[Linrad] Re: SDR-IQ + LINRAD

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

Hi Dom,

> tnx for reply Leif , now my Linrad copy is working.
> I'll have to run gain calibration. I have several hardwares runing , but 
> using the same PC and same Linrad. All of them are using SDR-IQ , but 
> sometimes , I use it on 2m,
> 23cm , 6cm or 3cm. Is it necessary to calibrate the full hardware chain , or 
> SDR-IQ is enougth?

It is enough to calibrate the SDR-IQ, and perhaps not even that because
you may use someone elses calibration file for SDR-IQ or SDR-14.

I could supply a set of calibration files for the most popular
decimation numbers if I knew what would be popular.

Someone might want
CIC2=16
CIC3=12
RCF=2
This would give a usable bandwidth of 166 kHz with a sampling rate 
of 173.61 kHz.

Someone else might want
CIC2=4
CIC3=4
RCF=24
The sampling rate would be the same, but the usable bandwidth would
be only something like 50 kHz.

The large bandwidth mode suffers from spurs at large separations
and would be useful in case you have a preselector or converter
that does not let very strong signals through at large frequency
separations. The small bandwidth mode would be for a wideband
input without preselector. The entire 0 to 30 MHz frequency span
when signals at the spur frequency could be very much stronger
(e.g. 7 MHz BC stations) than the signals of interest at e.g. 21 MHz.

The relation between decimation numbers and spur suppression 
is described in the AD6620 data sheet.

Most users would probably go for a compromise, but in case you 
want optimum noise blanking on 144 MHz you might want the largest
possible bandwidth. (That would require Linux....)

I know what Joe has recommended for MAP65-IQ:
M_CIC2 = 10
M_CIC5 = 10
M_RCF = 7
OL_RCF = 6

This would be a normal compromise between spur suppression and bandwidth.
The same sample rate could also be obtained with e.g. this set:
M_CIC2 = 14
M_CIC5 = 25
M_RCF = 2

More bandwidth and less spur suppression. A signal only about 50 kHz 
outside the band edge will cause a spur that is only suppressed by about
60 dB. Such spurs do never occur near the passband center however so if
one is interested in the center 80 kHz for weak signals these spurs are 
at -100 dB or below. The full bandwidth would still be of value for
the noise blanker.

> If it is necessary to get a different calibration on each band , as , if I 
> am rigth , there is only one calibration file possible , should I have on 
> the PC a Linrad for each band?
You might want several Linrads on your computer. Not necessarily for
different bands, but you might find it useful to have things like a
very large S-meter and wide bandwidth for noise measurements, another
with a large and extremely slow waterfall, one with extreme settings for
weak CW (=EME) and another with weak CW and delay times compatible
with terrestrial communication etc.
 
With linrad.exe, help.lir and errors.lir in your path you can
run linrad from separate directories and they will have their own
parameter files.

73

Leif / SM5BSZ


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