FO-20 PSK Modem Improvement for Microsat Use
	     --------------------------------------------
		       by James Miller G3RUH
			    1991 Feb 19
Introduction
------------
  Some users of my 1200 bps PSK modem have commented that data recovery
with the microsats is poor, difficult or in a few cases impossible.

  Personally I have no problem whatever with my standard modem and
standard FT736R, but persistent reports prompted further investigation.

  Fuji-Oscar-20 and AO-13 (400 bps) generate "pure" PSK.  That is, the
carrier phase is either 0 degrees or 180 degrees, and nothing else.

  Extensive investigation shows that all three microsats (AO-16, WO-18 and
LO-19) generate random phase noise.  I (G3RUH) found peaks of +/-27
degrees on occasions with +/-10 degrees RMS scatter typical.

  My modem, being designed "cold" and in wide circulation (1986) before
FO-12 had even transmitted a packet, expected clean PSK.  So its post
phase detector (i.e. data) filter is an RC network R30/C3.  This is
entirely  satisfactory for FO-20, but it hasn't really enough smoothing to
deal comfortably with the microsat phase jitter.

  The cure is to upgrade the data filter.  There is a spare op-amp on the
board, so by the adding just 4 resistors and 2 capacitors, a 3-pole filter
can quickly be implemented (15 minutes).  The filter offers some 30 db
attenuation to the effects of excessive phase noise, and you'll get
effortless 100% data recovery from PACSAT, WEBER and LUSAT.

Application
-----------
  This modification is for PCBs  marked "JAS-1/FO-12 MODEM (C)1986 G3RUH",
issues 1 & 2.

Circuit
-------
  Note that the existing R30 is removed, and the input of this circuit is
from its left-hand hole, close to the legend "VR2".  The existing C3
remains and becomes part of the new filter.  The circuit uses a spare
section of op-amp U5 so only 6 components   Ra,Rb,Rc,Rd,Ca,Cb are new.

		     Cb | |
	     +----------| |-------------------+
From         |       10n| |                   |
U7           |                 | \            |
pin 3        |                 |   \          |
       Ra    |   Rb          12|     \        |
>---/\/\/\/-++-/\/\/\/-+-------| +     \  14  |     Rd             To TP3
      27k   |   27k    |       |    U5   \--+-+---/\/\/\/---+----->
	    \          |     13|         /  |       15k     |
	    /          |    +--| -     /    |               |
	 Rc \      Ca  |    |  |     /      |      [  C3  -----  ]
	27k /        -----  |  |   /        |      [  10n -----  ]
	    \        -----  |  | /          |      [        |    ]
	    |      4n7 |    |               |               |
	    |          |    +---------------+              ===
	    |          |
	----+----------+----> 6v (U5 pin 3)       3rd Order Bessel Filter
						  =======================

Note: 10n = 0.01uf   = 10000 pf )  Use 10% or
      4n7 = 0.0047uf =  4700 pf )  better.