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- H H A A P N N 4800 b.p.s. Modem for the VADCG TNC
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- 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.
-
-
-
- .pa
-
- 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.
-
- .pa
-
- 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.
-
- .pa
-
- 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.
-
-