[Linrad] Re: A/D Margin 0.00

[Leif Asbrink <sm5bsz.com; leif@xxxxxxxxxxxxxxxx>]

Hello Stan,

> I am using Linrad-03.08 with Perseus. I have a 144 to 28 
> converter in front of the Perseus. I have discovered something 
> that surprises me. When pointing my antenna in 1 direction 
> the A/D amplitude margin starts in the 60's but in a short 
> period of time, (less than a minute) it drops to 0.00. 
> All of the other Amplitude margins remain good and I see 
> no trace on the waterfall display indicating a strong signal 
> had appeared and caused the A?D to drop to zero. Everything 
> appears to be working fine. Hitting the "Z" key rerstors the 
> A?D margin but  in less than a minute it drops to 0.00. 
> This is directional. When pointing mu antenna in other 
> directions the A?D does not drop..
The Perseus is saturated occasionally. Every time the A/D 
converter saturates the Perseus will set a flag and Linrad will
tell you it happened by setting the A/D margin to zero.

I suggest you disable the second FFT and set the sampling rate 
to 2 MHz with no waterfall averaging. You would then have no 
noise blanker and you would see wideband pulses as white horizontal
lineas. Change the frequency in steps of 1.5 MHz or so and see 
if you can locate a pulsed narrowband signal.

Occasional saturation of the A/D is not necessarily harmful.
Every strong signal in the passband will get an incorrect
amplitude value and therefore produce large wideband "keying
clicks" The Linrad noise blanker might remove them completely.

In case saturation lasts a little longer you might find false
responses and it could be a good idea to reduce the total gain.
In case your amplifier chain is already optimized for gain 
(the preamplifier dominates and increases the noise floor by 
about 17 dB) the only improvement you can add is in the form 
of filters.

In case saturation is caused by wideband pulses there is no reason
to worry about it. You could get rid of the problem by reducing
the bandwidth into the Perseus. If there is a pulsed transmitter
somewhere outside the band you should reduce it with a suitable 
filter. Maybe a cavity filter on 144 because the signal level 
required to saturate the Perseus is pretty high and probably not
within the linear range of your 144 to 28 converter.

73

Leif / SM5BSZ


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