Modulation Centering & Calibration; Voltage Reference; Detected Audio Hi/Lo Indication - Aeroflex NAV 2000R Operation Manual

4.5.11 % MODULATION CENTERING & CALIBRATION
The % Modulation Centering & Calibration circuitry adds the appropriate DC offset level to the audio
modulating signal and enables calibration. The DC level from the voltage divider of R41, R42, and R43 is
added to the audio modulation signal. The voltage divider is sourced by the stable +5.0 Volt voltage
reference, U15. By adjusting R42, the DC level can be set.
The DC level setting is not extremely critical due to the use of a VGA (variable gain amplifier) in the RF
modulator. It is important, however, to bias the modulating signal near the center of the VGA gain control
input range to optimize the use of the available dynamic range. During Calibration, R42 is adjusted so
that 0.85 Volts DC is present on pin 17 of J2 for a 1 kHz tone at 50% modulation. This ensures that the
VGA gain control input is biased at its optimum setting.
The modulation level calibration pot, R46, is adjusted during calibration so that a modulation analyzer
indicates 50.0% modulation for a 1kHz tone modulating a CW carrier at 156 MHz. An op amp, part of U9,
is a simple inverting buffer that produces a positive polarity signal for the percent modulation temperature
compensation circuitry on the RF modulation board.

4.5.12 VOLTAGE REFERENCE

The Voltage Reference U15 provides a stable +5.0 V DC voltage which is used to calibrate and stabilize
the NAV 2000R modulation performance. It is used by the Detected Audio Hi/Lo Indication, Audio
High/Low Indication, RF Level High/Low Indication, RF level set, % Modulation Centering & Calibration,
and Detector Bias networks.
In the Detected Audio Hi/Lo Indication, Audio High/Low Indication, and RF Level High/Low Indication
circuits use the voltage reference to calibrate the levels at which the indications will occur. The %
Modulation Centering & Calibration circuitry uses the voltage reference to center the DC operating point
of the modulating signal.
correction voltage which is then summed with the original voltage reference and fed to the RF leveling
loop circuitry on the RF Modulator Board.

4.5.13 DETECTED AUDIO HI/LO INDICATION

This circuit generates an error signal if the output of the RF detector on the RF modulator board is not
within acceptable limits. The error status is presented to the system bus through bus driver U2 and bus
transceiver U1.
This window comparator circuit checks the operation of the RF leveling loop and the RF detector and is
valid only for a single tone at 50% modulation. It is comprised of two sections of quad comparator U13
and both sections of retriggerable one-shot U17.
When a 50% modulated signal is detected, its positive peak must fall between allowable limits, as set by
divider network R51-R53. If the peak is too high or too low, the appropriate detected audio high or low
error status will be sent to the system bus.
This circuit is identical to the Modulating Audio High/Low circuit described in Section 4.5.9 except that the
limits are not as stringent.
NAV 2000R SIGNAL GENERATOR - REV. 0 – JULY 26, 2007 - PG 4-19
Aeroflex Operation Manual
The RF Level circuitry uses the voltage reference to derive a fine tune