Instruction manual

TM 11-6625-3017-14
I.F. output and frequency
(6) Connect the frequency meter and voltmeter to the
I.F. OUT socket and check that the i.f. is
approx.200 mV (with the meter reading at the
bottom ofthe black arc) and remains within 10% of
1.5 MHz when the modulation depth of the input
signal is varied from 0 to 80%o. (Keep the leads as
short at possible.) If the frequency is in error see
Sect. 5.4.10.
NOTE : Follow steps (4), (5), and (6) of section 2.5
to set the Function switch to F.M. SET
FREQ. and tune the oscillator frequency to
position the meter pointer to the SET line.
I.F. output impedance
(7) Adjust the LEVEL control to increase the voltage at
the I.F. OUT socket to 700 mV. Connect a 10 k
Ω
±
2% resistor across the voltmeter input and check
that reading drops to between 310 and 400 mV.
4.3.3 F.M. deviation accuracy
Test equipment:- b, d, e and g.
The internal f. m. calibrator provides a standard of
good long-term stability for checking and resetting the
deviation reading accuracy as described in Section 2.5
(8). The calibrator accuracy is largely dependent on the
frequency of its 200 kHz oscillator crystal, which is
unlikely to change significantly. If the calibrator accuracy
is suspected, the deviation reading should be checked by
making a normal measurement on a symmetrically
modulated signal of accurately known deviation set up by
the Bessel Zero method described below.
(1) Connect the signal generator, externally modulated
by the l. f. oscillator, to the inputs of the modulation
meter and the spectrum analyser, as shown in Fig. 4-3.
Fig. 4-3. Bessel zero measurement
(2) Adjust the oscillator frequency, checked against the
frequency meter, to 26.27 kHz. Slowly increase the
signal generator deviation from zero until the carrier
component, viewed on the spectrum analyser,
disappears for the second time. This occurs at a
deviation ratio of 5.52, which means that the
deviation is 145 kHz. (The MAX MOD
FREQUENCY should be 200 kHz.)
(3) Set up the modulation meter to measure this signal
as described in Sect. 2.5, using the 150 kHz DEV
RANGE.
(4) If the modulation meter reading differs from 145
kHz when switched to either DEV+ or DEV-, adjust
the SET CAL F. M. preset to give the best
compromise accuracy between the two
measurements. Then turn the
TABLE 4.2
F.M. Deviation Accuracy
DEV RANGE MAX MOD Deviation Modulating Carrier
kHz FREQ range kHz frequency disappearance
kHz kHz
500 200 480 55.46 3
200 360.7 150 1
200 145 26.27 2
150 200 145 26.27 2
15 48 8.696 2
50 15 48 8.696 2
15 14.5 2.627 2
15 14.5 2.627 2
15 4.8 0.8696 2
5 15 4.8 0.8696 2
1.5 15 1.4 0.5822 1
4-5