HP 70427A User Manual page 493

Block Diagram
HP 70427A Intermediate Frequency (IF) Amplifier
IF Output Switching
The IF output amplifier drives the IF Output available on the module's front
panel and the RF Analyzer Output, IF monitor selection. The IF Output is
always active, and should be terminated in 50Ω when not in use. Both
outputs may be used simultaneously.
The IF monitor path also has an auxiliary input which allows the RF
ANALYZER OUTPUT on the HP 11848A Phase Noise Test Set to be
connected through the rear panel of the HP 70427A to the HP 70427A
Analyzer Output.
IF AM and Peak Detector
The AM detector output amplifier drives the IF level peak detector, and the
AM detector path. The AM detector path can be further switched between
the AM noise measurement output, and the IF peak level output.
The AM detector employs a diplexed 40 MHz high pass filter to remove base
band noise and provide a constant input impedance.
The diplexed filter design is actually two filter inputs in parallel, a high-pass
maximally flat (Butterworth) filter and a low-pass maximally flat
(Butterworth) filter, designed with the same cutoff frequency. The result is a
diplexer with a constant input impedance. Signals above the cutoff frequency
pass through the high-pass part of the filter. Signals below the cutoff
frequency pass through the low-pass part of the filter and are terminated in a
Ω load resistor.
50
The filters were synthesized from 35 MHz, 4 pole designs, with 3 dB of
attenuation at the cutoff frequency. The theoretical input return loss is better
than –18 dB at all frequencies. The actual filter provides a return loss better
than –15 dB to beyond 1.5 GHz, with a 1 dB cutoff frequency of 40 MHz.
The output of the high pass section is also terminated in two 100Ω resistors
in parallel which provide a good low inductance 50Ω load for the diode
detector. The detector is a surface mounted 20 volt Schottky diode. The 20
volt version is chosen because it has the best noise performance of the
Schottky diodes. The detector capacitor is 3.3 pF, which is the same
capacitance used in the input AM detector.
The detector output is then switched between the AM noise output which is
connected to the AM Detector Filter, and the IF level amplifier.
The baseband bandwidth of the IF AM detector has been experimentally
adjusted to beyond 40 MHz. It is dependent on the input impedance match of
the HP 11848A noise input, the amount of coax between the two modules,
the compensation elements in the AM Detector Filter, and the stray
capacitance in the AM Detector Filter.
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