well done!

I tested the impulse response of my 35 Hz wide audio bandpass filter by
feeding an audio signal at the centre frequency to it, chopped by a relay
contact; the relay being driven from a multivibrator at a speed
corresponding to dots of the morse code. The output signal of the filter is
viewed on a oscilloscope. The timebase is triggered by the multivibrator so
that a steady picture is presented on the screen. Take care of proper source
and load impedances! My filter shows after the response to each audio block
ends a second response, but much smaller  than the main one. The slopes of
the blocks of audio become rounded of course. By increasing the repetition
speed one can observe where the "dots" run together, which is the limit of
the speed that can be handled. However it is more realistic to listen to the
output of the filter. It is then easy to determine what the maximum number
of words per minute  is the filter can pass.
You can do the same with some circuit analysis programs for the computer.
This is what Harry, PA0LQ, did who designed the prototype of the filter I
use.

The same test can be performed on a  receiver by chopping the output of a
signal generator connected to the antenna-input.

73, Dick, PA0SE