Applications
Frequency Modulation
For a modulation index of more than 1%:
1.
Set SEC/DIV and VAR so the average signal period covers five divisions.
2.
If deviation is less than 1 division in one cycle (20%), turn on X10 MAG.
3. Align .At cursors with the extremes of the deviation.
4.
The readout shows the peak-to-peak deviation in percent. If X10 MAG is
on,
divide the reading by 10.
For modulation indexes from
0.1%
to 2%:
1.
Measure the carrier period.
2.
Display the A Sweep with SEC/DIV between 1 and 2.5 times the period.
3. Turn off .At, pull SEC/DIV, and turn A REF for a DLY reading 10 times the
carrier period. If a B-Trigger mode indicator is on, select RUN AFT DLY.
4.
Turn and push SEC/DIV to display the B Sweep and set SEC/DIV at 0.2
times the period, using the switch and VAR. (The display will show one
cycle in five horizontal divisions.) If the DLY setting is less than 200 ns, turn
A REF to zero DLY, then turn A REF to move ten cycles past a point in
the display.
5. Align At cursors with the deviation extremes.
6. Divide the reading by 10 to determine the peak-to-peak deviation
in percent.
For modulation indexes from 0.01% to 0.2%, proceed as above for
0.1%
to
2%
except:
« at Step 2 set SEC/DIV between 10 and 25 times the period;
• at Step 3 set A REF for 100 times the carrier period; and
• at Step 6 divide the reading by 100.
The instrument adds about 0.01% spurious modulation to the display. This error
can be minimized by measuring the apparent frequency modulation of an
unmodulated carrier, at the same frequency and with the same control settings,
and subtracting that value from very low modulation-index readings.
3-4
2465B/2455B/2445B Operators
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