Operating instructions

LOCAL OPERATION
3-35
Modulation characteristics
Modulation frequency response and
Companding/limiting
The requirements of individual users will vary but the principle of the tests will be the same.
A signal having a known level and frequency is applied to the AF input of the transmitter and the
level of modulation produced is measured.
To test the modulation characteristics of a transmitter with any degree of accuracy requires that the
signal used to provide the modulation is accurately defined.
The Service Monitor has two AF generators for this purpose. The setting up instructions for
frequency, level and shape options of these is given under ‘Setting Up, AF Generators’ above.
The output from the AF generators is available at the AF output connector on the front panel. This
signal should be used to modulate the transmitter, by direct connection to the AF input of the
transmitter under test.
With the modulation meter options set to correspond with the modulation characteristics of the
transmitter, the signal can be examined.
Modulation measurement with the Service Monitor is made using a method which evaluates the
positive value and negative value as separate readings. This allows the results of both to be given
on the display. The bar chart is a dual display indicating the positive value on the top bar and the
negative value on the lower bar. The digital read-out is given in three parts. The normal size
legend displays the P-P/2 modulation level, while the individual levels are given in half size
numerals.
By making changes to the level of the applied AF signal, the linearity of the modulating circuits
can be checked, limiting circuits, companders and voice activated switching circuits can be tested
and adjusted.
The dual - positive and negative - readings are particularly useful when checking and adjusting the
symmetry of transmitters.
Similarly, by repeating the tests at differing frequencies, the frequency response of the modulating
circuits and filters can be plotted and adjusted.
By using both AF generators at different frequencies, or by using squarewave output,
intermodulation distortion levels can be explored and transient response examined.
*STANDARD OSCILLOSCOPE*
The digital oscilloscope facility can be used for modulation testing. When this facility is active
within the Tx TEST mode, the Service Monitor demodulator output is applied to the input of the
oscilloscope.
The input to the oscilloscope is taken from the demodulators through any selected AF filters.
Therefore modulation measurements made using the oscilloscope will include the characteristics
of the selected AF filter.
The Y ranges of the oscilloscope are directly calibrated in deviation frequency when FM
demodulation is selected and in percent modulation when AM demodulation is selected. The
ranges are from 200 Hz/ Div. to 10 kHz/Div. in a 1, 2, 5, sequence and 25 kHz/ Div. FM and 5%,
10% and 20%/ Div. AM.
The X ranges are from 50 µs/Div. to 5 s/Div. in a 1, 2, 5, sequence.
When the oscilloscope facility is selected, the modulation level bar chart is removed from the
display. The digital read-out of modulation level, including the positive and negative readings, is
retained. The Modulation frequency digital readout is also retained.
*EXPANDED OSCILLOSCOPE*
When the expanded oscilloscope is selected, all of the above relating to the standard oscilloscope
apply except that the positive and negative modulation readings are not displayed.