Giga-tronics GT 9000, Operation & Maintenance Manual

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One side of this differential signal is buffered by U26A for output to the rear panel as the
FM_SIG output. The voltage from DAC U4 and U5A controls the amplitude of the FM signal
via U14, depending on the deviation selected. The output of the variable attenuator U14 is a
differential signal combined by U15 to give a single signal output. This output is then switched
by U21 (the signal can be switched to be DC coupled or inverted to give an output which is
180 degrees out of phase with the input signal) to the input of U26B, which provides the FM
signal for the output PLL board A18.
U16 decodes chip select lines for other IC’s on the board. U17 thru U20 are required to latch
data for circuits on this board and also for circuits on the A6A1 PCA.
3.3.8 Function Generator — PC Assembly A6A1
The heart of this assembly is the Function Generator chip U1. This IC requires a 10 MHz clock
supplied external to the board but buffered on board by Q1 and Q2. The IC actually contains
two function generators which have been designated as gen A and gen B.
For gen A multiplexers, U2, U3 and U4 address RAMs U8, U9 and U10. If INT SINE wave is
selected, the computer stores data into the RAMs equal to the address being programmed.
However, if INT TRI wave is selected, the computer performs a function on the address data
which turns the data from sine data into triangle data and stores the new data into the RAMs.
Now, a sine or triangle wave is created when the function generator chip outputs data which
addresses the RAM, and the RAM data becomes DAC U15 input data. The output of this DAC
will be the sine or triangle wave (whichever was selected). The components L1, L2, C21, C22,
C23 form a low pass filter required to filter glitches out of the 10 MHz clock. The SINE CAL
pot R4 is used to adjust the amplitude of the sine wave which is then amplified by U16A. The
output is switched by analog switch U20 or U21. U21 can be switched to allow the sine wave to
be converted to a square wave, via U17. The duty cycle of U17 can be adjusted with R31 and
R32. The output of U17A drives the SQR CAL pot which allows adjustment of the amplitude of
the square wave. The signal then buffered by U16B before arriving at U20 analog switch. At
U20, the Sine or Square wave signal is switched for A_GEN output to the AM/FM Driver PC
assembly (A6).
Generator B operates identically to generator A and has its own set of multiplexers U5, U6 and
U7, RAM’s U11, U12, U13, DAC U18 and sine wave to square wave converter U17D.
3.3.9 Scan Modulation — PC Assembly A7
This circuit translates the internal or external modulating signal into a control signal for
regulating the attenuation of the scan module.
U10 and U35 decode certain control inputs from the computer. U10 enables and disables the
10 MHz clock (used to drive DACs and ADCs on this circuit board when they are needed).
U35 switches Q1 on and off to control the relay that switches the scan module into or out of the
RF output path.
U11B generates a reference voltage needed by the analog-to-digital converter U12.
U11A generates a reference voltage needed by U2B, U33A, and U33B.
U33A and U33B generate reference voltages needed by FETs in the scan module.
Model GT 9000 Microwave Synthesizer
3-22
Manual No. 120AM00250, Rev C, September 1998