1705A Spectrum Monitor 4-1
Section 4Theory of Operation
The materialin this section is subdivided into a general description (which is
supported by the mainblock diagram) and detailed circuit descriptions that use
the schematicdiagrams as illustrations. A thorough understanding of the
instrument startswith knowing how the major circuit blocks fit together,
followed by an understandingof 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 int o individual circuit
descriptions.

Overview

The 1705A Spectrum Monitor is a specializedspectrum analyzer, designed to
assist in locating satellites,and to help optimize communication with the
satellite. Itis capable of displaying the spectralplot of signals in the L--Band
(900 --1750 MHz), and the 45 --100 MHz range on the crt. Analphanumeric
frequencyreadout displays the frequency at the center of the intensified zone, in
FULL SPAN/DIV,on the crt. Inthe magnified SPAN/DIV ranges, the frequency
readout is the approximatefrequency under the readout marker.
Front-panel mode switching is accompl ished by push-button switches whose
status is constantly polledby a microprocessor. Inturn, the microprocessor
controls switching functionsand circuit gains so that the instrument can be used
to locate or monitor a specificset of frequencies.
The Low VoltagePower Supply is a high-efficiency switching type. TheHigh
VoltagePower Supply provides 13 kV acceleration potential.

Block Diagram

The 1705A has two separate signal inputs, L--Band(Sweep range 900 --
2000 MHz, calibratedrange 900 -- 1750 MHz) and 70 MHz(45 -- 100 MHz).
Selection of the frequency band to be displayed is acco mplished 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--BandTuner is self containedand consists of an RF Amplifier, Voltage
Controlled Oscillator,and a mixer stage. The gain, at the 1st IF frequencyof
489.9 MHz, is 10 dB. BothRF and IF gain can be adjusted. ItsVCO is

RF Input Circuits

(Diagram 1)