testing filters

[Andre' Kesteloot]

Dick Rollema wrote:

> 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