[Linrad] Re: Linrad and MAP65-IQ Calibration

[VK2KU <clearmail.com.au; vk2ku@xxxxxxxxxxxxxxxx>]

Fantastic, Leif.
This is something every SDR-IQ owner needs to do!

Linrad on my system was reading high by about 330Hz due to a slight
error in the sampling frequency of the SDR-IQ,
and the MAP65-IQ narrow waterfall was also a bit out.

This is what you do to fix it.

1. Find a reliable signal source of accurately known frequency.
GPS-locked SigGen, WWV or whatever.
I used my SigGen on 144.125MHz (or 28.125MHz).

2. Run SpectraVue and note the exact frequency measured by it.

3. Go into SDR-IQ setup in SpectraVue, and use the built-in calculator
to find the actual SDR-IQ sampling frequency.
SpectraVue will apply the correction automatically. My sampling
frequency turned out to be 66.665840MHz.
I had assumed, reasonably, that this correction was only made in
SpectraVue, but not so!
It is actually written to non-volatile RAM in the SDR-IQ.

4. Close SpectraVue and run Linrad. The frequency should now be
correct: 28.125MHz.

5. Open MAP65-IQ and set Fcal to give the correct reading on the
narrow waterfall.
In my case this would be -270Hz with the QSO frequency on 124.
i.e. 124 + 1.270 - 0.270 gives 125. I had to set Fcal to -376Hz.

There will still be a drift of a few Hz as the crystal in the SDR-IQ
warms up - no oven!

Thanks for pointing me in the right direction, Leif.

73  Guy  VK2KU

On Apr 2, 3:24 am, Leif Asbrink <l...@xxxxxxxxxx> wrote:

> There is another option. You may use SpectraVue to change
> the contents of the non-volatile RAM inside the SDR-IQ.
> I never did that and I do not know what effects it would
> have, but most likely it would be equivalent to change
> SDR14_SAMPLING_CLOCK
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