FREQUENCY STANDARD SECTION

TM 11-5820-917-13

4-7.

FREQUENCY

STANDARD

SECTION

4-8. FREQUENCY STANDARD 1A6 (figures FO-7 and FO-8) (S/N 400101 and on).

This module provides highly accurate 5 MHz outputs for use by the synthesizer and

timing clocks and also provides +5 volts DC (referred to as +5VB ) power to the

clock circuits. The +5VB supply is generated by a switching regulator (within the

1A6 module) that has a battery backup (1A8) so that time clock synchronization

can be maintained during an AC line power failure. The frequency standard has

three subassemblies Al, A2, and A3. Assembly 1A6A1 is a high stability, low noise,

oven-enclosed, 5-MHz crystal oscillator. It is constructed in a dewar flask to reduce

heat loss and thereby minimize oven power consumption. The oscillator has an in-

ternal voltage regulator whose output (8 volts) appears at A1J1-2 (figure FO-7).

This voltage is used to trim the frequency (coarse and fine) by applying an adjust-

able DC bias to A1J1-1. On figure FO-7, Q1, Q2, and CR3 comprise a voltage

regulator to power the unit during normal operation. In the event of primary power

failure, the battery supply on E15 is switched in by CR1. Diode CR2 prevents the

battery voltage from appearing on K1 during a primary power failure. This allows

K1 to close after a few milliseconds delay and bypass CR1 eliminating CR1 power

loss .

4-9. The switching regulator 1A6A3 (figure FO-8) provides +5VDC power (+5VB)

for all critical timing circuits involved with receiver synchronization. Regulation

is controlled by regulator U1. Switching transistor Q1 and clamping diode CR1

provide variable width drive pulses into storage inductor L1. L1-C2 provides the

phase shift necessary for oscillation and determines the switching frequency. L2 and

C3 form a ripple reduction smoothing filter. The switching regulator has a 1.5 amp

fuse on its input to prevent serious damage in case of a long-term short. Circuit

damage from momentary shorts is protected by internal current limiting (R2).

4-10.

SPECTRUM

ANALYZER

SECTION

4-11. SPECTRUM ANALYZER 1A1. The spectrum analyzer circuits process base-

band audio (0-500 Hz) analog input (from the 4028 HF receiver section) and supply

the resulting spectrum to the CRT display section. Sounder signals are received

by the 4028 receiver via different ionospheric modes and are separated into tones

of frequency proportional to their relative time delay. The resulting receiver multi-

tone output is transformed into a multi-mode sounder display (for the CRT) by the

spectrum analyzer, each tone representing a mode. The basic concept of the spec-

trum analyzer is to provide high resolution spectral analysis of the incoming base-

band signal from the receiver. That is, the spectrum analyzer resolves the composite

analog waveform into a group of distinct spectral lines (or tones) which make up the

received analog chirp signal. However, the analyzer must also perform this spectral

analysis rapidly to keep up with the continuously changing analog chirp signal.

Thus, the analyzer operates in "real time" to process the spectrum as quickly as

the spectrum changes. The rate of change of the spectrum is determined by the

nature of the chirp signal and the analysis bandwidth. In normal operation, the

spectrum analyzer is capable of processing the spectrum over six times faster than

the "real time" limit. To do so, the analyzer operates with a hybrid digital/analog

technique. The analog input signal is A-to-D converted and loaded into a continuous-

ly updating and continuously recirculating high-speed memory. The output of the

4-3