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[Linrad] Re: A/D Margin 0.00
- Subject: [Linrad] Re: A/D Margin 0.00
- From: aol.com; stanka1ze@xxxxxxxxxxxxxxxx>
- Date: Mon, 19 Oct 2009 19:36:00 -0400
Thanks for the ideas. I suspect it is wideband. I see the horizontal lines in the waterfall but looking 2 mhz wide from 27-30 I could find no narrow spikes. I think the noise blanker eliminates it good. I will probably be on a holiday until spring and won't be able to put the cavity in front of the preamp. I have used the cavity but it needs looser coupling as the response is too narrow.
Thanks again for the great program, Linrad.
From: Leif Asbrink <leif@xxxxxxxxxx>
To: linrad@xxxxxxxxxxxxxxxxxxxx style='visibility:hidden'>
Sent: Mon, Oct 19, 2009 5:18 pm
Subject: [Linrad] Re: A/D Margin 0.00
> 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
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.
Leif / SM5BSZ
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