[Linrad] Re: Frequency scale in Linrad

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

Hello Ermanno,

> Use Linrad as front-end for MAP65 and WSJT.
> To view an jt65 
> signal, on the waterfall of linrad (SSB), shift the BFO to -1270.5 and filter 
> to about 2.4 kHz.
> The frequency in Linrad corresponds exactly to the specified 
> frequency of MAP65 (+/- Doppler).
> The audio output from Linrad, which uses as 
> input for WSJT is perfectly focused (DF = 0) on the desired signal.
> 
> The 
> problem is that the scales of frequency of all windows of Linrad indicate a 
> different frequency, that is without compensation of the BFO.
> It is difficult 
> to click on the signal desired in high resolution spectrum, especially when 
> there are many JT65 signals in a few kHz.
> 
> How to get the right indication on 
> the scales-frequency of Linrad  ?
The frequency readout gives the frequency of the carrier. In SSB mode
that is the frequency of the BFO. In CW mode the carrier is assumed to be
placed at the center of the baseband filter. If you use AFC it will
be forced to the center if locking is sucessful.

You should run in CW mode. Then a carrier (or a JT65 sync tone) that
is placed at the baseband center would produce a frequency readout
that is in agreement with the frequency scale in all the graphs 
independently of the BFO setting. By placing the BFO 1270.5 Hz below
the passband center you can make the frequency of a signal at the
passband center appear at 1270.5 Hz in the output. In this way the
true frequency of the sync tone which would be shown in the frequency
readout as well as on all scales will differ from the MAP65 readout
by 1270.5 Hz. That is because Joe decided it is easier that way for 
people who use an ordinary radio. (I am NOT in favour of that, but now
it is the established "standard" for JT65. )

To make all the scales as well as the frequency readout show the true
frequency with a 1270.5 Hz shift you should make a users_hwaredriver.c
that sets a frequency that differs from the hardware frequency by 1270.5 Hz.

If you use the WSE units, edit users.c and make it contain your own version 
of wse_sdrxx.c which would differ from the standard version by having
int rx2500_fqcorr=1270.5; (Or perhaps minus...)

That way scales and readout would no longer show the frequency, they
would all show the true frequency plus (or minus) 1270.5 Hz.

In case you are not satisfied by having Linrad and MAP65 showing the
same frequency but want the frequency of both to agree with the true
frequency you would have to add a frequency correction to the 1270.5 Hz
and add the same correction to the frequency that you feed into MAP65.

73

Leif / SM5BSZ

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