HP 70427A User Manual page 502

Block Diagram
YIG-Tuned-Filter (YTF) and GaAs FET Amplifier Driver Assembly
+2 V, which corresponds to 1.3 V at the MMIC drains, if the –10 V supply is
more positive than –7.5 V.
The output of the noise filter voltage follower (U27) drives the output
transistor for each of the six drain supply outputs. Each output uses 2 or 3
parallel output transistors depending on the required output current. Each set
of output transistors uses both base and emitter resistors. The base resistors
prevent oscillation instability, the emitter resistors force equal current
sharing between devices.
Drain supply noise and spurs are reduced beyond 4.5 kHz by the R-L-C filter
at the output of each of the drain supplies. The series 55 μH inductors and
their corresponding 0.5Ω winding resistance are located at the output of each
of the drain supplies on the YIG-tuned-filter (YTF) and GaAs FET amplifier
driver assembly. The 22 μF shunt capacitors are located on the GaAs
amplifier connector assemblies. The inductors are designed to be low "Q".
The internal resistance keeps the noise from peaking near the filter corner
frequency of 4.5 kHz. Distributing the filter between the two printed circuit
board assemblies, minimizes the amount of 40 kHz spur energy induced on
the inter-connecting cables by the 40 kHz switching power supply.
The resistors between the output transistors' collector and the +10 V supply
are used to measure the drain current of the MMICs, and help reduce power
dissipation in the output transistors. Current is measured in a two step
process. First, the voltage is measured at the output transistor collectors by
the internal volt meter, and then the +10 V supply voltage is measured.
It is important to keep the change in junction temperature of the drain supply
output transistors to a minimum. The MMIC drain voltage is set by the
reference voltage at the noise filter output, minus the base-emitter voltage
drop of the drain supply output transistors. The base-emitter voltage is
typically 0.7 V, with a –2 mV/°C temperature sensitivity.
A small change in drain voltage will produce several tenths of a dB change
in output power when operating in drain bias control mode, especially at low
output levels.
The sensitivity of output level to drain bias voltage also makes minimizing
drain voltage noise crucial in producing good AM noise performance.
GaAs FET Amplifier Gate #1 Bias Supply
The gate #1 voltage controls the bias current through the MMIC amplifiers.
The voltage range is –2.5 V (Drain current pinch-off) to 0.0 V (I ). The
dss
gate #1 noise voltage must be kept to a minimum to prevent drain current
modulation, which produces AM noise.
The Gate #1 voltage is supplied by the –10 V supply. The supply is
monitored by the drain voltage shutdown circuit, which protects the MMICs
in case of –10 V supply shutdown.
HP 70427A/HP 70428A User's Guide 13-67