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Service
Model 8901B
in the A6 AM Demodulator Assembly (in Service Sheet BD3). Special Function 3 controls the IF
frequency selection (as well as the 5.25 MHz High-Pass Filter in the RF Input Assembly).
(The next three paragraphs apply
to 2642A and above.)
The
IF
frequency response for most measurements is determined by the
IF
filters and the
IF
amplifiers
here and in the A18
IF
Amplifier Assembly. The 455 kHz Wide Bandpass Filter in the A17 Input
Mixer Assembly determines the response of the 455 kHz IF except in the case of the Selective Power
measurements. The 455 kHz Wide Bandpass Filter is switched in automatically for input signals in the
range of 2.5 to 10 MHz. (In the Selective Power measurement mode, the narrower channel filters in
the A72 Channel Filter and A71
IF
AmplifierDetector Assemblies determine the frequency response.)
When the 455 kHz
IF
is chosen, the 455 KHZ
IF
annunciator lights.
Automatic IF filter selection provides the optimum
IF
frequency and
IF
filter selection for each
measurement mode. The frequency response of the
IF
filter is determined by the RF input blocking
capacitors (not shown), 4 MHz Low-Pass Filters 1 and 2, and (principally) the 2.5 MHz Low-Pass
Filter in the A6 AM Demodulator Assembly (in Service Sheet BD3). Special Function 3 controls the
IF
frequency selection (as well as the 5.25 MHz High-Pass Filter in the RF Input Assembly).
The First and Second
IF
Amplifiers function as buffers and provide the same IF gain for the assembly
as in older instrument configurations.
Buffer Amplifier Assembly (A16) (2314A to 2636A)
The Buffer Amplifier contains two amplifers used to split the IF signal from the Input Mixer. The
upper path is a unity gain amplifier used to drive the A18
IF
Amplifier. The lower path has a gain of
9dB and used to provide
IF
input to the A55
IF
Channel Filter when the Modulation Analyzer is in
the Selective Power Measurement mode.
IF
Amplifier Assembly (A18)
The
IF
Amplifier increases the signal from the Input Mixer Assembly to a level suitable to drive the
AM
and FM Demodulators. The
IF
strip is designed for low noise, linear phase shift vs. frequency
(that is, constant group delay) to minimize FM distortion, and for flat frequency response to minimize
incidental
AM
(that is, AM occurring as the result of FM).
Local Oscillator
The Local Oscillator consists of the LO Divider Assembly (A19), LO Control Assembly (A20), Low
Frequency VCXO Filter Assembly (A21), Low Frequency
VCXO
Assembly (A22), Sampler Assembly
(A23), and High Frequency VCO Assembly (A24). The overall operation and different tuning modes of
the LO are described in the Principles
of
Operation for Service Sheet BDl.
High
Frequency VCO Assembly (A24)
The High-Frequency, Voltage-Controlled Oscillator
(HF
VCO) has a nominal frequency range of 320 to
650 MHz. The output is buffered by two Output Buffer Amplifiers. One output drives the LO Divider,
the other drives the Sampler. The tune input to the
HF
VCO has a switchable lead-lag network (Tune
Voltage Filter) to reduce phase noise. The network is switched out while the
LO
is tuning and is
switched in when tuned.
LO Divider Assembly (Al9)
The signal from the HF VCO, after passing through the LO Divider Assembly, is the LO drive to
the Input Mixer. The LO Divider Assembly has one Doubler stage (640 to 1300 MHz LO range), one
through path (320 to 640 MHz range), and eight LO Dividers (1.25 to 325 MHz ranges). Each divider
is
a
high-speed, divide-by-two device. The Divider Output Gates enable and cascade the appropriate
dividers for the range selected. The first three dividers are always enabled. The 40 to 81.25 MHz output
of the third divider is the LO (HF VCOt8) input to the Counter.
8D-18
Service Sheet BD2
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