NOISE MEASUREMENTS

TM 11-6625-3017-14

TABLE 2.2

2.9 NOISE MEASUREMENTS

By connecting an external meter to the OUTPUT

Spurious deviations with increasing % a.m.

terminals, noise measurements limited only by the noise

level generated within the instrument can be made. The

1. LEVEL control set for meter reading at top end of arc.

internal meter, being peak reading, is not suitable for

measuring noise and, ideally, an r.m.s. responding meter

2. A.M. frequency = 1 kHz

should be used. However, sufficiently accurate results

are usually obtained with an average reading meter by

Spurious deviation

applying the appropriate correction factor.

A.M.

15 kHz bandwidth

200 kHz bandwidth

Typical meters which can be used are as follows:

75 Hz

750 Hz

(a)

R. M. S. valve voltmeter capable of measuring

to the necessary accuracy (1% deviation on any

150 Hz

2 kHz

range is approximately 7.75 mV). Errors due to

crest factor and zero shift can be avoided by

operating the meter at mid-scale by means of an

attenuator.

TABLE 2.3

Spurious deviations with increasing a.m. frequency

(b)

Marconi Instruments Sensitive Valve Volt- meter,

type TF 2600.

1. LEVEL control set for meter reading at top

end of arc.

2. A.M. depth = 80%.

(c)

Marconi Instruments Distortion Factor Meter,

type TF 2331.

Spurious deviation

(b)

and (c) are average reading and a correction of

A.M. freq.

15 kHz bandwidth 200 kHz bandwidth

+1 dB should be applied.

1 kHz

150 Hz

2 kHz

(1)

The modulation meter has two selectable f.m.

bandwidths and the appropriate filter must be

10 kHz

2 kHz

3 kHz

selected when making the test.

100 kHz

7 kHz

A capacitor should be connected across the

OUTPUT terminals - 0.

014 ~F for the 15 kHz

2.8 MEASURING A.M. ON F.M.

bandwidth, or 0. 0013 ,F for 200 kHz. These corrective

capacitors are necessary because of the design of the

The modulation meter may be used to indicate amplitude

filters (see section 3. 9).

modulation in a frequency modulated signal, provided

that the deviation is less than 100 kHz The procedure is

For optimum results in the 15 kHz bandwidth position

that of a.m. measurement, section 2.6.

a low-pass filter as shown below should be used

between the OUTPUT terminals and the external meter

In general, the spurious a.m. Indication will be

instead of the capacitor.

proportional to the deviation. The i. f. amplifier is set up

for optimum phase response for f.m. deviation

measurement rather than a maximally flat response for

measuring a.m. on f.m.

Below 100 kHz deviation, the internally generated

spurious a.m. is less than 2% approximately.

2-6