Tektronix 492, 492P, Руководство по обслуживанию

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Theory of Operation—492/492P Service Vol. 1 (SN B030000 & up)
Other Systems
The power supply system provides regulated dc power
for all parts of the instrument. The switching supply is capa­
ble of regulation over wide line frequency and line voltage
ranges.
The cooling system consists of an intake on the bottom
of the case, air passages within the instrument, a fan, and a
rear panel exhaust. Air is routed to all sections of the instru­
ment in proportion to the heat generated by that section.
Internal temperature rise is small for reliable operation.
Signal, power, and control connections between sections
are accomplished by a mother board distribution system.
Most circuit boards plug onto the mother board from the top
side. Components on the RF deck underneath the mother
board are also connected to the mother board through
smaller connectors.
For Further Information
The systems in the 492/492P are described in nine sec­
tions as shown by the Functional Block Diagram. Nine block
diagrams representing these systems follow the Functional
Block Diagram.
For more detailed information, the instrument is divided
into circuit diagrams for each assembly or part of an assem­
bly. Each schematic is accompanied by a detailed block dia­
gram and a parts location illustration. These are printed in
the Diagrams section with look-up tables to aid in finding
components on either the schematic or parts location
illustration.
DETAILED DESCRIPTION
The following description is arranged by sections or sys­
tems; such as 1st Converter, 2nd Converter, etc., followed
by circuit analysis of the circuits within that section. Each
system/section is introduced with a description of the sys­
tem using the block diagram found in the Diagrams section
of the manual. This is then followed with a description of
each circuit board or major circuit within the system.
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1ST CONVERTER CIRCUITS
The 1st Converter mixes the incoming RF signal with a
tunable local oscillator signal to produce intermodulation
products. All of these are filtered out except the 2072 MHz
and 829 MHz IF signals, which are applied to the 2nd Con­
verter circuit.
The 1st Converter consists of the following major
segments:
1) the RF Attenuator, which sets the input power to the
analyzer;
2) the Preselector (Option 01 only), which provides the se­
lectivity required to eliminate spurious responses and image
frequencies;
3) the 1st LO (Local Oscillator), which provides a tunable
signal for the 1st Mixer;
4) the Power Divider, which splits the signal from the 1st
LO for application to the 1st LO OUTPUT front-panel con­
nector, and 1st Mixer. (The 1st LO signal passes through
the Phase Gate to the 1st Mixer if Option 03 is included);
5) the Phase Gate, which couples a portion of the oscillator
signal to a phase gate that compares the phase of the oscil­
lator signal against a strobe signal from the phaselock
system;
6) the Transfer Switch, which permits the use of an external
mixer with the analyzer;
7) the Filter and Diplexer circuits, which select only the
2072 MHz and 829 MHz IF signals for application to the 2nd
Converter;
8) the RF Interface circuits, which select the input RF at­
tenuation, control the selection of the IF frequency (2072 or
829 MHz), and control the transfer switch.
The input RF is fed through a 0 to 60 dB decade
attenuator to the 1st Mixer (if Option 01 is not included) via
a 3 dB attenuator. The attenuator matches mixer imped­
ance and protects the mixer diodes from spurious or static
signals.
REV NOV 1982 5-3