(2) Output circuit terminal impedance.

(a) Connect Telephone Test Set AN/USM-181 to the circuit under

test as shown in B of figure 1-12.

(b) Tune the oscillator to 1,000 Hz and adjust the

AMPLITUDE

control for the calculated value of 0 dBm0. Close switch S1.

(c) Measure the circuit output voltage at Station B across the

600-ohm resistor (V1) with S2 closed.

(d) Open switch S2 and measure the output voltage (V2).

(e) Calculate the output terminal impedance (Zo) of the circuit.

(f) This test differs from the previous test in that its purpose

is to determine the output impedance of the circuit by using the voltmeter.

In the event the circuit presents an output impedance seriously at variance

with the 600-ohm input impedance of the terminating equipment, the circuit

conditioner will have to insert a matching device such as a transformer or

pad.

Only in this way can he assure himself of impedance compatibility

between the transmission facility and the terminating equipment. The output

impedance tolerance is 10 percent.

(3) Accuracy check.

To insure that the oscillator has an output

impedance (Ro) of 600 ohms, its open circuit output level (V2) should be

measured, then a 600-ohm resistor should be placed across the oscillator

output and the output level (V1) should be recorded. If the oscillator has

the required 60b-ohm output impedance, V2 should be 1.550 volts and V1

should be 0.775 volt, a ratio of 2/1.

If the ratio is not equal to 2,

calculate the oscillator output impedance from:

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