SP KI6QE @ KI6QE < G3RUH
Better UO-22 Decoding

KI6QE de G3RUH 1992 Feb 13
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                       Better UO-22 Decoding
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Dave - you are right.  UO-22 is less than optimum.  The problem 
starts in the satellite which does not have a transmit spectrum 
extending to DC, nor even to the desirable 30 Hz.  In fact it is 3db 
down at 100 Hz.  The effect of this is to cause "droop" on short 
runs of 1s or 0s.  It can clearly be seen on a scope.  Display the 
eye diagram, and slow the sweep speed down so that a dozen or so 
bits is visible.  Looked at another way, the poor LF performance 
introduces wobble on the trace, and this blurrs the eye.  So if the 
receive system was so-so (say with UO-14) then it may well be very 
error prone from UO-22.

The cure is to make the receive system have as good an HF 
performance as possible, and a good LF performance.  Having a good 
HF response ensures a good eye, and thus a better margin to cope 
with the LF wobble.  And having a good LF response minimises and 
additional self noise from the RX/modem interface.

On the modem increase C25 to 1 uf.  This is the RXAudio input 
coupling capacitor.

On an FT736R:
 1. Use a CFW455B (or C or D) IF filter in the RX UNIT.

 2. On the RX UNIT, remove C82.  This is a little ceramic capacitor 
    tucked in close to the grey cube marked "455D".  Bend it back and 
    forth until the legs snap off.  You can reach it by removing 
    the radio lid only.  DO THIS!

When you have done these changes, TX selection 10 transmitting to an 
FT736R gives a virtually perfect eye.  Since UO-22 also transmits 
selection 10, you can see the extent of the LF aberration as a 
blurring at the "eye" convergence point.  However you should now 
have reliable decoding.

Other radios seem not to be as reluctant as the FT736R, probably 
because they have a better basic HF response.  However, changing 
modem C25 should help.

I am evaluating the feasibilty of implementing LF equaliser to 
rectify the UO-22 LF problem.  The perfect project for all you DSP 
freaks.  I'm on holiday for two weeks.  I expect one of you lot to 
have done it by the time I get back.  No kidding.

73 de James G3RUH @ GB7DDX.#22.GBR.EU
/EX