Lpf & Integrator; Scaling & Rf Level Set; Rf High/Low Indication - Aeroflex NAV 2000R Operation Manual

Lowpass network L4 and C27 prevent modulated RF from entering the audio circuitry but have negligible
effect on the detected audio waveform. R58 and C54 form an additional lowpass filter at the input to
U9B. The op amp U9B buffers the temperature compensation circuitry output and provides drive to the
audio level processing circuitry.
The calibrated detected audio is provided as an output to the audio board. The calibration of this signal is
accomplished using the Detected Audio High/Low circuitry on the Audio Board and is discussed above.
The output level is adjusted by pot R44.
4.5.21 LPF & INTEGRATOR
To obtain a signal, which is proportional to the average RF power level from the detected audio signal, it
is necessary to find the average DC level. This is done for all valid audio frequencies above 10 Hz by a
lowpass filter and integrator.
Resistor R57 and Capacitor C49 form the first pole of the leveling loop filter. Op Amp U9C buffers this
first pole and drives the loop integrator formed by Op Amp U8, Capacitors C45, C46, and C44, and
Resistors R48 and R47. The output of U8 drives the voltage control pin (U2-8) of RF amplifier U2.
4.5.22 SCALING & RF LEVEL SET
The RF level is set and controlled by adjusting the voltage at pin 3 of U8. At output frequencies greater
then 25 MHz the leveling loop described above is closed. The high gain of U8 at DC maintains the DC
value of the detector output at the voltage set at pin 3. Maintaining this voltage, integration amplifier U8
drives the control pin of RF amplifier U2 to adjust the RF level seen by the detector, thereby fixing the
output level of the modulator.
Voltage reference (J1-16) is scaled by resistors R41, R42, R37 and pot R39 to the appropriate detectors
voltage. RF level is calibrated by R39. Minor detector frequency response corrections are accomplished
by software which adjusts the voltage reference by means of the audio board.
At output frequencies below 25 MHz, the change in output level do to frequency and temperature is
minimal. Therefore, the modulator can be ran open loop. Software adjusts the voltage reference to
compensate minor frequency response variations, as it does at higher frequencies. To open the leveling
loop, control signal DET SEL (pin 4 of J1) is taken high. This disconnects the RF output from the detector
using relay K1, reduces the DC gain of U8 to one by connecting across the integrator components with
analog switch U7, and selects resistor network R40 and R38 in place of R37. Potentiometer R38 allows
changing the voltage reference scaling to calibrate this lower frequency band output level.
The diodes CR3 and CR4 clamp the AGC gain control voltage at a maximum of +1.4 Volts. The gain
control input of the leveling VGA gives maximum gain near 1 Volt. Since the negative supply of U9 is
ground, the minimum output is just above ground.

4.5.23 RF HIGH/LOW INDICATION

The RF High/Low Indication circuitry indicates only that the leveling loop is operating within established
limits. It does not, however, indicate that the RF output power level is within a certain percentage of the
calibrated level. The indication circuitry consists of two window comparators in U6. The leveling loop
control voltage is compared to two voltages derived from the voltage reference. The comparison voltages
are 80 mV for an RF High indication and 1.05 Volts for an RF Low indication.
If the leveling loop gain control voltage attempts to lower the RF output by developing a voltage of less
than 80 mV, "RF level too high" is indicated. Conversely, if the leveling loop gain control voltage attempts
to raise the RF output by developing a voltage greater than 1.05 V, "RF level too low" is indicated. Both
error indications are +5 VDC.
NAV 2000R SIGNAL GENERATOR - REV. 0 – JULY 26, 2007 - PG 4-23
Aeroflex Operation Manual