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Overview
Looking at the Block
Diagram
Block Diagram
HP 70427A AM Detector Filter
HP 70427A AM Detector Filter
The HP 70427A AM Detector filter (Assembly A3) prevents the DC voltage
component of the demodulated signal from saturating the measurement
system's low noise amplifier (LNA) in the HP 11848A Phase Noise
Interface. The AM detector assembly also measures the peak input carrier
level to set the input attenuation level on CW and pulsed signals.
AM Detector Filter
The IF AM detector and microwave peak detector (Input AM Detector)
outputs are connected together at the input of the detector filter block.
Detector selection is made by the presence of an RF carrier on the detector's
input. The diode detector is best modeled as a current source which produces
a current proportional to the carrier level. If the carrier signal is removed, the
detector acts like an open circuit. The switching is therefore accomplished
by selecting which detector is connected to an RF signal.
The combined input signal is switched between two paths. The AM Detector
Filter path removes the DC component from the detector output for AM
noise measurement. The level detector path measures the peak DC
component which is proportional to carrier level.
The AM Detector Filter is a 0.1 Hz, one pole, high pass filter with a flat
frequency response to beyond 40 MHz. The filter is implemented as a 3,000
μF blocking capacitor. The value of this capacitor sets the lower frequency
limit of the demodulated output. The cutoff frequency can be decreased by
increasing the value of the DC blocking capacitor.
The 511Ω resistor in the AM detector filter sets the DC bias current in the
diode detector. The AC load is 50Ω, set by the input impedance of the HP
11848A noise input. The 50Ω load increases the detector bandwidth to
greater than 40 MHz.
The detector output high frequency roll-off is greatly reduced by
compensation inductors. The inductors compensate for excess capacitance to
ground by peaking the response. The result is a low pass filter network with
a cutoff frequency greater than 40 MHz.
The output voltage of the AM detector filter is limited to ± 1 volt maximum.
Silicon protection diodes are in parallel with the filter output. The protection
diodes prevent possible destructive discharges from the DC blocking
capacitor from overloading the HP 11848A's Noise Input.
Carrier feed through in the detectors may be excessive for frequencies below
a few hundred megahertz. The measurement system's LNA in the HP
11848A interface is protected from saturation by its internal filters used to
13-60 HP 70427A/HP 70428A User's Guide
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