Table of Contents
Advertisement
absorb phase detector feedthrough and unwanted mixer products. This limits
the maximum carrier offset frequency to 2 MHz, for input frequencies of less
than 95 MHz, and 40 MHz for carriers above 95 MHz.
Detector Bias Amplifier
The detector bias amplifier is a DC coupled, 12 dB gain amplifier with a
26 Meg-Ω input impedance. The ratio of amplifier input impedance to the
511Ω detector DC bias resistor results in 94 dB attenuation of noise coming
from the amplifier's input into the AM detector signal path. The amplifier
allows the measurement of the DC voltage component of the demodulated
signal without degrading the noise measurement. This feature is only used
for diagnostics.
Level Detector Amplifier
The diode detector outputs are amplified by the level detector amplifier
during the input attenuator setting operation. The amplifier provides a 100Ω
load for the detector outputs, and adequate gain and frequency response to
operate the peak-hold detector under pulsed conditions.
The level detector amplifier is a 165 MHz, 3000 V/μs, trans-impedance
op-amp, set for a gain of 10, and back-matched with a 50Ω resistor. The
output voltage range is ±11 V. An auxiliary output is provided through a
500Ω resistor which connects the DC-coupled AM signal to the diagnostic
multiplexer system.
Peak-Hold Detector
The peak-hold detector is a buffered diode peak detector followed by a
monolithic peak-hold circuit. The peak-hold detector measures and stores
the peak amplitude of the microwave detector diode until it can be read by
the diagnostic multiplexer system.
The diode peak detector rise time is about 50 ns for full output from a single
pulse, which will decay with a 33 μs time constant. The detector output is
buffered through a unity gain non-inverting FET-input op-amp amplifier,
which is followed by a monolithic peak-hold detector. The peak-hold
detector stores the fast diode detector output until the level can be read by
the module's internal volt meter. The acquisition time of the peak-hold
detector is 2.5 μs/V. The droop rate is 15 mV/s at room temperature.
The internal microprocessor uses the peak-hold level to set the input
attenuator for an output level of –5 dBm to –10 dBm. The input attenuation
setting routine is initiated by the
softkey. The auto attenuator setting command, initiated after a signal is
present, first sets the input attenuator to maximum, and then steps the gain
up in 5 dB steps until the desired output range is achieved. The input
attenuator is set back to maximum if the output level of –5 dBm to –10 dBm
HP 70427A AM Detector Filter
softkey or the
AUTO RANGE
HP 70427A/HP 70428A User's Guide 13-61
Block Diagram
AUTO ATTEN
Advertisement
Table of Contents
