Section I. GENERAL INFORMATION

TM 11-5820-917-13

SECTION 1

GENERAL

INFORMATION

INTRODUCTION

1-1.

1-2. This manual provides operating and service instructions for the RCS-4B

Receiver. The information is presented in seven sections. Section 1 provides a

brief description of the equipment and operating specifications. Unpacking instruc-

tions, site requirements, cabling data, and installation instructions are included in

Section 2. Section 3 provides operator controls and indicator information and

describes operating procedures. In Section 4 are functional descriptions of

receiver circuits. Section 5 describes preventive and corrective maintenance

procedures for the receiver and provides a performance verification test. Section

6 contains the wire lists for reference during maintenance. All oversize drawings,

such as schematic diagrams, are grouped at the back of this manual as foldout

sections. Section 7 presents procedures for managing HF circuits using the

supplied equipment.

1-3.

GENERAL

DESCRIPTION

1-4. The RCS-4B receiver (figure 1-1) is one part of an HF Radio Sounder Set

(also designated the TFMS) used for frequency management of HF radio circuits.

It is a sweeping HF receiver that tunes through the HF spectrum at a 50-kHz per

second or 100 - kHz per second rate beginning at 2 MHz. When synchronized with

a TCS-4B transmitter located at the other end-of an HF circuit, the receiver pro-

vides a measurement of ionospheric propagation conditions for the circuit. A TCS-4B

transmitter at one end of the HF circuit emits a CW signal which starts at 2 MHz and

sweeps upward in frequency at a constant linear rate (50 kHz /second for a 2-16

MHz sweep; 100 kHz/second for a 2-30 MHz sweep.) An internal clock in the RCS-4B

receiver starts the receiver sweep synchronously with the transmitter and precisely

tracks the sweeping transmitter signal. Any radio frequency that can propagate

over the HF circuit is received by the RCS-4B. The radio energy propagates by

various modes (e. g. surface wave, one- hop, t we-hop, etc.) which have different

propagation delays. The time delay created by the travel of the RF signals from the

transmitter to the ionosphere and to the receiver causes the ionospherically returned

signal to arrive "late" or slightly behind the exact tuned frequency of the receiver.

That is, since the transmit and receive sweeps are started at exactly the same time,

the receiver sweep has advanced to a slightly higher frequency by the time the

transmit signal arrives at the receiver. Thus, the receiver is tuned to a slightly

different (higher) frequency than the arriving transmit signal. This received

frequency difference is amplified in the receiver IF and converted to a "baseband

audio" tone. A tone at zero hertz indicates no time delay and tones of increasing

audio frequency (up to 500 Hz) indicates increasing time delay of the ionospherically

returned signal. In practice, multiple tones are present in the sounder's baseband

audio output which represent the various delays caused by the different layers,

or modes, of the ionosphere. These multiple tones are processed by the sounder's

spectrum analyzer to separate the tones into discrete components which identify

corresponding ionospheric modes. A speaker and headphone jack are provided for

audio monitoring. The output spectrum analyzer is displayed on the 6025 CRT display

versus the RF sweep of the sounder. The CRT vertical axis indicates received

1-1