Instruction manual

TM 11-6625-3017-14
then causes the meter reading to fluctuate, carrier shift is
present. The extent of the shift may be measured by
connecting a counter to I. F. OUT. In cases of severe
carrier shift, the modulation meter must be set up with
modulation on.
2.13 USE OF L.F. OUTPUT TERMINALS
CAUTION Application of d. c. to these terminals will
cause damage - see section 3.11.
These terminals enable the modulation wave-
form on the input signal to be monitored or analysed.
They also enable a sensitive external meter to be used to
extend the deviation range down to the level of residual
noise within the instrument.
The terminals are fed by an independent output
stage in the 2nd 1. f. amplifier and therefore the internal
meter is unaffected by loads connected to them.
Depending on the position of the Function
switch, a.m. or f.m. demodulated outputs are obtained.
The output impedance is approximately 600 0 and the
open circuit level approximately 1.5 V for full-scale
deflection on the internal meter. On the 1.5 kHz
deviation range the output level will only be 0.1 mW. On
all other deviation ranges the output level will be 1 mW.
Note : For most purposes, the output can be
terminated with 600
Ω
or unterminated. In the latter
case, a voltage due to leakage of C11 may appear at the
output, causing difficulty if a d. c. coupled oscilloscope
is used at high sensitivity.
The 1. f. response is substantially level up to
200 kHz. The output is also available with switched 50
millisec or 75 pisec de-emphasis to restore the
modulation characteristic of signals that have had pre-
emphasis applied.
2.14 USE OF I.F. OUT SOCKET
This socket can be used to measure carrier shift
by connecting a counter to it - see section 2.6 - or to view
the a.m. envelope on an oscilloscope.
It may also be used to measure amplitude
modulation above 50 kHz which normally would not be
passed by the a.m. detector. This is an extended use of
the modulation meter which in practice would probably
be limited by the attenuation of the 10 IQ output
resistance and the input capacitance of the measuring
instrument.
2.15 CRYSTAL SELECTION
Crystal control of the local oscillator reduces
microphony and pick-up from surrounding equipment
where these effects cannot be avoided. At higher
frequencies, distortion due to noise in the oscillator can
be significantly reduced. Up to three crystals can be
plugged into sockets on the oscillator front panel and any
one can be selected by the red coloured section of the
oscillator RANGE switch.
A recommended crystal is a 3rd overtone series
resonant Marconi type Q01670F150/A/S with a specified
frequency between 22 and 44 MHz. Thus, the crystal is
used on its fundamental on Range 3 and appropriate
harmonics on the other and higher ranges. These
crystals are in hermetically sealed, style D miniature
cases conforming to British Standard and U. S. Style
HC6U.
The following simplifies the procedure for est-
ablishing the required crystal frequency for any specified
r. f. input between 20.5 and 1600 MHz.
1) Add 1.5 MHz to the r. f. input frequency
2) Divide this sum by the local oscillator harmonic factor
found from Table 2.4 (shown opposite the range
applicable to the r. f. input). This result is the crystal
frequency.
3) Expressing 1) and 2) as a formula:
(R. F. + 1. 5) MHz
Harmonic factor
TABLE 2.4
Crystal Selection
Range R.f input frequency Local oscillator
no. MHz harmonic factor
1 4 - 9.5 No crystal
control on this
range
2 9.5 - 20.5 No crystal
control on this
range
3 20.5- 42.5 1
4 42.5 - 86.5 2
5 86.5 - 174.5 4
6 174.5 350.5 8
7 350. 5 702.5 16
1001.5 1406.5 32
8 702.5 1001.5 24
1406.5 1600 48
Crystal frequency =
2-10