[Linrad] Linrad and MAP65-IQ Calibration

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

Leif,

I finally got around today to making some careful frequency
measurements
on a known frequency source. So that you understand what I was doing,
the signal source is a GPS-locked signal generator set to 144.126000
MHz.
My XV144  transverter has its 116MHz LO locked to 10MHz from the GPS.
The 28MHz transverter output is teed out to:
1. SDR-IQ, then to Linrad + MAP65-IQ (both running on Computer 1),
2. FT1000MP Mk5 tuned to 144.125000 MHz into WSJT6 (running on
computer 2).

The FT1000MP is not GPS-locked, but seems to be within a few Hz.

I note the following frequencies:

Linrad: 144.126330 MHz (high by 330Hz),

MAP65-IQ with QSO frequency set to 125kHz: DF = -320Hz,
which translates to 125 + 1.270 - 0.320 = 125.950 kHz (50Hz low),

FT1000MP + WSJT6 on true frequency scale: 1000 Hz (spot on!).

It is possible that there are compensating errors in the FT1000MP and
computer 2,
but they are surely small.
It appears to me that the SDR-IQ + Linrad are 330Hz high,
but that MAP65-IQ (even with Fcal = 0) is by chance mostly
compensating for this!

330Hz is not a big error in WSJT terms, but it is enough to be a pain
when working CW and transferring a frequency from Linrad into the
FT1000MP
with a narrow audio DSP filter. It involves some quick mental
arithmetic,
or a serious search to find the CW signal on the FT1000MP!
Yes, perhaps I should be using Linrad as the receiver, but the
FT1000MP
has to be set anyway for transmitting.

I would like to understand this properly before I start making
changes!
If I have understood you correctly, I first need to set
SDR14_SAMPLING_CLOCK
in sdrdef.h (and recompile) to correct the Linrad reading.
Then I need to set Fcal in MAP-65-IQ to bring that into line.
Is this correct?

If this is the right approach, then may I request that the next
version of Linrad
brings the sampling clock frequency out into the setup in some form,
perhaps just as correction of up to +/-1000Hz if that is enough.
Recompiling is ok, but it needs to be done for every new version of
Linrad,
and there are lots of SDR-IQ users out there who would not wish to do
that,
but who would like to correct this sampling frequency error.

Linrad + SDR-IQ is a great combination - all thanks to you and Joe.

73  Guy  VK2KU


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