CHAPTER 2 FUNCTIONING

TM 11-6625-667-45/NAVAIR 16-30APM123-21TO 33A1-3-367-22

CHAPTER 2

FUNCTIONING

2-1. Block Diagram

ated in the decoder section for proper pulse posi-

tions. Paragraphs 2-2 through 2-5 cover the opera-

a. The test set evaluates the minimum perfor-

tion of these sections for system, identity, and

mance of aircraft installed transponder sets on a go-

emergency tests. Mode 4, self-test, and frequency-

no-go basis. It checks the receiving, decoding, cod-

power operations are described in paragraphs 2-7

ing, and transmitting functions and provides either

and 2-8, respectively. The power supply section is

an accept or reject indication. A coded interrogation

described in paragraph 2-9.

signal is transmitted, and the transponder reply is

analyzed. Incorrect or no replies result in a reject in-

2-2. Encoder Circuit

dication, and an accept indication will result when

(fig. 8-1)

all transponder functions are performing within

limits, With internally generated pulse-pairs, the

The encoder is located in the transmitter section

test set can interrogate in modes 1, 2, 3/A, TEST,

and generates the modes 1, 2, 3/A, TEST, and C

and C. In addition, a sidelobe-suppression test can

pulse-pairs to modulate the transmitter rf output

circuit. The interrogation sidelobe suppression

be performed during interrogations. Mode 4 inter-

rogation pulses are supplied by external mode 4

(ISLS) pulse also can be selected for transmission,

During mode 4, however, the encoder circuit is used

equipment for transmission. Radiation tests are

performed with an adjustable antenna mounted on

only to shape interrogation pulses from external

equipment. Certain circuits are shared with the de-

the front panel. Tests can also be performed by di-

coder section and are described in the following

rect coupling, by using Cable Assembly, Radio Fre-

paragraphs. Figure 2-1 illustrates the timing.

quency CG-409H/U (radiofrequency (rf) cable) and

a. Prf Generator. The prf generator controls the

Fixed Attenuator CN-1088A/U (attenuator), or us-

ing Test Hood, Antenna MX-4396/APM-123(V).

repetition rate of the encoder circuit, and all test set

Replies are analyzed for minimum power level, fre-

operations. It produces 2.17-millisecond pulses at

230 + 510 pulses per second (pps). These pulses

quency, coding, and pulse position limits. A reply

rate evaluator is included to provide an accept indi-

are applied to encoder line-drive generator ISSI and

encode enable IFFI, and to readout DFF5 during

cation when 80 percent or more transponder replies

self-test operations.

are correct. The evaluator gives a reject indication

when 50 percent or less transponder replies are cor-

b. Pulse-Pair Generator.

(1) Encoder enabling. Encode enable IFFI en-

rect. When the percentage of correct replies falls be-

tween 50 and 80 percent, the accept-reject indica-

ables the P1-P3 pulse gating circuits. The trailing

edge of the 2.18 millisecond pulse from the prf gen-

tors may give unsteady (flickering) indications. The

reply evaluation is provided on a continuous basis

erator sets this flip-flop, and a positive enable volt-

and requires approximately 1/8 second (50 pulse repe-

age is then applied to the gates.

tition frequency (prf) periods) to change from an ac-

(2) Pulse-pair generation. The pulse-pair gener-

cept to a reject indication.

ation operation is started simultaneously with the

b. Figure 8-1 illustrates a general block diagram

encoder enabling. Encoder line-drive generator

ISSI is triggered by the trailing edge of 1, the 2.18-

of the major functional sections and their major cir-

millisecond pulse from the prf generator. A nega-

cuits. The power supply section provides direct cur-

tive 0.7-microsecond pulse is produced, and is gated

rent (dc) power to all other sections. The transmit-

ter section produces the coded interrogations for

and inverted by line drive gate 2 (OR). (This OR gate

is also used during decoding operations.) The gated

transmission to the test set. The receiver section

encoder line-drive pulse is coupled by an emitter fol-

evaluates the test set reply frequency and power.

The receiver video pulse train output is then evalu-

lower (EF) to delay line A6DL1, which is also com-

Change 4

2-1