Instruction manual

TM 11-6625-3017-14
The oscillator takes about two minutes to
stabilize after turning on, but this is only of any
significance at very high frequencies.
2.10.2 Crystal-controlled oscillator
Plug in a crystal of the required frequency as
selected in section 2.15.
Select the appropriate position on the RANGE
switch and set the oscillator dial to the crystal frequency.
Set the CRYSTAL selector to the crystal position
number.
Slight tuning on position TUNE OSCILLATOR of
the. Function switch may be necessary to obtain
maximum sensitivity at the higher frequencies. DQ not
tune far off the original setting to avoid any possibility of
mode jumping.
Proof that the oscillator is under crystal control
can be obtained by setting the Function switch to SET
FREQ and rocking the tuning control. If the oscillator is
under control the meter reading will not vary.
In general, the tuning control is set to the
required oscillator frequency, but provided the crystal
frequencies do not differ too widely, any of the crystals
can be selected without re-setting the tuning control
between selections.
2.10.3 External oscillator
If a stable frequency source is available, it may
be preferable to use it rather than the internal oscillator.
Feed the external oscillator signal into the EXT OSC
socket and turn the RANGE-switch to EXT. An input
level of 100 mV into 50 n is required at a frequency 1.5
MHz above the input signal frequency.
The primary use of this facility is for measurement on
r. f. signals which contain harmonics of less than 20 dB
down on the fundamental. Because the internal
oscillator signal also contains harmonics, mixing two
such signals can give rise to spurious results. A ’pyre’
local oscillator signal prevents this happening.
Secondly, if an external crystal oscillator is available,
it may be preferred to use this, rather than to crystal
control the local oscillator, for low level noise
measurements.
2.10.4 Use above 1000 MHz
It is possible to use the instrument above 1000
MHz using internal or external oscillator.
With either the variable or crystal controlled
oscillator, higher harmonics are sufficiently present to
cover the range 1000 to 1600 MHz without loss of
sensitivity. For 1000 to 1400 MHz, use range 7 with a
scale indication of half the wanted frequency (500 - 700
MHz). Similarly, for 1400 to 1600 MHz use part of range
8 (700 - 800 MHz).
Using a suitable external oscillator the carrier
frequency range can be extended to about 2500 MHz,
but the sensitivity may deteriorate.
2.11 MEASUREMENT IN i TO 2 MHz RANGE
The instrument can accept signals in the band 1
to 2 MHz, applied directly to R. F. IN, with the oscillator
switched off. Input sensitivity is the same as usual.
Ensure the modulation frequency does not take the
frequency beyond the band limits quoted, so as to avoid
introducing distortion caused by the reduced gain outside
the pass band.
Operating the instrument in this way cuts out the
frequency inversion caused by the heterodyne system
normally used. The polarity of the DEV + and DEV -
positions of the Function switch is therefore reversed.
2.12 ASYMMETRIC MODULATION AND CARRIER
SHIFT
The presence of asymmetric modulation usually
indicates distortion in the equipment under test, and is
revealed by unequal meter readings when the Function
switch is turned between PEAK and TROUGH or DEV+
and DEV-.
To make sure that the asymmetry is due to the
input signal, and not introduced by the instrument, retune
the oscillator to the lower peak, 1.5 MHz below the
carrier frequency: If the asymmetry is reversed, i.e., the
original DEV + reading now appears at DEV -, and vice
versa, the asymmetric modulation can be attributed to
the input signal.
In position SET FREQ, the output from a counter
circuit in the limiter is measured by the meter. When the
meter indicates SET, the i. f. is centered on 1.5 MHz
exactly. If modulation
2-9