Instruction manual

TM 11-6625-3017-14
2nd harmonic: (fl -f2): 2 kHz
3rd harmonic: (2f2 -fl): 12 kHz
and express them in dB’s relative to 0 dB. Add +6 dB to
the 2nd harmonic reading and +9 dB to the 3rd harmonic
reading. The r. m. s. sum of the two levels (see below
for method of calculation) should be lower than -54 dB
(0. 2%. Distortion limits for other values of deviation and
modulation frequency are given in Sect. 1.2 - Data
Summary.
Summation of dB levels
The r. m. s. sum of two dB levels can be obtained
by increasing the level of the larger one by an increment
between 0 and 3 dB; the value of increment depends on
the difference between the two levels as shown by the
graph. For example, if the two levels are -40 dB and -46
dB, giving a difference of 6 dB, the increment is 1 dB; the
sum of the two levels is therefore -40 +1 dB = -39 dB.
4.3.7 A.M. rejection
Test equipment:- a, f and m.
(1) Set up the equipment as shown in Fig. 4-7. Adjust
the signal generator to give a c. w. output of 100 mV at
any convenient frequency. Adjust the I. f. oscillator
frequency to 1 kHz at a level which gives 80%
modulation as measured on the TF 2300A.
(2) On the TF 2300A set the Function switch to TUNE
and the OSCILLATOR TUNE control to give maximum
meter reading. Then adjust the LEVEL control for a
reading within the black arc.
Fig. 4-7. A.M. rejection measurement
(3) Switch to F. M. - SET FREQ and adjust the
OSCILLATOR tune control to bring the meter reading to
the SET mark.
(4) Switch to TUNE and adjust the LEVEL control to
give a reading just below full-scale on the
black arc.
(5) Turn the DEV RANGE switch to 5 kHz and the MAX
MOD FREQ switch to 15 kHz.
(6) Switch to DEV+ and DEV- in turn and check that
the residual reading on the meter is less than about
150 Hz.
4.4 CLEANING AND LUBRICATING
Rotary switch contacts
These should be cleaned once or twice a year,
depending on usage, with benzine or white spirit (not
carbon tetrachloride). After cleaning wipe the contacts
with a suitable lubricant such as a 1% solution of
petroleum jelly in white spirit.
Oscillator unit
Excessive lubrication must be avoided but the
moving parts should be cleaned and lubricated at least
twice a year:
(a) Worm shaft: Use Rocol anti-scuffing paste.
(b) Carriage slide: Use Aeroshell 4 oil.
(c) Scale drum mounting: Apply a few drops of
Aeroshell 4 oil to the felt washer on the spigot
mounting.
(d) Range switch bevel gears: Use Rocol anti-
scuffing paste.
Note: Sealed bearings at end of worm shaft will not
require lubrication within the life of the instrument.
Fig. 4-6. Summation of dB levels
4-8