Tektronix 1705A - Manual (Page 74)

1705A Spectrum Monitor 4- 1
Section 4
Theory of Operation
The material in this section is subdivided into a general description (which is
supported by the main block diagram) and detailed circuit descriptions that use
the schematic diagrams as illustrations. A thorough understanding of the
instrument starts with knowing how the major circuit blocks fit together,
followed by an understanding of the individual circuit’s functions. These
discussions of the 1705A Spectrum Monitor begin with a brief, fundamental
overview, then proceed on to the block diagram, and then into individual circuit
descriptions.
Overview
The 1705A Spectrum Monitor is a specialized spectrum analyzer, designed to
assist in locating satellites, and to help optimize communication with the
satellite. It is capable of displaying the spectral plot of signals in the L--Band
(900 -- 1750 MHz), and the 45 -- 100 MHz range on the crt. An alphanumeric
frequency readout displays the frequency at the center of the intensified zone, in
FULL SPAN/DIV, on the crt. In the magnified SPAN/DIV ranges, the frequency
readout is the approximate frequency under the readout marker.
Front-panel mode switching is accomplished by push-button switches whose
status is constantly polled by a microprocessor. In turn, the microprocessor
controls switching functions and circuit gains so that the instrument can be used
to locate or monitor a specific set of frequencies.
The Low Voltage Power Supply is a high-efficiency switching type. The High
Voltage Power Supply provides 13 kV acceleration potential.
Block Diagram
The 1705A has two separate signal inputs, L--Band (Sweep range 900 --
2000 MHz, calibrated range 900 -- 1750 MHz) and 70 MHz (45 -- 100 MHz).
Selection of the frequency band to be displayed is accomplished by pushing a
front-panel, momentary, push-button switch, which is monitored and acted upon
by the microprocessor. When one input is selected, the other has its supply
voltage interrupted.
The L--Band Tuner is self contained and consists of an RF Amplifier, Voltage
Controlled Oscillator, and a mixer stage. The gain, at the 1
st IF frequency of
489.9 MHz, is
¶10 dB. Both RF and IF gain can be adjusted. Its VCO is
RF Input Circuits
(Diagram 1)