AUTO SYNC 1A5

TM 11-5820-917-13

4-19. On activation of the AUTO SYNC front panel switch, the AUTO SYNC lamp

flashes (U20, figure FO-11/1), and a signal is sent to the receiver and spectrum

analyzer to increase the bandwidth to 5 kHz (from 500 Hz). In addition, another

signal (BSL) is sent to the advance control circuits (figure FO-11/2). This (BSL)

signal sets in motion the following sequence:

a. A burst of extra clock pulses producing a very rapid timing advance (E63,

FO-11/2) moves the 3-path programmer timing clock and synthesizer sweep one-second

ahead of the nominal time.

b. A series of short clock interruptions (E66, FO-11/2) to the basic 100 kHz

clocks of the 3-path timer and synthesizer which effectively steps or retards the

receive system back in time to a point one-second before the nominal start time.

Between each retard- step the chirp sweep and timer clocks are allowed to proceed

normally for brief periods varying from 90 to 210 milliseconds.

c. During these periods the output of the spectrum analyzer is examined by the

auto sync circuitry to determine if a coherent (i.e., same sweep parameters) chirp

signal is present:

d. A burst of extra clock pulses that again places the system one-second ahead

of the nominal start time; and

e. Retard timing slips as in b above. This sequence is repeated until a coherent

chirp signal is detected.

NOTE

The transmitter that the receiver is searching for must be within the

second window if the auto sync is to work properly.

To detect the presence of a coherent chirp signal, the output of the spectrum

analyzer is sampled by the data level detector circuit (U62, figure FO-11/3). If the

sampled spectrum crosses an adaptive threshold level (indicating the presence of

a chirp signal), then logic "1's" are loaded into a 1024-bit MOS memory (U54).

The position of the "1's" in the memory indicate the location of the coherent tone

in the spectrum analyzer output scan. If this occurs, a second sample is taken to

check that the "1's" are still in the same positions to verify that the first detection

is not noise. If this checks out, a third (and final test) is run to determine the

presence of at least one recurring "1" to verify the coherency of the detected sig-

nal. If all three tests prove positive, the receiver is left in the time frame in which

the test occurred, a fine adjustment timing slip is made, and the auto sync circuit

is disabled.

4-20. If the signal "1's" are not detected and verified during the first test (which

is usually the case), then the receiver is retarded (moved) to the next time period.

chirp sweep (50 bands of 40 milliseconds each for 100 kHz/second sweep rate; 25

bands of 80 milliseconds each at 50 kHz/second). A 2-second burst advance takes

place at the end of 25 (50) of these periods if no synchronization occurs, and the

test sequence starts over. Each test spectrum is looking for a fixed tone within

the receiver baseband. Since the receiver is sweeping through the HF band and

crossing many non-sweeping signals, the only fixed tone that should be seen by the

4-8