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Block Diagram
YIG-Tuned-Filter (YTF) and GaAs FET Amplifier Driver Assembly
The gate #1 bias supply is implemented by first filtering the –10 V supply
with a capacitance multiplier. The capacitance multiplier corner frequency is
7.2 Hz. The output of the capacitance multiplier is then fed to the six gate #1
output controls.
22 μF shunt capacitors are located on the GaAs amplifier connector
assemblies. The capacitors form a low-pass filter with the output resistance
of the gate #1 controls. Distributing the filter between the assemblies,
minimizes the amount of 40 kHz spur energy induced on the
inter-connecting cables by the 40 kHz switching power supply. GaAs FET
Amplifier Drain Current Adjustment
The drain current must be checked and adjusted whenever the
YIG-tuned-filter (YTF) and GaAs FET amplifier driver assembly or the
GaAs FET amplifier assemblies are altered.
The current in MMIC stages #1 to #4 must be set for 100 mA, ±2 mA.
Stages #5 and #6 are set to 150 mA, ±2 mA.
The current is measured without RF signal present (Main DAC 100), and the
Output Level DAC set to maximum (DAC= 4095). The module may be in
either second gate control or drain voltage control modes. The current may
be read using the diagnostic multiplexer system in service mode (see chapter
15).
GaAs FET Amplifier Level Detector Amplifier
The 470 pF peak-hold capacitor at the detector amplifier input filters the
detector output. The parallel 10 kΩ resistor across the capacitor produces a
4.7 μs discharge time constant.
The detector amplifier (U20A) is a non-inverting high input impedance
amplifier with a gain of 20.6. The amplifier output is connected into the
diagnostic multiplexer system.
YIG-Tuned-Filter (YTF) Current Driver
The YTF Current Driver is a precision current sink which controls the YTF
frequency. Frequency is set by the dual 12-bit DAC U6. One section of the
DAC adjusts the main frequency tuning. The main DAC tune sensitivity is
7.32 MHz/step. Fine tune adjustments are made by the other DAC section.
The Fine tune DAC sensitivity is 85.2 kHz/step.
Each 12-bit DAC section uses the +10.0 volt reference to produce an output
from 0.0 volts to –10.0 volts. The output is then shifted and gain adjusted to
produce the desired output.
The main tune DAC output is first inverted by the unity gain amplifier U16
to produce an output between 0.0 and +10.0 V. The reference and DAC noise
is then removed with an adaptive noise filter.
13-68 HP 70427A/HP 70428A User's Guide
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