Rf Modulator Board Operation; Programmable Step Attenuator Circuit - Aeroflex NAV 2000R Operation Manual

4.3.11 RF MODULATOR BOARD OPERATION

The RF Modulator Board circuitry provides two basic functions: AM modulation and automatic RF
leveling. Both functions are achieved using variable gain amplifiers (VGA's). Temperature compensation
is also provided to correct for gain variations of RF amplifiers with temperature.
The audio signal, conditioned to achieve a desired percent modulation by the audio board, is applied to
the Modulating Signal Temperature Compensation circuitry. The modulating signal is then applied to the
first VGA's gain control input to amplitude modulate the CW RF signal applied to the RF input of the RF
modulator. The Modulating Signal Temperature Compensation network removes the gain temperature
dependence of the first VGA stage.
The second VGA amplifier is part of the automatic leveling loop. It is followed by three stages of
amplification, which boost the signal to the desired level. At the output of the final fixed gain amplifier, a
portion of the signal is detected by a temperature stable detector.
The Temperature stable Detector circuitry detects the modulation of the RF carrier and corrects for the
temperature dependence of the detector diode. The detected modulating signal is then applied to the
Detected Audio Hi/Lo Indication circuitry and to the Low Pass Filter & Integrator circuitry. The Detected
Audio Hi/Lo Indication circuitry is on the audio board and is discussed above. The LPF & Integrator
circuitry filters and integrates the detector output to determine its average DC level. This signal is an
input to the RF leveling output leveling loop.
In the Scaling & RF Level Set circuitry, the DC signal from the LPF & Integrator circuitry is used in
conjunction with the voltage reference provided on the audio board to generate the gain control voltage
for the second VGA. This closes the RF leveling loop. If the detected RF output level is too high, the
gain control voltage on the second VGA is reduced; if too low it is increased.
The RF High/Low Indication circuitry indicates the ability of the second VGA to level the RF output power.
If the output level falls to a low level which is very small due, for example, to an amplifier malfunction, the
gain control voltage would go to its maximum value in attempting to restore the RF output level. Similarly,
if for some reason the detected RF output level was exceedingly high, the VGA control voltage would
drop in attempting to lower the RF level. In either case, the indication circuitry would set bits indicating
the leveling loop is out of its control limits.
The RF level high/low indication bits do not mean that the RF output power is within a certain window of
its desired level. They simply indicate proper operation of the leveling loop in leveling RF output level.

4.3.12 PROGRAMMABLE STEP ATTENUATOR CIRCUIT

The programmable step attenuator provides control over the output power level. It provides the ability to
have outputs from 0 dBm to -127 dBm in 1/10th dB steps. The drivers for each of the step attenuators,
along with the 1/10th dBm control circuit, are found on the audio board.
NAV 2000R SIGNAL GENERATOR - REV. 0 – JULY 26, 2007 - PG 4-9
Aeroflex Operation Manual
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