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Model
8901B
Service
Service Sheet BD3
BLOCK
IF
PRINCIPLES OF OPERATION
Genera I
The
IF
Block Diagram documents the circuits that further amplify, filter, level detect, and demodulate
the
IF
signal. The diagram also documents the AM and FM calibrators.
NOTE
The following two paragraphs contain operating principles for both the
A54, A55, and A71, A72 Option Series 030 assemblies. The A54 and A55
Assemblies are used in instruments with serial prefixes prior to 2636A; the
A71 and A72 Assemblies are used in instruments with serial prefixes 2642A
and above.
Channel Filter Assembly (A55 or A72, Option Series 030)
The A55 (A72) Channel Filter Assembly and A54 (A71)
IF
AmplifierDetector Assembly are used only
in the Selective Power measurement mode. The assemblies are installed only in Option Series 030
(the Selective Power Option) as defined in Table 8D-1. The Adjacent-Channel Power measurement is
controlled by Special Function 24.
The 455 kHz
IF
signal is routed through an
IF
Step-up Transformer to one of two channel filter paths.
The channel filter path is determined by Special Function 2 4 First Channel 1 Filter (the "wide" filter)
is selected by Special Functions 24.1 and 24.2; First Channel 2 Filter (the "narrow" filter) is selected by
Special Functions 24.3, 24.4, 24.5, and 24.6. Four filters are available (specified by the option number)
and two filter options must be chosen. Of the two filter options, the wider-bandwidth filter becomes
First Channel 1 Filter; the narrower-bandwidth filter becomes First Channel 2 Filter. (Again, refer
to Table 8D-1.) The total
IF
measurement bandwidth is the series combination of the First Channel
Filters and the Second Channel Filters in the A54
IF
AmplifierDetector Assembly.
Each channel has an input channel buffer (0 dl3 gain) and an output channel amplifier (6 dl3 gain).
The channel output has a selectable gain of 0
or
20 d E 3 (the Channel Output Attenuator followed by
the Channel Output Amplifier).
IF Amplifier/Detector Assembly (A54 or A71, Option Series 030)
The 455
kHz IF
signal from the
A55
(A72) Channel Filter Assembly is further amplified and filtered by
the A54 (A71) IF AmplifierDetector Assembly, which also converts the IF level to a dc voltage which
is then measured by the Voltmeter.
The
IF
gain is determined by the combination of selectable Attenuators
1
through 4 and fixed-gain
Amplifiers 1 through 3. Attenuators
1
through 3 have 0 and 20
dB
steps. Attenuator 4 has 5
dB
steps
from 0 to 15
dB.
The Second Channel
1
Filter is always a 33 kHz wide bandpass filter. It
is
selected by Special Functions
24.1, 24.2, 24.3, or 24.4. The Second Channel 2 Filter is always a 2.5 kHz wide bandpass filter and has
the same nominal bandwidth as the Option 037 filter in the A55 Channel Filter Assembly. It is selected
by Special Function 24.5 or 24.6 for
SSB
noise measurements. The Channel Output Amplifier has 33
dI3
gain. The
IF RMS
Detector converts the 455 kHz level to its dc rms level equivalent.
Service Sheet BD3
8D-25
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