Giga-tronics GT 9000, Operation & Maintenance Manual

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A.15.2 Option 27 Theory of Operation
The modulating waveform can be applied from an external source, or (if Option 24 is installed)
generated internally. Internal modulating signals are transmitted through a shaping circuit which
adapts them to the input requirements of the scan circuit. The modulating signal is then applied,
through a switch, to either of two input paths: a DC-coupled path for the scan mode or variable
attenuator mode, and an AC-coupled path (including a logarithmic amplifier) for the linear AM
mode.
The modulation input is applied to an analog-to-digital converter, and converted to a binary
number (see Figure A-3). This number (in the form of a set of parallel data lines) is applied to
the address bus of a RAM. The RAM is loaded with correction data stored in ROM (there are
several tables of correction data in ROM, for different frequency ranges; the appropriate table is
loaded into RAM when Scan modulation is selected or when the frequency changes). When the
RAM is addressed by the output of the ADC, the data stored at the relevant address is furnished
to the input of a digital-to-analog converter. The DAC converts this correction data to an analog
signal which drives the scan module. As the input waveform changes, different storage areas in
the RAM are addressed, so that the correction factors retrieved from memory constantly track
the input. The correction data is tailored to the individual scan module installed in the
instrument; it is needed to linearize the response of that module to the modulation input.
The scan module contains a pair of voltage controlled attenuator circuits; each has a dynamic
range of 30 dB, and since they are placed in series, the dynamic range of the whole is 60 dB.
Each attenuator is driven by a loop amplifier, which provides correction of its attenuator’s
response (the first corrects for linearity, the second for input/output impedance matching).
During modulation, the two variable attenuators of the scan module are switched into the RF
output path of the instrument. The modulating input to the scan module controls the attenuation,
and therefore the RF output level. When modulation is deactivated, this module is switched out
of the RF path.
Figure A-3. Scan Modulation
Model GT 9000 Microwave Synthesizer
A-14
Manual No. 120AM00250, Rev C, September 1998