HP 70427A User Manual page 472

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.
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 occur at +10 dBm output level.
The disadvantage of drain bias control is in level accuracy and level
switching speed. The level accuracy is about ±3 db. The switching speed is
slow because the output level is dependent on drain voltage driver transistor
junction temperature and MMIC device temperature. In ALC ON mode,
time is taken to allow the thermal transients to decay, while constantly
re-adjusting the output level. The switching time from +20 dBm to 0 dBm is
as much as 5 seconds.
Table 13-15 GaAs Amplifier's Typical Residual Phase Noise @ 9.6 GHz:
Carrier Offset (Hz)
Residual Phase
Noise
Table 13-16 GaAs Amp's Typical AM Noise @ 9.6 GHz (Second Gate control):
Table 13-17 GaAs Amp's Typical AM Noise @ 12 GHz (Drain Volt control):
Carrier Offset (Hz)
AM Noise @ 10 dBm
1
–110
Carrier Offset (Hz) 1
AM Noise @ 10 –93
dBm
1 10
–105 –116
HP 70427A/HP 70428A User's Guide 13-37
HP 70428A Microwave Deck Assembly
10 100 1 k
–120 –128 –137
10 100 1 k
–103 –112 –123
100 1 k 10 k
–126 –139 –150
Block Diagram
10 k >1 M
–144 –158
10 k >40 M
–132 –155
>1 M
–161