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HP 437B
Service Sheet BD1
8-18
SERVICE
Range attenuator | and range attenuator 2 work together to create five 10
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dB power ranges whose values are determined by the power sensor being
used. The higher the power at the input, the more it is attenuated.
This
needed resolution for each range.
The variable gain amplifier provides between 23 and 39 dB of gain
depending on the control signals from the Digital Section. The gain of the
variable gain amplifier is digitally adjusted when the power sensor is
calibrated to a reference power (the CAL function).
The bandpass amplifier provides filtering and amplification of the signal
from range attenuator 2, The passband frequency is 220 Hz +20 Hz witha
gain of approximately 45 dB.
The synchronous phase detector has an alternating gain of +1 and -! which is
synchronized with the 220 Hz multivibrator.
This alternating gain provides
full-wave rectification of the 220 Hz signal from the 220 Hz bandpass
amplifier.
The rectified 220 Hz signal
from the synchronous phase detector is fed into
the low pass filters. The low pass filters section consists of a fast filter and a
slow filter. The fast filter has a response time of 45 milliseconds, while the
slow filter has a response time of 800 milliseconds.
When the digital filter is
set to 1, 2, 4, or 8 averages, the fast filter is selected.
When the digital filter is
set to 16, 32, 64, 128, 256, or 512 averages, the slow filter is selected.
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The internal digital filter is not a hardware component.
It is a software
routine that averages together readings from the low pass filters. This
averaging produces a more stable and accurate power reading.
The number
of readings averaged together by the digital filter can be selected
automatically or manually.
In auto filter mode, the Power Meter automatically sets the number of
readings averaged together to satisfy the filtering requirements for most
power measurements.
The number of readings averaged together depends
upon the resolution and the power range in which the Power Meter is
currently operating. The following table lists the number of readings
averaged for each range and resolution when the Power Meter is in auto
filter mode.
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