CRYSTAL SELECTION

TM 11-6625-3017-14

then causes the meter reading to fluctuate, carrier shift is

2.15

CRYSTAL SELECTION

present. The extent of the shift may be measured by

Crystal control of the local oscillator reduces

connecting a counter to I. F. OUT. In cases of severe

microphony and pick-up from surrounding equipment

carrier shift, the modulation meter must be set up with

where these effects cannot be avoided. At higher

modulation on.

frequencies, distortion due to noise in the oscillator can

be significantly reduced. Up to three crystals can be

2.13

USE OF L.F. OUTPUT TERMINALS

plugged into sockets on the oscillator front panel and any

one can be selected by the red coloured section of the

oscillator RANGE switch.

CAUTION Application of d. c. to these terminals will

A recommended crystal is a 3rd overtone series

cause damage - see section 3.11.

resonant Marconi type Q01670F150/A/S with a specified

frequency between 22 and 44 MHz. Thus, the crystal is

These terminals enable the modulation wave-

used on its fundamental on Range 3 and appropriate

form on the input signal to be monitored or analysed.

harmonics on the other and higher ranges. These

They also enable a sensitive external meter to be used to

crystals are in hermetically sealed, style D miniature

extend the deviation range down to the level of residual

cases conforming to British Standard and U. S. Style

noise within the instrument.

HC6U.

The following simplifies the procedure for est-

The terminals are fed by an independent output

ablishing the required crystal frequency for any specified

stage in the 2nd 1. f. amplifier and therefore the internal

r. f. input between 20.5 and 1600 MHz.

meter is unaffected by loads connected to them.

1) Add 1.5 MHz to the r. f. input frequency

2) Divide this sum by the local oscillator harmonic factor

Depending on the position of the Function

found from Table 2.4 (shown opposite the range

switch, a.m. or f.m. demodulated outputs are obtained.

applicable to the r. f. input). This result is the crystal

The output impedance is approximately 600 0 and the

frequency.

open circuit level approximately 1.5 V for full-scale

3) Expressing 1) and 2) as a formula:

deflection on the internal meter. On the 1.5 kHz

deviation range the output level will only be 0.1 mW. On

(R. F. + 1. 5) MHz

all other deviation ranges the output level will be 1 mW.

Crystal frequency =

Harmonic factor

Note : For most purposes, the output can be

terminated with 600 Ω or unterminated. In the latter

TABLE 2.4

case, a voltage due to leakage of C11 may appear at the

output, causing difficulty if a d. c. coupled oscilloscope

Crystal Selection

is used at high sensitivity.

Range

R.f input frequency

Local oscillator

no.

MHz

harmonic factor

The 1. f. response is substantially level up to

200 kHz. The output is also available with switched 50

- 9.5

No crystal

millisec or 75 pisec de-emphasis to restore the

control on this

modulation characteristic of signals that have had pre-

range

emphasis applied.

9.5 - 20.5

No crystal

2.14

USE OF I.F. OUT SOCKET

control on this

range

This socket can be used to measure carrier shift

by connecting a counter to it - see section 2.6 - or to view

20.5-

42.5

the a.m. envelope on an oscilloscope.

42.5 -

86.5

86.5 -

174.5

It may also be used to measure amplitude

174.5

350.5

modulation above 50 kHz which normally would not be

350. 5

702.5

passed by the a.m. detector. This is an extended use of

1001.5

1406.5

the modulation meter which in practice would probably

702.5

1001.5

be limited by the attenuation of the 10 IQ output

1406.5

1600

resistance and the input capacitance of the measuring

instrument.

2-10