Giga-tronics GT 9000, Operation & Maintenance Manual

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(LGAIN) pot. The output of U17B is the level voltage which goes to the level driver board
(A10A1) to drive the PIN leveling diodes.
The reference signal is generated on this board. The 2.4596 MHz oscillator is composed of U1
A, B, C, and Y1. Its square wave output is turned into a sine wave via filter L1, L2, C10, C11,
and C9. The R3 and R4 pots allow a variable amplitude of this signal to be tapped off and
amplified. U2A and B switch in the appropriate pot depending on whether the frequency
selected is > or
∞
2 GHz. The signal is amplified and fed to modulator U8 (which functions as
a variable attenuator controlled by a voltage which may be a composite of different signals).
The output of the modulator is sent via U10 to the Detector Buffer module to be amplified and
returned to the leveling board as REFX20.
The main control voltage for the modulator ICs (U8, U28, and U9) is generated by the U24
DAC. This voltage changes as the power level changes and controls the gain of U28. U28
adjusts the level correction voltage which is switched in by U16A and amplified by U7A. The
amplitude of this signal is adjusted by the modulator U28 according to the power level the
instrument is set to. U29A amplifies the modulator output and sums it with the standard main
control voltage at U29B. The output of U29B is inverted at U13B to control the modulator U9,
which adjusts the amplitude of the AM signal when enabled by U2A. R5 calibrates the AM
signal which is then translated into a differential signal by U6B and U7B and applied to the
modulator U9 input. The outputs of U9 are summed by U6A, and the AM signal amplitude has
been adjusted according to the control voltage. The result is that the AM signal level is correctly
calibrated for any power level. This AM signal is then summed with its control voltage at U5A
and inverted at U5B, forming a composite signal used for appropriately modulating the
reference signal at U8. VR3 and R51 are set to limit the control voltage so that it does not
overdrive the modulator.
U31 is a DAC programmed by the computer to give an output voltage at U32B. This voltage is
the appropriate offset required for the PIN leveling diode drivers on the A10A1 level driver
board. The U26 Reference provides a 5 volt reference, which is filtered by L3 and C66 and
C67. This filtered 5 volt reference is also amplified by U32A to give a 10 volt reference. The
5 volt reference is also divided down by pot R48 to give a 3.2 volt reference.
U20 provides decoding for the chip select lines. U21, U22, and U23 provide the necessary
buffers for the bus interface lines.
3.3.11 Level Driver — PC Assembly A10A1
The level driver contains amplifiers and filters which drive the levelers (variable attenuator
circuits) in the microwave module.
LEVELV, the output of the Level PC board (A10), is amplified by the circuit. The amplifier
circuit has four levels of AC gain activated (in order of decreasing gain) as the AC input level
increases and switches on the diodes CR9, CR7, and CR6. The SCANSIG input is not used in
this instrument. The amplifier output at U6-6 (LVL) is supplied to the level drivers associated
with particular bands.
Inside the microwave module, the leveler for Band 0 (the downconverted range, below 2 GHz)
consists of a pair of FETs which act as variable attenuators. Separate driver outputs are required
to control the series FET and the shunt FET. For Band 0, the LVL signal is converted by a
differential amplifier consisting of U1A, U3A, and U3B into two outputs. The BAND0LVL VP
output is applied to the series FET, and the BAND0LVL VS output is applied to shunt the FET.
Both outputs are transmitted through low-pass filter elements (see L1 and L2) to the microwave
Model GT 9000 Microwave Synthesizer
3-24
Manual No. 120AM00250, Rev C, September 1998