Aeroflex NAV 2000R Operation Manual page 64

Aeroflex Operation Manual
The Percent Modulation Level Set Attenuator attenuates the audio input signal by an amount which
corresponds to a desired percent modulation. Subsequently, the audio input signal is added to a DC
level in the % Modulation Centering & Calibration circuit. Assume that the DC level is normalized to 1.
Then, after adding the audio input with no attenuation the resultant signal should give 100% modulation.
For 100% modulation, the largest peak reaches the normalized value of 2 and deepest null swings to
zero. The required attenuation values used to achieve from 0% to 100% modulation are stored in the
NAV 2000R system computer. Therefore, to achieve 50% modulation, the modulating signal applied to
the audio board will have its amplitude reduced by a factor of one half. Note that percent modulation is
given by
% mod = [(1+m)A-(1-m)A]/[(1+m)A+(1-m)A] * 100 = m * 100%,
where m is the modulation index and A is the carrier amplitude. The modulation index parameter controls
the relative proportions of sideband to carrier levels. The factor m is also the factor by which the input
signal should be attenuated to achieve a desired percent modulation, i.e. for 50%, m=.5 and for 25%,
m=.25, etc.
The % Modulation Centering & Calibration circuitry adjusts the DC level of the composite modulating
signal and calibrates the audio modulating signal. Before being applied to the RF modulator, the audio
signal must have the proper AC and DC levels. The DC level determines the range of RF levels which
the Voltage Gain Amplifier of the RF modulator board will produce. This DC level is set so that the
variable gain input to the VGA provides the optimum dynamic range. Due to the wide linear dynamic
range of the gain control input, the DC component of the audio signal is not highly critical. However, the
AC component of the audio signal applied to the VGA must be calibrated very accurately, as it
determines the % modulation. Calibration of the AC component is achieved by setting the % modulation
attenuator to provide 50% modulation modulating signal. Then, the centering and calibration circuitry with
the voltage reference adjusts the resultant modulated RF output to be 50.0% (measured using a
modulation meter).
The Programmable Step Attenuator Control circuits provide control signals which set the attenuation of
the Programmable Step Attenuator, respectively. The Programmable Step Attenuator has eight control
lines which provide values of attenuation in one dB steps. The step attenuator program word is not
sequential. Each control line activates either a one, two, four, eight, sixteen, thirty two, or sixty four dB
attenuator. Thus, to obtain a given value of attenuation the proper values of attenuation must be selected
so that the sum total of the attenuation is that which is desired. As a result, the control word, which
controls the value of attenuation, being non-sequential, is stored in NAV 2000R system computer
memory.
To achieve 1/10th dBm steps, a circuit is added to allow the step attenuator setting to be varied. This
circuit uses a DAC circuit to vary a reference signal to the leveling loop.
The detected Audio Hi/Lo Indication circuitry indicates whether or not the RF detector is working properly.
It is set up to measure the peak of a single tone modulating signal at 50% modulation. If the peak is
greater than the acceptable high level or less than the acceptable low level then an appropriate indication
is made. A high or low indication could mean that the detector or leveling loop are functioning improperly
or that the RF calibration level has become uncalibrated.
NAV 2000R SIGNAL GENERATOR - REV. 0 – JULY 26, 2007 - PG 4-8