[Moon-net] Linrad + SDR-IQ. Please help

[Leif Asbrink]

Hello Gabriel,

I answer here on moon-net, to also announce the Linrad 
mailing list:
http://groups.google.com/group/linrad
(The old list at Antennspecialisten is closed)
Please send further questions to the Linrad list.

> The results with Linrad and the SDR-IQ were absolutely disappointing, but
> I'm sure that it must be because I have wrongly configured something, so I
> will appreciate your help.
> 
> I have noticed three major problems:
> 
>  1.- The signal repeatedly seems to drop for some seconds and then return to
> the normal level. I looks like if someone was switching on and off the
> preamplifier all the time.  This does not happen with SpectraVue or WinRad,
> so I don't think it's a problem of the SDR-IQ. You can see a screenshot of
> this effect at http://www.vhfdx.net/temp/lines_linrad.jpg
Your noise blanker setting is extremely aggressive. The dumb blanker, yellow,
is 6mm above the noise floor on my screen. The red line which marks the 20 dB
point is at 27 mm so you are running with the blanker threshold 3.7 dB
above the noise floor. The yellow number in the upper left corner of
the high resolution graph (1 on your image) shows the percentage of time when
the signal is gated out. During the intervals where the signal has dropped,
the blanker level is presumably a little lower, and then the percentage of 
signal removed would grow.

For weak signal work on VHF you should click on the yellow A to go to manual
mode. In auto mode, Linrad tries to keep the threshold at a constant height
above the noise floor. The noise floor is however the noise of the signal
output from the blanker so when the blanker operates on noise only,
the noise floor itself is reduced causing the level to stay 3.7 dB above
a level which is actually below the noise floor.

In manual mode you can set low blanker thresholds. Normally there is noo need,
but it could be helpful during static rain.

I suggest you set the yellow line 10 dB above the noise floor in manual
mode to start with. You can try to optimize it later, it will make a difference
only if you have very small pulses that occur extremely often.

>  2.- Signals are drifting up. In the baseband waterfall I can see how all
> signals I receive are always drifting up. The same signals received with the
> TS-790 were absolutely stable.
This must be due to the SSB-Electronics 144 MHz downconverter.  Try
to listen on 28 MHz with the TS-790.

>  3.- Signals in the middle of the receiver bandwidth are noticeably stronger
> that near the edges. You can also see this effect in the screenshot at
> http://www.vhfdx.net/temp/lines_linrad.jpg
> 
> I used M_CIC2 [16], M_CIC5 [13], M_RCF [2], OL_RCF [7].
The parameters you have selected can provide the best possible
flat bandwidth in Linrad, but only if you perform the amplitude
calibration with a pulse generator. The reason for the non-flat
response is that decimating with only 2 in the RCF filter will
use a very wide portion of the output from CIC5 which is not
flat over such a wide range. The digital filter that Linrad
will add when it is calibrated will correct for this.

In case you do not want to do the calibration, then I suggest
you change to M_CIC2 [16], M_CIC5 [9], M_RCF [3], OL_RCF [7].
That will not give a quite flat noise floor, but it will be
less curved than the one you have now.

For calibration you need a square wave at about 100 Hz - but
not exactly (to avoid it to be phase stable to 50Hz hum)

73

Leif / SM5BSZ