Nautel XR12, Руководство по устранению неисправностей, Страница: 83 / 283

XR12 Troubleshooting Manual Detailed Circuit Descriptions
Issue 3.0 2009-07-28 Page 2-15
Modulation level detector
The modulation level detector circuit monitors the on/off times of the Forward Power Sample input
(J13-3) for excessive on times. It automatically inhibits a portion of any modulating audio's positive
going half-cycle that would cause excessive 'on times' and results in the stress current threshold of the
transmitter's RF power amplifier stages being exceeded. The circuit consists of comparators U32A,
U32B, U32C, U32D, U33A, U33B and associated components. It provides a discharge path to -15 V
for C137 when the 'on time' of the F
orward Power Sample input will result in modulation peaks that
exceed safe operating levels.
The Forward Power Sample input, applied to the inverting inputs of each comparator, is a varying
positive dc voltage directly proportional to the RF output's modulation envelope. The audio
component is synchronized with the audio output of U33C and is the most positive at the peak of the
modulating audio's positive half-cycle.
Each comparator has a reference voltage applied to its non-inverting input that represents the RF
stress current threshold for a specific carrier level/audio amplitude/audio frequency combination
(see
Table 2.1 ).
MOD PROT THRESHOLD
potentiometer R165 is adjusted to precisely set the
threshold voltages, when the PWB is installed in a transmitter.
When the voltage representing the modulation envelope exceeds the RF stress current threshold of a
comparator, that comparator connects its output resistor to -15 V and provides a discharge path for
C137. The time for C137 to discharge to a negative voltage is dictated by the number of resistors
connected to -15 V (modulation amplitude) and the period of time they are connected (audio
frequency). C137's discharge time (dictated by the R/C time constant) is progressively faster as the
modulation depth increases (more comparator thresholds are exceeded) and it has a longer time to
discharge as the audio frequency decreases. See
Table 2.1 for the duration of a positive half-cycle at
the lowest unaffected audio frequency for specific modulation depths.
Table 2.1: Peak Modulation Limit Threshold
Mod Depth
Peak %
Lowest Unaffected
Freq (Hz) Duration (ms)
85 10 50.0
95 11 46.6
105 20 25.6
115 32 15.4
125 56 8.9
135 96 5.2