Audio Modulated Amplifier; Rf Leveling Amplifier - Aeroflex NAV 2000R Operation Manual

4.5.17 AUDIO MODULATED AMPLIFIER

The audio modulated amplifier amplitude modulates a CW carrier at its RF input with the audio
modulation signal delivered by the Modulating Signal Temperature Compensation network.
The CW carrier at pin 3 of U1 is at a level of about -30 dBm, at RF frequencies from .15 MHz to 450 MHz.
Operating the VGA at -29 dBm input minimizes the audio distortion of the modulated output. The
required input to the RF Modulator is -20 dBm. After the 9 dB pad, the RF power at the VGA input should
be about -29 dBm.
The audio modulating signal applied to the AGC gain control input (U1 pin 8) varies the gain of the
amplifier in proportion to the audio signal level, generating amplitude modulation.
available AGC voltage dynamic range of U1, a DC level, at room temperature of around 0.25 Volts is
maintained. This level is adjusted by R42 in the modulating centering circuit on the audio board.
About 0.9 Volts of gain control voltage at U1 pin 8 produces maximum gain (for all temperatures). For
100% positive modulation, the instantaneous peak control voltage, referenced to the DC operating point
of will be doubled. The control voltage directly relates to voltage gain so now the peak RF level will be 6
dB above the average RF level. Similarly, for 100% negative modulation, the null of the modulating
waveform should extinguish all RF level. This is approximated since the audio modulating signal will, for
100% modulation, swing from .25V to .5V for the peak and from .25V to 0.0V for the null.
With nearly 0.0V applied to the AGC input, there is maximum attenuation of the input CW RF carrier.
Although the RF is not completely cut-off, it is adequately attenuated. Decreasing the DC bias point of
the modulating signal would be to use less of the linear dynamic range of the VGA. It is important that the
VGA be operated in its linear region to avoid audio distortion. Using a lower bias point would mean that
the audio signal would need to be smaller and thus noise would become more significant resulting in
increased distortion.
There are two RF inputs to the VGA and two RF outputs for balanced operation, if desired. In this
application, the RF signal is applied unbalanced with the other input terminated in a 50 Ohm resistor.
This termination provides the same impedance as found on the RF input. The output is taken balanced
and feeds the RF Leveling Amplifier balanced inputs through a 16 dB pad.
With Vagc, the gain control input, biased at about .25 volts, the average RF power gain should be about
11 dB so that the carrier level of the modulated output is at a level of about -22 dBm.

4.5.18 RF LEVELING AMPLIFIER

The RF leveling amplifier is used to level the output power of the RF Modulator board. Without leveling,
the RF level would vary due to the RF frequency response and temperature dependence of the RF
circuitry. The leveling amplifier receives a gain control voltage that corrects for differences in output
power level from a set level. The leveling loop can correct for output level errors up to +/-5 dB from the
set level.
The nominal RF carrier power level received at each of the balanced RF inputs (U2 pins 3 and 5) is about
-35 dBm. This input level minimizes audio distortion at the output. The nominal output of the first
modulated VGA with a DC bias of 0.25 volts on its Vagc input (U1 pin 8) is about -22 dBm. The 14 dB
balanced pad attenuates the signals delivered to each of the inputs of the RF leveling VGA to -36 dBm.
NAV 2000R SIGNAL GENERATOR - REV. 0 – JULY 26, 2007 - PG 4-21
Aeroflex Operation Manual
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