[Linrad] Re: SDR-IQ + LINRAD

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

Hi Joe,

> I notice that without a calibration file there is an increase in
> average noise just above the center frequency in the waterfall in my
> configuration.  I think an appropriate calibration file will take care
> of that discontinuity and other irregularities I haven't discovered
> yet.
NO. The response should be symmetric. Calibration (on the SDR-IQ) will
be totally symmetric and can not change the amplitude differently on
different sides of the center frequency.

Try to step the center frequency. Presumably the structure you 
mention will nicely shift by as many kHz as the frequency shift 
you entered.

You may have narrowband noise (manmade from computers etc.) or you
may have a gain in previous stages that varies with frequency.
Switch off the preamplifier or add attenuators at various places
to invertigate the origin of the non-flat noise floor.

> I think these two sets of decimation numbers would be very interesting
> to experiment with (especially with the very bad noise at this
> location).  Hopefully the larger bandwidth will help reduce the noise
> while, as you suggest, the spurs remain low around the center
> frequency.  Although with all the computer, plasma TV, etc. generated
> junk, a few low level spurs may never be noticed.
OK. Some spurs are not really a problem. A good operator will know
where they are and they can always be avoided by a small change of the
center frequency.

On the other hand, more bandwidth will not help against computer, 
plasma TV, etc. generated junk since all of that is narrowband. It would
however be helpful against powerline QRN, static rain, ignition QRN and
everything else that is truly wideband.


> > I know what Joe has recommended for MAP65-IQ:
> > M_CIC2 = 10
> > M_CIC5 = 10
> > M_RCF = 7
> > OL_RCF = 6
> >
> 
> > M_CIC2 = 14
> > M_CIC5 = 25
> > M_RCF = 2
> >
> 
> If it's not too much to ask, would you mind generating the appropriate
> SDR-IQ calibration file for these sets of decimation numbers?

Before doing that I would like to know what other sets of decimation
parameters are in use with Linrad and SDR-IQ/SDR-14. After having set
up the hardware I could as well record a couple of more files the same time.


73

Leif  / SM5BSZ


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