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Model 8901B
Service
Service Sheet BD5
BLOCKS
0
Digital
0
Power Supply
PRINCIPLES OF OPERATION
General
The Digital and Power Supply Block Diagram documents the Controller, Counter, Keyboard and
Display, Remote Interface (HP-IB), and the Power Supply.
Counter
Assembly (A1 1)
The Counter consists of a 10 MHz Reference Oscillator, Time Base Dividers, Input Selector, four
counter stages, and counter control circuits. The input to Stages 2 through
4
is
selected by the Input
Selector switch. When the LO frequency is counted, counter Stage 1 is enabled and fed into Stage 2.
The input to Stage
1
is the High Frequency VCO signal divided by eight (HF VCO
4).
When the
Voltmeter or Audio Counter input is selected, the Selected 10 MHz Time Base Reference (from the
Time Base Select Switch), gated by the Voltmeter/Audio Gate (the VOLTMETER/AUDIO line), is
the input to Stage 2. Other signals which
can
be counted are the
IF,
the 10.1 MHz VCO of the FM
Calibrator (the CALIBRATED FM 2 line), three 10 MHz Time Base References (Internal, External, or
Selected). Counting of the Internal and External Time Base Reference
is
useful as a Counter self-check.
Special Function
46
controls the Input Selector.
The Counter counts in binary. Stage
1 is
a divide-by-eight, and Stages 2 through
4
are divide-by-
sixteens. Additional stages of counting are contained in the Microprocessor (in A13), which counts the
output carries from Stage 4 via the Counter Output and Time Base Gate.
At the end of a count sequence, the Time Base disables the Counter via the Counter Gate Control. The
Counter Transfer Logic then transfers the count of the individual stages in parallel to the Controller
(A13) via the Counter Output and Time Base Gate. First, the output from Stage
4
is transferred.
Then the output from Stage 3 is loaded into Stage
4,
and the output from Stage
4
is again transferred.
The transfer process is again repeated with Stage 2 loading into Stage 4 via Stage 3, and transferring.
Finally, Stage 1 is loaded into Stage
4
via Stages 2 and 3, and transferred. (The output from Stage 1
is used only when counting the LO frequency.)
To make a voltage measurement, the Voltage-to-Time Converter (in the A5 Voltmeter Assembly of
Service Sheet BD4) generates a pulse whose time interval is proportional to its dc input voltage.
During this time interval, the Counter counts the Selected 10 MHz Time Base Reference (via the
VOLTMETER/AUDIO line). The count thus accumulated is proportional to the input voltage. The
count is initiated when the Counter Gate Control enables the Input Selector and the Voltage-to-Time
Converter (via the Ramp Gate). The Voltage-to-Time Converter then closes the Voltmeter/Audio Gate
(via the Stop Count line) and the Selected Time Base Reference passes into Counter Stage 2. When
the time interval ends, the Voltmeter/Audio Gate is opened. The Controller disables the Counter and
transfers the accumulated count to the Controller.
The Audio Counter also makes use of the Stop Count line to control the counting of the Selected Time
Base Reference during the count sequence. (See the discussion of the Audio Counter in Block Diagram
BD4.)
The Selected Time Base Reference is either the standard 10 MHz Reference Oscillator, the Option
002
10 MHz High-Stability, Crystal Oscillator, or an external input from the rear-panel TIME BASE 10
MHz INPUT connector. For Option
002,
the crystal from the 10 MHz Reference Oscillator
is
removed
and the oscillator's circuitry is driven by the High-Stability, Crystal Oscillator. In either case, when
Service Sheet BD5
8D-47
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