Cable Traffic Detector, Asynchronous Mode (684091)

TM 11-5820-695-35

condition that exists when the pcm (cable traffic) signal

parts used to attain dc temperature tracking are: R33,

is present. The voltage at E13 then alternately exceeds

C9, Q13A, R34, R35, R36 and C10.

and falls below the voltage at E14 and the output (pin 7)

of A6 switches from high to low at the pcm rate. This

(2) The output of the dc temperature

signal coupled through isolating resistor R32 alternately

tracking circuit is added to the crystal output. This

turns Q12 on and off. When Q12 is off, C8 charges to a

composite signal is fed to the noninverting input pin 2 of

plus voltage and when Q12 is on, it discharges through

the comparator A6 while Q13B bias is applied to the

Q12 Thus, when the pcm (cable traffic) signal is present,

inverting input pin 3. The output of the comparator will

Q12 switches on and off and prevents C8 from holding a

be low for small amounts of cable traffic, due to the 10%

charge (logic one). The source of the charging voltage

offset supplied by R34 and R36 (in the dc tracking

for C8 is in A3E. When the pcm signal is absent, the

network). When the traffic is normal the comparator

signal inputs to A6 at E13 and E14 are determined by

output will be switching and turn on the transistor

the noise and transients, but no switching takes place.

integrator (R32, Q12 and C8). The integrator will allow

C8 can then charge up until its output is logic one (high).

small gaps in the pcm cable signal to go undetected by

the alarm gating circuits.

d.1.

Cable Traffic Detector, Asynchronous

(3) Cable traffic amplifier A5 is a high

Mode (684091). The cable traffic detector comprises a

gain video amplifier. The supply voltage at pin 9 is

differential amplifier, integrated circuit Al, and a voltage

derived by dividing the +12 volts supply through resistor

comparator, integrated circuit A2. The +12 vdc supply

R29 and zener diode CR9. This voltage is filtered by

voltages for Al and A2 are filtered by L4 and C8 (Al) and

capacitor C6. It is also the collector voltage for Q13.

L6 and C18 (A2) and the -6vdc supply is filtered by L7

Capacitor C17 determines the low frequency cutoff and

and C19 (A2). Bias voltage for the input circuits of Al is

C18 determines the high frequency cutoff. Capacitor C5

provided by voltage divider R29 and R30; the voltage is

and resistor R30 from a high pass filter. Dc blocking

applied to input pins 1 and 3 through isolating resistors

capacitor C7 isolates the high pass filter so that dc

R31 and R32. Tune-a circuit L8 and C7 determines the

voltage at pin 1 of A5 is not changed. The cable traffic

bandwidth oil the input circuit of Al. Bias voltage for the

signal from cable digital regenerator 1A12A2 is

input circuits of A2 is provided by voltage divider/filter

amplified by A5 and the output signal is connected to

networks R38, R41 and C10 (pin 3) and R39, R42 and

crystal filter Y1 and to Q13A and Q13B. Crystal filter Y1

C13 (pin 2).

offers a low impedance to a very narrow frequency band

centered at 2304 kHz and a high impedance outside the

(1) The input signal to Al is capacitatively

passband. Thus, the signal at the output side of Y1 is

coupled through C5 and C6 which also isolate the dc

the pcm signal minus most of the noise and transients.

bias of the input circuit from the external circuit. When

Q13A and Q13B are emitter followers which pass the

the signal voltage (XCPCM) at pin 1 goes more positive

pcm signal to the inputs of A6 but in the process they

than the bias voltage, the output at pin 7 goes positive

generate an average dc bias from the signal. Q13A has

and remains positive until the input signal falls below the

a high frequency cut off at the input comprised of R33

bias level. At this time the signal at pin 2 is negative

and C9. Q13B has a similar input circuit, R37 and C13,

with respect to the bias and there is no output at pin 6.

but is has a higher cut-off frequency and thus, Q13B

When the input signal is negative-going at pin 1, the

passes more high frequencies that Q13A. The emitter

signal at pin 2 is positive-going and a positive pulse

load resistance of Q13A is R34 and R36. Since the

output is produced at pin 6. In the output circuit of Al,

output is coupled to A6 through isolating resistors R35

the cathodes of diodes CR9 and CR10 are connected to

and R31, only 90% of the output signal of Q13A is used.

a fixed dc voltage provided by voltage divider R34-R35.

The emitter load resistance of Q13B is R39. Capacitor

Because of the positive bias voltage at the cathodes,

C10 and C14 become charged to the average dc level

diodes CR9 and CR10 rectify only the upper half of the

of the input signal and determine the dc level about

positive pulse outputs from pins 6 and 7 of A1. The

which the pcm signal varies. The dc level of the signal

resulting current pulses in resistor R35 produce positive

at E13 is lower than the dc signal at E14 because the

voltage pulses at the signal frequency (4915.2 kHz).,

output taken from Q13A is only 90% of the available

This signal is the input to A2.

signal. The pcm signal at E13 is greater than at E14

because of the signal coupled through Y1. This is the

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