Giga-tronics GT 9000, Operation & Maintenance Manual

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amount equal to the desired output frequency). These two signals are applied to a mixer. The IF
output of the mixer (which equals the difference between the two inputs, and has a range of 40
to 2000 MHz) is applied to the second Divide-by-4 circuit, which reduces it to an output in the
range of 10 to 500 MHz. Because two YIG oscillators are required for downconversion, two
PLL circuits are needed. The 8 to 20 GHz YIG is tuned by the Output PLL circuit, and the 2 to
8 GHz YIG (which generates a fixed 8005 MHz output) is tuned by the Downconverter PLL
circuit. Feedback coupled from the 2 to 8 GHz YIG output is applied to a sampling mixer, and
combined with a timebase-derived 8000 MHz harmonic to produce a 5 MHz IF, which in turn is
applied to the variable input of the Downconverter PLL. The PLL tunes the 2 to 8 GHz YIG to
8005 MHz.
In instruments which do not include the range below 500 MHz, the downconverter mixer and
the downconverter PLL are not used; all frequencies up to 2000 MHz are generated by the 2 to
8 GHz YIG and divided down to the desired output.
A second downconverter scheme bypasses the frequency dividers, and uses the downconverter
mixer to generate the entire 10 to 2000 MHz range. In this scheme, the 8-20 YIG is tuned to a
frequency in the range of 8015 to 10005; this frequency, mixed with 8005 MHz output of the 2
to 8 GHz YIG, produces an IF in the 10 to 2000 MHz range. Ordinarily, this no-divider scheme
is not used, because the dividers improve the phase noise characteristics of the YIG output, and
the mixer alone does not. However, when certain special functions are used, the dividers can be
bypassed in order to produce faster frequency changes.
The output frequency (in the range of 10 to 2000 MHz) passes through a leveler, pulse
modulator, and amplifiers on its way out of the downconverter module. A separate module
contains the low pass filters which remove harmonics from the output of the downconverter
module.
Theory of Operation
Manual No. 120AM00250, Rev C, September 1998
3-13