Applications
Frequency Modulation
For modulation index 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
~t
cursors with the extremes of the deviation.
4.
The readout shows the peak-to-peak deviation
in
percent. If
X1
0 MAG is
on, divide the reading by 10.
For modulation index from
0.1
% to 2%:
1 . Measure the carrier period.
2.
Display A-Sweep with SEC/DIV between 1
and
2.5 times the period.
3.
Turn off
~t,
pull SEC/DIV, and turn
~
REF
for a DLY reading 10 times the
carrier period. If a B-Trigger mode indicator
is
on,
select
RUN
AFT
DL
Y.
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 DL Y setting
is
less than 200
ns,
turn
~
REF
to zero
DL
Y,
then turn
~
REF
to move ten cycles past a point
in
the
display.
5.
Align
~t
cursors with the deviation extremes.
6.
Divide the reading by 10 to determine the peak-to-peak deviation
in
percent.
For modulation index 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
~
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.
2465A/2455A/2445A Operators 3-3
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