PEAK READING METER (Unit A10-Fig. 7-4)

TM 11-6625-3017-14

Q2; one via a switch, so that on the 5 to 500kHz

output impedance of Q3. This gives the circuit a wide

deviation ranges the meter circuit is connected to

band response which is flat to within the limits of the i. f.

terminal 7, and the other from terminal 9 to the DE-

amplifier. MR1 feeds into a resistive load and a fraction

EMPHASIS switch. On the 1.5 kHz deviation range the

of the voltage developed across the load is tapped off

meter circuit is connected to the output of the amplifier

and fed to a 50 kHz low-pass filter.

by terminal 11.

When the Function switch, SB, is in positions

The remainder of the amplifier is the same as the

TROUGH or PEAK, the detected signal is fed via

first 1. f. amplifier and its output is taken to the front

terminal 12 to the range switch, the 2nd i. f. amplifier and

panel terminals. Headphones may be used here, or a

the meter circuit.

distortion factor meter, external meter (or oscilloscope).

The output impedance is 600 Q1, and is isolated by C11.

When making an a. m. measurement, the internal

Do not allow d. c. to reach the OUTPUT terminals so as

meter is switched directly to terminal 7 by the Function

to avoid damaging C11, which is a low voltage

switch, SB, and the d. c. component adjusted by RV1,

electrolytic capacitor.

SET AM control, until the meter reads SET.

The d. c. component is also used for setting the

3.12 PEAK READING METER (Unit A10-Fig. 7-4)

input level to the i. f. amplifier, thus ensuring correct

loading and good signal-to-noise ratio. On position

For reading modulation depth and deviation, and for

TUNE OSCILLATOR AND ADJUST LEVEL of the

setting the f. m. calibration, the peak reading meter

Function switch, a fixed resistor, R1, is switched in place

circuit is brought into use.

of RV1 and the correct level set up by adjusting the input

attenuator until the meter indicates within the black arc.

Q1 and Q2 amplify the signal from the 2nd 1. f.

amplifier and RV1 presets the gain of the amplifier. The

3.14 BETWEEN-UNITS CIRCUITRY (Unit A0-Fig. 7-1)

remainder of the circuit consists of two similar pairs of

semiconductors arranged in push-pull, driving the meter

The A. M./DEV RANGE switch, SC, controls three

via rectifier MR1. The polarity of the meter can be

Tr pads of 10, 20 and 20 dB respectively. These

switched to read trough or peak on a. m., the positive or

attenuating pads are inserted in steps as follows:

negative deviation on f. m. Feedback is used to stabilize

the gain, reduce distortion and broaden the fre-quency

Range

Attenuation

response. The feedback loops include R3, R12, C14,

R16, R19 and R24.

(1)

1.5kHz f. m.

(2)

5 kHz f. m.

The meter is also used to measure the d. c.

(3)

15 kHz f. m., 30% a. m.

10 dB

component of the output from the a. m. detector for i. f.

(4)

50 kHz f. m. , 100% a. m.

20 dB

level monitoring and tuning, and in the position SET

(5)

150 kHz f. m.

10 dB +20 dB

FREQ it reads the voltage developed by a counter circuit

(6)

500 kHz f. m.

20 dB + 20 dB

in the limiter thus monitoring the frequency from the

mixer for fine tuning purposes.

If should be noted that the meter has two deviation

scales; ranges 2, 4, 6 are associated with the bottom

scale and the other ranges with the upper.

3.13 A.M. detector (Unit All-Fig. 7-5)

Also on the chassis is a single attenuator pad for

The diode detector, MR1, is preceded by Q2 and

the calibrator. This is switched in place of the range

Q3 which are arranged in a composite emitter follower

attenuator on the SET CAL position of the Function

configuration to give a large peak-to-peak signal without

switch. This pad incorporates a variable resistor, RV3,

distortion, and thus allowing MR1 to operate over the

which is mounted on the top of the chassis immediately

linear part of its forward characteristic. The input to the

to the rear of the calibrator board. RV3 standardizes the

detector is via a common-emitter amplifier, Q1, stabilized

calibration circuit to the meter.

by negative feedback through R6 and R7.

MR1 is fed directly from a tuned circuit, C4 and L1,

which is heavily damped by the low

3-6