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Block Diagram
YIG-Tuned-Filter (YTF) and GaAs FET Amplifier Driver Assembly
Analyzer LO module. The spectrum analyzer uses this output to control the
HP 70601A Preselector Microwave Filter.
Output Level Control
The output level control sets the output level on the 2.4 GHz to 26.5 GHz
GaAs amplifiers. Level setting can be done in two ways, depending on the
noise performance vs level stability requirements.
Second Gate Control
Second gate control adjusts gain by controlling the gate bias of the second
MESFET of each of the cascode stages in the MMIC. The second, third, and
fourth amplifier MMIC's second gate control ports are connected together
and can be controlled by the GaAs amplifier driver assembly. The control
range is –7 dB to +8 dB gain per MMIC, which allows an overall output
level range of 0 to +20 dBm.
Second gate control is used in the module whenever frequency switching or
calibration is taking place. The advantage of second gate control is fast and
repeatable power level setting. The disadvantage is a 20 dB degradation in
AM noise over drain bias control.
The second gate control is produced by forcing the MMICs' second- gate
bias network from its nominal +2.5 V operating point (@ 7.0 V drain
voltage) over the range of –2.5 V to +2.5 V. The drive current required is 17
mA/MMIC (50 mA total). The second gate control voltage is generated by
off setting the – 10 V to 0 V DAC output and adjusting the gain. The drive
current is supplied by an emitter follower placed on the offset/gain adjust
amplifier output (U13), but still connected inside the amplifier's feedback
loop. Four sections of a quad FET switch (U26) are used to connect the
Second Gate Control driver to the microcircuits. A 2.3 kHz L-C filter
follows the FET switch output to reduce control line noise.
The second gate control requires the drain voltage be set to +7.0 V.
Drain Bias Control
Drain bias control adjusts the output level by controlling the drain voltage on
each of the six MMIC amplifiers. The level adjustment range is –15 dBm to
+16 dBm, but is only usable from 0 dBm to +16 dBm. The level adjustment
sensitivity increases rapidly at lower output levels.
Drain bias control is used in the module's normal low noise mode. AM noise
is suppressed by drain voltage saturation limiting. The limiting occurs in the
last several MMIC stages at lower microwave frequencies, and provides a 20
dB improvement in AM noise. The amount of limiting and noise suppression
decreases with increasing frequency as the comb frequency power decreases,
and the MMIC gain decreases. Optimum noise performance and level
accuracy occurs at +10 dBm output level.
HP 70427A/HP 70428A User's Guide 13-65
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