Audio High/Low Indication; Modulation Level Set Attentuator - Aeroflex NAV 2000R Operation Manual

4.5.9 AUDIO HIGH/LOW INDICATION

The Audio High/Low Indication circuitry is on JPN Drawing 02-9520-00. This window level comparator
monitors the positive peak level of the sum of the external and internal modulating signals to insure they
are within acceptable limits. There are three possible states of the comparator:
[1]
[2]
[3]
If the positive peaks of the modulating signal do not exceed the bias on U13 pin 7, pin 1 will remain high
and fail to trigger U14 pin 9. Therefore, U14 pin 12 (Q bar) will remain high, indicating a low modulating
audio level.
If the positive peaks are high enough to produce pulses at U13 pin 1 but not so high as to exceed the DC
bias at U13 pin 5 (a higher level than pin 7), pin 1 will generate triggers to U14 pin 9 and U14 pin 12 will
go low. U14 pin 13 will remain low. With both U14 pins 12 and 13 low, an acceptable modulating signal
level is indicated.
If the positive peaks of the modulating signal exceed the DC bias on U13 pin 5, pin 2 will generate trigger
pulses at U14 pin 1, causing pin 13 to go high. This indicates a high modulating audio level. At the same
time, of course, triggers from U13 pin 1 will cause U14 pin 12 to be at a low state.
The RC time constants in both sections of retriggerable one-shot U14 are sufficiently long that the outputs
will remain triggered at the lowest anticipated modulating frequency, if signal levels at the inputs to
voltage comparator U13 exceed bias levels for any fraction of the positive-going modulating waveform.

4.5.10 % MODULATION LEVEL SET ATTENTUATOR

This system attenuates the audio level from the summing amplifier to achieve the desired modulation
percentage. It is comprised of digital-to-analog converter (DAC) U7, parts of gates U5 and & U6, and part
of op amp U9.
When address decoder U3 generates outputs on its 0 or 1 output lines, bus data forms the MSB or LSB
input, respectively, to multiplying DAC U7. Audio from the summing amplifier is applied to the Vref input
of U7.
Attenuation data written to U7 causes attenuation to be applied to the modulating audio signal. For
example, when the bus data written to U7 causes an attenuation factor of .50 to be applied to the
modulating audio, the resulting percent modulation will be 50%. For 99% modulation data is written to
the attenuator to provide a 0.99 attenuation factor to the audio modulating signal. In this way, modulation
percentage may be controlled to the nearest percent by the bus data. There is a direct relation between
the data written to the attenuator and the resultant attenuation. The system computer stores the data to
be written to the DAC to achieve the various attenuation's which produce the various % modulations.
The DAC audio output is buffered by part of op amp U9 and applied to the percent modulation centering
and calibration circuit.
NAV 2000R SIGNAL GENERATOR - REV. 0 – JULY 26, 2007 - PG 4-18
Aeroflex Operation Manual
Inadequate audio level - U14 pin 12 (Q bar) will be high; pin 13 will be low,
Acceptable audio level - U14 pins 12 & 13 are both low, and
Excessive audio level. U14 pin 13 will be high, pin 12 will be low.