TINY-2/MICRO-2 Technical Ref., 2. Ed 1989
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Modem Receive Bias Adjustment
Trimpot (R9 100k) is the modem bias adjustment. The factory procedure below is used to adjust
the modem bias. It normally results in a value of 2.65 to 2.70 VDC on pin 7 of the TCM3105 mo-
dem chip (U16). The setting does not seem to drift over time but may need to be changed it a dif-
ferent modem chip is substituted. If it is necessary to adjust the modem bias, the following proce-
dure should be used.
1. Prepare to perform an audio loopback by jumpering pins 1 and 4 of J2, the radio connector.
2. Give the commands: CAL<CR>, KD<CR> to provide a 'dithered' transmit signal for loopback
demodulation. When the adjustment is completed, type 'Q' to discontinue the tone. NOTE: The
watchdog timer will cause the PTT circuit to reset approximately one minute after the 'KD' Keys
are depressed. If you need a longer adjustment time, type 'Q'<CR> to return to the cmd:
prompt and restart the procedure.
3. Observe the signal output on pin 8 of the TCM3105 (U16) with an oscilloscope. It may be nec-
essary to increase the modem output level by adjusting trimpot R12 to get a strong enough
signal for decoding. The signal should be a square wave. Adjust R26 to cause the positive and
negative portions of the square wave to be of equal duration.
4. Enter Q, remove the jumper from J2 and readjust R12 if needed.
Carrier Detect Level
Trimpot R26 should be adjusted to cause the DCD LED to illuminate when a weak input signal to
the modem is present and to go out when no signal is present. See the section on audio input lev-
els.
Crystal Oscillator Adjustment
You may adjust the oscillator with the variable capacitor C17, to within a 5 kHz range (typical) from
its 4.9152 MHz nominal frequency to move a harmonic a few kilohertz off of a favorite operating
frequency. The setting of this frequency is not critical. A 30 kHz error at the oscillator frequency
will result in only a .6 % error at 1200 baud.
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