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Block Diagram
HP 70428A Microwave Deck Assembly
The active devices used in the two amplifier microcircuits are
monolithic-microwave-traveling-wave-amplifiers. These GaAs
monolithic-microwave-integrated-circuits (MMIC) are designed for high
gain and excellent input/output match to 50Ω over the full 2 GHz to 26.5
GHz band. The seven MESFET cascode stages provide a minimum S
of
21
7 dB per MMIC, and remain relatively low noise even while operating in
gain compression.
The pre-amplifier microcircuit and power amplifier microcircuit contain a
total of six MMIC gain stages, with three stages in each package. The
amplifier was packaged in two microcircuits to greatly reduce internal
microcircuit shielding requirements, and to re-use existing package and
substrate designs. The two microcircuits make up one six stage amplifier.
The microcircuits are designed for ±5 dB gain flatness from 2.4 GHz to 26.5
GHz, with gain being rolled off below 2 GHz. The low frequency roll-off is
accomplished by using small value coupling capacitors between MMIC
stages. The low frequency gain roll-off is necessary to prevent noise below 1
GHz from being mixed up onto the microwave output for output frequencies
above 20 GHz. The sloped gain attenuator between stage 2 and 3 greatly
improves gain flatness.
The output power level is detected in the microcircuit at the output of the
sixth stage using a low-loss GaAs directional bridge. The module's internal
voltmeter is then used to measure the power level. The power level is then
compared to the power level curve-fit data, and the power is then adjusted
accordingly.
Amplitude calibration data is stored in the EE PROM on the microprocessor
assembly. The calibration data contains polynomial curve-fit coefficients for
power vs frequency at 10 dBm output, and power vs detector voltage at 9.6
GHz. Coefficients for the second set of GaAs amplifiers are also stored on
the microprocessor assembly for the HP 70428 opt 002, when necessary. The
module can measure its own frequency calibration data by connecting an
external HP 70100A or HP 437B power meter to the instrument, and
accessing the YTF Calibration screen (see chapter 15).
NOTE
The power meter must be manually calibrated and zeroed before calibration
is started.
Output power and gain are controlled in two different modes which are
selectable from the Miscellaneous screen.
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 gate
control ports of the second, third, and fourth MMIC amplifiers are connected
together and can be controlled by the GaAs amplifier driver assembly. The
13-36 HP 70427A/HP 70428A User's Guide
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