TAPR1.1

DEMODULATOR SIDE MODIFICATION:

1.  HOW:    Change U4 from a TL084 to a TLC274 op-amp.
            Replace D2 (1N4148) with a germanium diode (Bvr > 15 volts)
            or if not available with a 1N3600 silicon diode.

    WHY:   Noise margin.  The data slicer output is extremely dirty.  An
increase in noise margin represents improved bit recovery performance.

The TLC274 can pull its output nearly to ground, the TL084 cannot.  This
becomes important in U4A, where the op-amp is used as a schmitt-trigger
comparator (data slicer).  The op-amp output drives a digital input to ground
through diode D2.  D2, a 1N4148, adds 0.9 volts offset from ground, and a
TL084 will add from 0.5 to 0.8 volts to this offset. Since a 74HCxx series
part must see less than 1.4 volts (VCC=5.0 v) to  sense a logic 0, the
resulting noise margin may be as low as 1.4-(0.8+0.9)= -0.3 volts.  This will
cause bit recovery errors and intermittent dropout of DCD (i.e. many retries)
With the TLC274 and 1N3600, noise margin is 1.4-(0.05+0.6)= 0.75 volts - much
better-.  Germanium diodes, now somewhat rare, add only 0.35 volts offset and
will boost noise margin even further to about 1.0 volts.

A trade off is that the TL084 slew rate is twice as fast as the TLC274, making
it a faster comparator.  However, increased noise margin seems to be the
critical item in my modem.
CONTINUED IN TAPR1.2