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H H A A P N N 4800 b.p.s. Modem for the VADCG TNC
RECEIVER INTERFACE
==================
The tap point in the receiver for the Rx audio should be chosen at the F.M.
detector. Tapping must be done before the detected signal is de-emphasized.
Most rigs use a resistor-capacitor combination to de-emphasize the signal
very close to the detector stage -- this circuit is often difficult to find.
It may masquerade as a bypass capacitor, or a bias resistor.
Most radios use a squelch circuit called a "noise-operated squelch". It
gets its operating signal from the F.M. detector, before de-emphasis. The
point at which this squelch circuit joins the F.M. detector is exactly the
point where you should tap for the modem's Rx audio. The input circuit of
the modem is A.C. coupled, and is high impedance (100k ohms). It can
accomodate signal amplitudes from 2 mv. to 200 mv. r.m.s.
Some integrated-circuit detector chips include an audio preamp that may
supply too much signal for the modem. You can accomodate these larger
signals by soldering a resistor (10k to 33k) in parallel with R16.
You should use shielded wire between the receiver interface point and the
modem. Keep the length as short as possible -- a run of more than five feet
is to be avoided. If you must keep your radio at some distance from your
T.N.C., a buffer stage should be constructed at the detector so that it is
not loaded by the capacitance of a long run of shielded cable.
TRANSMITTER INTERFACE
=====================
The transmit audio part of this modem can be driven from either RS232
levels, or TTL levels. As for the rig interface, both frequency modulators
(F.M.) and phase modulators (P.M.) are supported. The modem is compatible
with synthesized radios since the transmit data contains no D.C. component
that could pull the synthesizer from the channel centre. Use shielded cable
to connect the modem to your radio.
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MODULATOR TYPE - F.M. or P.M.
=============================
You must determine which modulation type your radio uses. If phase-
modulated, install jumpers on the modem at J1 and J2. Leave these jumpers
open if your rig is frequency modulated.
Many manuals include a block diagram -- this is a useful guide in
determining which modulation type your radio uses. F.M. is always applied to
an oscillator circuit, either a crystal controlled oscillator, or a voltage
controlled oscillator (VCO). The frequency modulator on the schematic
diagram will be a voltage-variable-capacitance diode coupled closely to the
frequency-determining inductors and capacitors (or crystal) of the
oscillator circuit.
Phase modulation is always applied to a stage following an oscillator -
never to the oscillator itself. P.M. could be produced using either a
voltage-variable-capacitance diode, or a transistor stage. "Reactance
modulator" is another name for phase modulator.
Refer to appendix ? for examples of transmitter interface.
ADJUSTING YOUR MODEM
====================
There are four variable resistors to be adjusted by the user. These are:
Rx level
Tx level
Carrier detect level
Clear-To-Send delay (CTS delay)
RX LEVEL - R15
==============
This control adjusts the gain of the receive part of the modem. Connect the
radio to the modem and turn to a clear channel. Monitor the D.C. voltage at
TP 1 or TP 2 with a voltmeter or oscilloscope. These points are marked on
the circuit board. The voltage should be about 9 to 10 volts (+9 volts at TP
1, -9 volts at TP 2).
Have another station send you some 4800 baud packets. His radio should
already have been set to modulate at the proper deviation. Adjust R15 so
that the voltage at TP 1 is about 6 v. D.C. If the packets are so short that
your voltmeter cannot settle, have him lengthen CTS delay to maximum.
CARRIER DETECT LEVEL - R45
==========================
This modem includes a noise-operated carrier detect circuit. A light-
emitting-diode (L.E.D.) is provided to show when the channel is active or
inactive. It should light up when someone transmits voice, 1200 baud packets
or 4800 baud packets. It will also light up if the radio is disconnected or
turned off. You can adjust R45 as you would the "squelch level" on your
radio (by observing the L.E.D.), or you can set R45 so that the voltage at
the junction of R49 and R50 is about zero volts (for a clear channel).
The squelch control on your radio is completely independent of the modem's
carrier detect control.
TRANSMIT LEVEL - R4
===================
This control adusts the voltage level going out to the rig's modulator. You
could call it a 'deviation control'. Because the interface point into your
radio bypasses the circuits that prevent overmodulation, it is critical that
this control be properly set. Modulation should be no more than 6 Khz. peak-
to-peak (+- 3 Khz.).
You will need someone to monitor your transmissions, preferrably with an
oscilloscope. It should be connected to the F.M. detector of the monitor
receiver.
Since your radio (when used for voice transmissions) properly limits the
modulation to fit available channel space, you can 'calibrate' the
oscilloscope by noting the amplitude produced by voice-modulating your
radio. Don't forget to unplug your modem while making these calibration
transmissions. Plug the modem back in, send some long packet frames, and
adjust R4 so that the peak-to-peak amplitude is the same as it was for the
voice transmissions.
If you have a synthesized radio that can transmit in 5 Khz. increments, the
oscilloscope on the calibration receiver can be calibrated by sending a
carrier 5 Khz. above and below the channel centre. The oscilloscope must be
D.C. coupled directly to the F.M. detector.
Many 1200 baud packet transmissions are overmodulated. It is probably a
mistake to set R4 so that your transmissions are equal in level to those of
1200 baud modems.
You will likely find that your rig must be disconnected from the modem when
you want to use it for voice transmissions. A voice transmission with the
modem connected will likely result in very weak modulation, or no modulation
at all.
CLEAR-TO-SEND DELAY - R52
=========================
This control adjusts a time delay that holds off the T.N.C. from sending
data when it wants to make a transmission. It can be adjusted to match the
time it takes for your rig to activate its transmitter, and get R.F. power
out to the antenna.
Since most T.N.C.'s also provide a delay, this control may be redundant. In
this case set the delay to the minimum value.
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PART 2: CIRCUIT DESCRIPTION
===========================
This modem can be logically divided into three blocks - receive data
processing, transmit data processing, and interface circuits. Appendix A and
B show the circuit diagram and circuit waveforms.
RX CIRCUIT
==========
The first stage (U1b) is a variable gain amplifier, with high input
impedance. The signal from your radio is coupled in through a D.C. blocking
capacitor, C8.
The following two op-amps (U1a, U1d) form a four-pole, low-pass filter
which eliminates high frequency noise. This filter has a gain of about two.
The waveform at the output of U1d should swing symetrically about zero
volts, with an amplitude of 12 volts peak-to-peak.
D6 and C14 detect to peak height of the positive-going pulses. This D.C.
voltage is available at test point #1 (TP1) to aid in setting the voltage
gain at R15. R28 and R27 divide the pulse peak height to provide a positive-
pulse threshold level for U3d, the positive pulse slicer. This threshold
level is one-half to two-thirds of the positive pulse peak height. Output of
U3d is a squared-up waveform, made TTL logic-level compatible by R35 and
R39. A similar circuit detects negative-going pulses (D5, C13, R25, R26,
U3a, R36, R37).
Besides going to the pulse detectors, the output from the filter also goes
to a pulse-position detector, U3b and U3c. These comparators find the exact
centre of each positive or negative pulse.
C15 and R29 differentiate the pulses so that the voltage at R29 goes
through a zero-crossing at the centre of each pulse. U3b and U3c form a
zero-crossing window detector. Their outputs are or-tied to give a very
short positive pulse when their inputs go through the zero-crossing. R34 and
R38 make these pulses TTL logic-level compatible.
U4 is wired as a simple J-K fip-flop to recover the original data. U4a and
U4b lock out invalid zero-crossing pulses coming from the window
comparators. U4c and U4d make up the latching part of the flip-flop, setting
high (logic 1) on positive pulses and resetting low (logic 0) on negative
pulses.
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TX CIRCUIT
==========
U1c performs quite different functions, depending on the status of J1 and
J2. For a radio that has a phase modulator, J1 and J2 are jumpered. In this
mode, U1c works as a unity-gain buffer. The output of U1c swings
symetrically about zero volts, following the square wave input.
For a radio using a frequency modulator, J1 and J2 are left open. Now U1c
works as a 'digital differentiator'. D1 and D2 limit the input voltage to
+/- 0.6 volts. U1c slews between +5.6 and -5.6 volts at a rate determined by
C1 and R3. D3 and D4 limit the slewing to +5.6 and -5.6 volts. C2 and R4
differentiate the slewed square wave. The waveform at R4 gives a short
positve pulse whenever the data stream goes from logic level 0 to 1, and a
short negative pulse when the data stream goes from logic level 1 to 0.
U2b and U2a form a four-pole, low-pass filter which eliminates unwanted
harmonics, while minimizing inter-symbol interference.
INTERFACE CIRCUITS
==================
These circuits include carrier detect, watchdog timer, clear-to-send timer,
and RS232 level converters.
CARRIER DETECT
==============
After amplification by U1b, the signals from the radio should still have
lots of high-frequency information. U2c is a high-Q, high-pass filter (11
Khz.) that amplifies the high-frequency noise. Noise amplitude is detected
by D7 and C19. When a carrier appears on the channel, the high-frequency
noise ceases, and voltage at R45 goes to zero.
U2d is a low-pass filter that smooths variations in the noise amplitude. It
responds in 10 to 15 milliseconds. Its output swings between zero volts
(carrier present) to +2.5 volts (no carrier present). U5b is a Schmidt-input
logic gate that provides a clean, quick transition for the carrier detect
signal.
The watchdog and clear-to-send (CTS) timers are both monostable
multivibrators (U6). The CTS timer is variable up to about 500 milliseconds;
watchdog is fixed at 5 seconds.
When request-to-send (RTS) is asserted by the T.N.C., the CTS monostable
pulses, tripping the watchdog on its trailing edge. The CTS and watchdog
timers are or-ed together to drive the push-to-talk (PTT) line of the radio.
If the T.N.C.'s RTS line is active for more than five seconds, the watchdog
timer will time out, disabling PTT, which shuts off the radio transmitter.
The watchdog, when it times out also resets CTS going back to the T.N.C.
ADDENDUM
========
It was found, after the circuit board was laid out and tested, that U2a had
a tendancy to oscillate about 400 Khz. Because this signal directly
modulates that F.M. transmitter, spurs could be generated outside the
channel. R57 and C29 were added to the board to cure the problem.
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