BRUEL & KJAER 2006 - Modulation Measurements

twice the IF, i.e. 22.6 MHz. This condition is valid for signals with frequencies
between 22.6 MHz and 230 MHz. The frequency scale of the instrument is
calibrated such as to be correct at the higher mode, i.e. the higher of the
two readings on lhe frequency scale is the correct one. For the measurement
or analysis of unknown frequencies the operation procedure should thus be
continued as follows:
6. Note the reading of the meter, when tuned in.
7. Adjust the frequency scale pointer to a frequency which is 22.6 MHz
above (or below) the frequency first found. Tune for maximum deflection
and note this value.
This is to ensure that it is the right signal which is found. The two readings
must be of equal value and the distance between them 22.6 MHz. As mention-
ed before it is now the higher mode which is the correct one (see Fig. 3.1).
Note:
The loudspeaker may often be of great help for the proper identifica-
tion of the signals obtained under items 6 and 7.
Modulation Measurements.
When it is desired to measure the degree of modulation of a signal the
carrier signal should be tuned in as described above.
Then proceed:
1. Set the METER AND LS SWITCH to position "Adj.".
2. Adjust the LS AND MOD. ADJ. knob for full scale deflection on the
meter. (If full scale deflection cannot be obtained by means of the LS
AND
MOD. ADJ. knob, turn the VOLTAGE RANGE switch to a more
sensitive range).
3. Set the METER AND LS SWITCH to AM or FM depending upon the type
of modulation to be measured.
The reading on the meter is now, in the case of AM signals, directly the
percentage of amplitude modulation, while in the case of FM-signals it is
the frequency swing in kHz.
When measuring AM modulation at modulation frequencies higher than 1
kHz the BANDWIDTH switch should be set to "Wide". When measuring FM
modulation the BANDWIDTH switch should always be set to "Wide".
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