Crystal-controlled oscillator

TM 11-6625-3017-14

The oscillator takes about two minutes to

2.10.4 Use above 1000 MHz

stabilize after turning on, but this is only of any

significance at very high frequencies.

It is possible to use the instrument above 1000

MHz using internal or external oscillator.

2.10.2 Crystal-controlled oscillator

With either the variable or crystal controlled

oscillator, higher harmonics are sufficiently present to

Plug in a crystal of the required frequency as

cover the range 1000 to 1600 MHz without loss of

selected in section 2.15.

sensitivity. For 1000 to 1400 MHz, use range 7 with a

scale indication of half the wanted frequency (500 - 700

Select the appropriate position on the RANGE

MHz). Similarly, for 1400 to 1600 MHz use part of range

switch and set the oscillator dial to the crystal frequency.

8 (700 - 800 MHz).

Set the CRYSTAL selector to the crystal position

Using a suitable external oscillator the carrier

number.

frequency range can be extended to about 2500 MHz,

but the sensitivity may deteriorate.

Slight tuning on position TUNE OSCILLATOR of

the. Function switch may be necessary to obtain

2.11

MEASUREMENT IN i TO 2 MHz RANGE

maximum sensitivity at the higher frequencies. DQ not

tune far off the original setting to avoid any possibility of

The instrument can accept signals in the band 1

mode jumping.

to 2 MHz, applied directly to R. F. IN, with the oscillator

switched off. Input sensitivity is the same as usual.

Proof that the oscillator is under crystal control

Ensure the modulation frequency does not take the

can be obtained by setting the Function switch to SET

frequency beyond the band limits quoted, so as to avoid

FREQ and rocking the tuning control. If the oscillator is

introducing distortion caused by the reduced gain outside

under control the meter reading will not vary.

the pass band.

In general, the tuning control is set to the

Operating the instrument in this way cuts out the

required oscillator frequency, but provided the crystal

frequency inversion caused by the heterodyne system

frequencies do not differ too widely, any of the crystals

normally used. The polarity of the DEV + and DEV -

can be selected without re-setting the tuning control

positions of the Function switch is therefore reversed.

between selections.

2.12

ASYMMETRIC MODULATION AND CARRIER

2.10.3 External oscillator

SHIFT

If a stable frequency source is available, it may

The presence of asymmetric modulation usually

be preferable to use it rather than the internal oscillator.

indicates distortion in the equipment under test, and is

Feed the external oscillator signal into the EXT OSC

revealed by unequal meter readings when the Function

socket and turn the RANGE-switch to EXT. An input

switch is turned between PEAK and TROUGH or DEV+

level of 100 mV into 50 n is required at a frequency 1.5

and DEV-.

MHz above the input signal frequency.

To make sure that the asymmetry is due to the

The primary use of this facility is for measurement on

input signal, and not introduced by the instrument, retune

r. f. signals which contain harmonics of less than 20 dB

the oscillator to the lower peak, 1.5 MHz below the

down on the fundamental.

Because the internal

carrier frequency: If the asymmetry is reversed, i.e., the

oscillator signal also contains harmonics, mixing two

original DEV + reading now appears at DEV -, and vice

such signals can give rise to spurious results. A 'pyre'

versa, the asymmetric modulation can be attributed to

local oscillator signal prevents this happening.

the input signal.

Secondly, if an external crystal oscillator is available,

it may be preferred to use this, rather than to crystal

In position SET FREQ, the output from a counter

control the local oscillator, for low level noise

circuit in the limiter is measured by the meter. When the

measurements.

meter indicates SET, the i. f. is centered on 1.5 MHz

exactly. If modulation

2-9