Components R2l, RZL, and Cl7 form a simple R-C 'low-pass f ilter
for the discrirninated signals. lf only 60 wpm (t+5 baud) op*ration is
anticipated, the value of Cl7 should be set to.058 mfd, c.orresponding
to a -3dn response of approximately 30 Hz. This value wil I qive enhanced
iow-pass f ilter performance for 60 wpffi, but will not allow satisfactory per-
formance at 100 wpm. lf operation at 100 wpm or both 50 and 100 wpm is
anticipated, a better choice is to set C17 equal to .039 mfci, corresponding
to a -3dg response of approximately 50 Hz. The change in 60 wpm performance
under these condi tions is sl ight and general Iy not noticeable. Moreover,
equalization for 100 wpm allows compatibility with other commonly used
keying speeds (60, 66, and 75 wpm) whereas the 60 wpm equalization does not.
Operational amplifier OAZ, also a 709N, is used as a very high-gain
slicer amplifier to "square-up" the filtered keying waveform from the
discriminator. The output of the slicer (pin I0, 0A2) is a square-wave with
positive voltage corresponding to a mark condition and negative to space.
This keying signal, through R27, switches the high-voltage keying transistor
(42), keying the TTY loop. Diode D9 prevents negative breakdown of the
2N5655 base-emitter junction. CI5 and R2B are a transient supPression net-
work to protect QZ from the high-voltage transients that can be induced by
the selector magnets of the teleprinter.
A convenient tuning meter circuit is provided with the ST-5. Diodes
D3 and Dq provide positive half-wave rectification of the mark and sPace
signals respectively. The detected outputs are summed in the base of Ql
with C6 and Rl7 providing filtering of the a-c component. Transistor Ql
is an emitter-fol Iower current ampl if ier that drives the meter through Rl5
and Rl8. Since both mark and space signals provide a positive current to
the meter, it wiil always read up-scale. However, the current to the meter
will be a maximum only when a signal is matched in frequency to the discrim-
inator mark or space filters. ln fact, if both the mark and space frequencies
of the input signal match the discriminator tuning, the meter will have
maximum def Iection for both tones and wil I not f luctuate when the signal
changes from mark to space. Therefore, correct tuning is indicated on the
meter as an up-scale reading that does n6t chaqge when the signal changes
state. lf the shift of the incoming signal does not exactly match that of
the discriminator, the signal can still be tuned so that the meter does not
fluctuate, but the steady reading obtained will not be as great as before
since the signal has been "straddle-tuned" and is not peaked on either filter"
This "plus-plus" meter signal from Ql is also used to drive the ST-5AS Auto-
start opt i on.
Provision is also made in the discriminator circuit to use an
oscilloscope as a tuning indicator. The a-c signals f rom each discriminator
f ilter are connected to J5 through isolating resistors R29 and R30. lf these
two signals are then connected to the X and Y axis inputs of the oscilloscope,
a conventional "cross-type" of RTTY tuning indicator can be used and the
receiver tuned so as to maximize the signal on each axis. This technique
works very well rvith older demodulators with sharply tuned discriminator
filters and works to a limited extent with the purposely wide bandwidth
discriminator f ilters in the ST-5. To achieve the highly desirable Iinear
discriminator response, the ST-5 filters are considerably broader than those
