HP 8901B Operation And Calibration Manual page 25

Model 8901B
General Information
LO Circuitry
The
LO
drives the high-level port of the Input Mixer and is one of several inputs
to
the Counter. The
LO has
four main modes of operation:
a
tuning to the frequency required to down-convert a signal whose frequency is entered from the
keyboard (manual tune mode),
a
automatically searching for an input signal, then tuning the LO to the frequency required to
down-convert the signal (automatic tune mode),
a
automatically searching for an input signal, then configuring the
LO in
a feedback loop that
automatically tracks the signal (automatic tune track mode), and
a
tuning
to the frequency required to down-convert a signal whose frequency is entered from the
keyboard, then configuring the LO in a feedback loop that automatically tracks the input signal
(manual tune track mode).
The manual tune track mode is useful when it is desired to follow an unstable signal in the presence
of other signals. The non-track modes are used when the
LO
noise (residual FM) must be minimized.
IF Circuitry
The gain of the
IF
Amplifier is fixed. The
IF Filters determine the frequency response of the IF. When
the 1 . 5 MHz IF is selected, the
IF
filter consists of a 150 kHz
to
2 . 5 MHz bandpass filter (with a
nominal center frequency
of
1 . 5 MHz). When the 455 kHz
IF
is selected, the
IF
filter is the Wide
455 kHz Bandpass Filter (with a bandwidth of 200 kHz).
In instruments with Option Series 030, the IF signal is further processed by the Channel Filters (which
also include a precision, variable-gain amplifier) and detected by the IF RMS Detector. The Channel
Filters set the
IF
bandwidth and gain for the Selective Power measurement. The measurement is made
by entering a series of Special Functions which establish an
IF
reference in the center of the Channel
Filter, then allow the relative
IF
level
to
be displayed as the IF frequency is detuned by a pre-determined
offset.
Audio Circuitry
The modulation on the
IF
is demodulated by either the AM or the FM Demodulator. Phase modulation
is recovered by integrating the demodulated FM in the Audio Filters and Gain Control circuitry.
The demodulated signal is amplified and filtered in the Audio Filters and Gain Control circuitry. The
filters are selected from the front panel, and for FM, the filtering may also include de-emphasis. The
processed signal is passed to the front-panel MODULATION OUTPUT/AUDIO INPUT connector
and the voltmeter.
The audio signal from the Audio Filters and Gain Control is converted to a dc voltage by the Audio
Peak Detector, the Audio Average Detector or the Audio RMS detector. The Audio Average and RMS
Detectors are used primarily for measuring noise. The output from the detectors is routed into the
Voltage-to-Time Converter.
The Voltage-to-Time Converter within the voltmeter converts the dc input into a time interyal. During
the interval, the 10 MHz Time Base Reference is counted by the Counter, and the resultant count
represents the dc voltage. Other inputs
to
the voltmeter, which are not shown, include outputs from
an audio level detector and the AM calibrator.
The Distortion Analyzer measures the distortion of either the internal demodulated signal or an audio
signal applied externally to the MODULATION OUTPUT/AUDIO INPUT connector. The frequency
of the input signal must be either 1 kHz or 400 Hz. The distortion on the signal is determined by
measuring the amplitude of the signal before and after a notch filter that is set
to 1 kHz
or 400
H z .
The
two ac signals are converted to dc by a the Audio RMS Detector and then measured by the voltmeter.
Distortion is computed as the ratio of the voltage out of the notch filter to the voltage into the filter.
(The Audio RMS Detector can also be used to measure the demodulated AM, FM, or OM internally
or the ac level of an external audio signal applied to the MODULATION OUTPUT/AUDIO INPUT
connector.)
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