Hal Communications ST-5 User manual

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INSTRUCTION
MANUAL
HAL COMMUNICATIONS CORP.
BOX 365
URBANA, ILLINOIS 61801
QU ALITY COM M U NICATIONS EQ U I PM ENT

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Copyright @ 1973 by HAL Communications Corp.
i n the Un i ted States of Amer i ca. Al I r i ghts
publ ication may not be reproduced in any form
sion of the copyright owner.
, Urbana, lllinois. Printed
reserved. Contents of th i s
without the wr i tten permis-
December,1979

HAL ST-5 RTTY DEMODULATOR KIT
CONTENTS
! NTRODUCT I ON I
2
4
B
l2
SPECIF!CATIONS
oPERAT!0N 0F THE ST-5
ST-5 KIT CONSTRUCTION
TEST AND ALIGNMENT
I NTERCONNECTI ON WI TH OTHER EQUI PMENT
D I AGRAMS r5
t7
)q
IB
lg
20
2l
22
23
2\
25
26
27
2B
PARTS LI ST
Figure I
Fi gure 2
Fi gure 3
Fi gure 4
Figure 5
Figure 6
Figure 7
Fi sure B
Figure 9
Fi gure I 0
Figure I I
ST-5 Schemat i c
ST-5 Power Supply
ST-5 Power Supply
ST-5 Demodulator
ST-5 Demodu I ator
Loop Plug Preparat
I nterconnect ion to
I LLUSTRAT I ONS
Layou t
Board Layout *.
Board . . -.
ton
Other Equ i pment
Toroid \^/iring . .
Toroid Mount ing
Mechan i ca I Layout fo r CI)ZI l0A Box
P I ug Connect i ons

I NTRODUCT I ON
The model ST-5 RTTY Demodulator is a high-performance, all solid-
state demodulator for reception of radio teleprinter signals. The ST-5
features a very high gain I imiter stage capable of a 70dB dynamic range
with Iow zero- Ievel cross-over d istort ion. The d iscr iminator c i rcu its
of the ST-5 are broad-band and linear in response throughout the required
bandwidth. The Iinear discriminator allows reception of signals with
non-standard f requency shif t and assures rel iable recovery of weak signals
under noisy conditions. Discriminator bandwidth is switch-selectable to
provide enhanced reception of the two standard frequency shifts (tZO Hz
and 850 Hz). A high-gain slicer stage assures that keying pulses from
the sol id-state keyer have the fast switch ing t imes necessary for Iow-
distortion keying of a teleprinter. The internal power supply of the
ST-5 furnishes regulated plus and minus 12 volts to operate the denrodulator,
and includes a 175 volt, 50 ma loop supply. A tuning meter is also furnished
with the ST-5, thus reducing the number of external pieces of equipment
reguired to receive RTTY signals. All parts necessary to construct and
interconnect the ST-5 are furnished with the kit with the exception of a
cabinet. The ST-5 can be installed in a user-supplied cabinet or in a
Bud Minibox available separately f rom HAL Communications Corp.
This manual contains basic operation instructions, construction
procedures, and a guide for servicing the ST-5. Please read the complete
manual before beginning assembly of the ST-5.

2, SPECIFICATIONS
I nput Frequency Range : 2125 2975 Hz (Standard)
1275 - 2125 Hz ("Low-Tones")
lnput lmpedance: 500 ohms, uhbalanced
Limiter Performance: (at 2125 tlz)
Limiting Threshold - l.OmV rms input
Maximum Signal lnPut - 3.5V rms
D i scr im inator Perf ormance:
Mark Frequency, BII shifts - 2125 Hz (Standard)
1275 Hz ("Low-Tones")
Space Frequency, 170 Hz shift - 2295 Hz (Standard)
I Ir45 Hz (" Low-Tones" )
Space Frequency, 850 Hz shift - 2975 Hz (Standard)
2125 Hz ("Low-Tones")
Discriminator Filter Bandwidth 140 Hz
Minimum Usable Frequency Shift - (Standard Tones): 850 Hz 50 Hz
170 Hz - l0 Hz
Maximum Usable Frequency Shift - (Standard Tones): BSO Hz - ll00 Hz
I70 Hz - 350 Hz
Low-Pass Filter Cut-Of f Frequency: 30 Hz (60 wPffi, l+5 baud)
50 Hz (tOO wpffi, 75 baud)
(Setectable by proper choice of component values)

0utputs:
L00P: An internal 175 volt, 50 ma loop power supply is keyed by the
demodulator and connected to a panel connector to which the
selector magnets at the teleprinter can be directly connected
FSK: A bi-polar keying voltage output is supplied that is EIA
(nS-232) compatible. This voltage is keyed by either the
demodulator itself or by interruption of the loop circuit
by external devices such as a keyboard or tape transmitter.
This output can be used to key an FSK circuit for transmitting
RTTY or to drive the HAL RVD-1005 Solid State Visual Display
System, or the HAL AK-l AFSK osci I lator.
Nominal Vol tages: Ma rk:
Space:
KEY: A telegraph key may be connected to the FSK system to provide
narrow-shift CW-lD transmission. A separate potentiometer is
provided to allow adjustment of the CW shift.
SCOPE: AC voltages proportional to the signals in the discriminator
f ilters are provided to allow use of an oscilloscope to aid in
tuning the receiver. Since a tuning meter is provided with the
ST-5, use of the'rscopeil output is optiona'l . Approximately 4V
peak-to-peak (at the center frequency of each discriminator
filter) is furnished with an internal impedance of I megohm.
Miscel laneous Data:
Fuse Protect ion:
Power Requ i rement:
0.5A fuse in AC power I ine
l2o/2\o vAC, 50- 60 -Hz, 1 lo%
(Power cords are furnished for U.S. standard
three-wire 120 VAC connections; other connectors
must be user-suppl ied i f requ i red. )
Power Consumption: I4 watts maximum
Shipping Weight: 5 lbs.

3. OPERATI ON
The ST-5 demodulator is constructed on two printed circuit boards
as is shown in the schematic diagram (Figure l). Audio input signals are
connected to jack Jl and therefore to point rrAil on the demodulator board.
Resistor Rl provides a 560 ohm termination for the receiver output.
lf at all possible, the receiver output impedance should be matched
to 500-500 ohms by either proper selection of output transformer windings
or with an external voice-coil to I ine transformer. The ST-5 will work
with audio signals f rom the 4-B ohm voice-coil output, but only with the
sacr if ice of 30 to 40db of I imiter dynamic range. Also note that res istor
Rl is rated for % watt; this should be quite adequate for most operations;
use of more receiver output power is unnecessary and will result in damage
to Rl and possibly to the I imiter, 0AI.
Diodes Dl and DZ in conjunction with R2 provide transient protection
of the limiter operational amplif ier f rom static bursts.
Operational amplif ier 0Al, a type 709 integrated circuit, serves
as the Iimiter stage of the demodulator. Resistors R5, R7, and RB provide
adjustment of the limiter switching level for minimum distortion. Proper
adjustment of R5 wi ll be discussed in Section 5. Components C4, C5, and
Rll provide extended high-frequency compensation of the 709. Components
C2, R9, C3, and Rl0 provide power supply decoupl ing at the limiter.
The output of the limiter stage (pin 10,0A-l) is connected through
a resistor balancing circuit (ntZ, Rl3, Rl4, Rl5) to the discrimirrator
fi lters. Proper adjustment of Rl3 (aiscussed in Section 5) al lows balancing
of the amplitude and bandwidths of the mark and space filters of the
discriminators. Capacitor C7 (with C7A) tunes Ll to the mark frequency
(2125 Hz) and CB (with cBA) tunes Lz to the *space f requency (2295 Hz or
2975 Hz). Diodes D5 and D6 provide full-wave, neg€tive detection of the
mark signal and, Similarly, D7 and DB provide a positive output for a
space signal. The two detector outputs are summed at pin 4 of 0A2 through
R2l and R22. lf the two filters are balanced in both amplitude and band-
width and the input to the demodulator is noise, equal but opposite currents
are produced in R2l and R22, resulting in zero volts at pin 4 of 0A2. Since
noise f Iuctuates randomly in both amplitude and f requency, pin 4 will osci I-
Iate from plus to minus but with an average value of zero. When a signal of
mark frequency (ZIZS Hz) is applied to the ST-5 input, D5 and D5 generate a
strong negative signal that forces the input of 0A2 (pin 4) negative.
Similarly, a space signal (2295 or Z97S Hz) will force pin 4 positive. Thus
the two f ilters and associated diodes provide discrimination between various
audio frequencies. Proper choice of the balancing resistors Rl2, Rl3, Rl4,
and Rl5 will result in a frequency to voltage response at the input of 0A2
that is maximum (but of different sign) "t the center frequencies of LI and
LZ and linear in response for intermediate f requencies. This linear discrim-
inator characteristic along with the wide dynamic range of the Iimiter stage
are the major factors in the superior weak-signal performance of the ST-5
over other demodu I ators.

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

constructed of tuned, but unloaded, BB mhy toroids. The effect on the scope
output is that straight, crossed I ines are no Ionger formed, but rather,
the traces are ell ipses with considerable opening. These ell ipses will ,
however, with sufficient operator experience, provide as much infornration
regarding proper tuning with the ST-5 as with other demodulators. AIso,
since the ST-5 uses semiconductor devices, the scope outputs are only of
the order of 4 volts peak-to-peak, thus a stage of amplification may be
required in the oscilloscope.
The power supply of the ST-5 furnishes zener regulated +12 volts
and -12 volts for the demodulator as well as ll5 volts @ 60 ma for the TTy
Ioop. Approximately -15 volts unregulated, partially f iltered power is
available at point AD for use with the ST-5AS Autostart option. lt is not
recommended that this point be used to power any other accessories. Suffi-
cient additional low-voltage power capability is provided that both the
ST-5RS anci Al',.- I opt ions can be powered f rom the ST-5, but any f urther
accessories should obtain their power from another source.
The "l{a in I ine" f loat ing- loop supply is incorporated in the ST-5
to produce a loop-keyed voltage (FSK output) that is negative for mark
and positive for space. ln mark state, q2 is conducting and point 'rLn
is held close to qround potential. Since there is little voltage drop
in the TTY machine, point "Y", the positive terminal of the Ioop supply
is also close to ground potential. Therefore, most of the output voltage
of the loop supply (175 volts) appears across Rl07 and the loop current
is therefore control Ied by RI07. The junct ion of Rl05 and Rl07 is there-
fore at approximately -155 vo'l ts and the junct ion of R104, Rl05 and Rl05
at approximately \ that, -80 volts (t 5 volts). This is further divided
by Rl05 and R l0B in para I lel wi th any externa I Ioad across P in 3 of JZ so
that approximately -15 volts is presented to a lK Ioad in the mark state.
Conversely, if the Ioop is open (qZ not conducting or machine not conducting)
no current flows through Rl07 and point "Y" is allowed to,'float-up', to
the f ul I output voltage of the unloaded loop supply, +175 volts. The
divider Rl04-R105 and Rl05-Rl0B again reduces the voltagl at the FSK out-
put to approximately l5 volts, but this timg positive. This minus to plus
transition is very convenient to use to key cliode-FSK circuits and is
compat ible wi th the input key ing requ i rements of the AK- I AFSK osc i I 'lator.
This output is also compatible with RS-232 standards and can be used
directly to drive the HAL RVD-1005 visual Display system.
Another convenient feature of the FSK output of the ST-5 is that
it is keyed by either the incoming signal or by the TTY keyboard. This
means that if an AK-l is connected to this point, it will regenerate an
AFSK signal from the ST-5 processed incoming signal, producing a very
clean output s ignal that cculd be repeated via a vHF I ink, etc. conversely,
if the RVD-1005 is connected to the FSK output, anything printed at the
machine wi ll be displayed on the monitor screen, be it of loca'l -loop or
off-the-air origin.
Resistor Rl09 and Pin 5 of JZ provide a means to use narrow-shift
CW identification when a diode FSK keying circuit is used.

A special transformer that has both low-voltage and loop-voltage
windings as well as a 120-2\0 volt primary is furnished with the ST-5
Kit. Connections for either primary voltage are shown in Figure l.
Swi tch 53 al lows defeat of the keyer stage , Q2, when no RTTY
signals are received or when it is desired to transmit using the FSK
output. A DPDT switch is furnished so that this switch could also be
used to control the transmitter.