Df
p-p
–
Press SETTINGS ® STAT and set No.of
samples to 1000 or more.
–
Press Meas Time and select a low value.
Press STAT/PLOT and watch Df
p-p
.
DDfff f
pp-
=-=´
max min
2
.
Errors in f
max
,f
min
, and Df
p-p
A measurement time corresponding to
1
10
cy
-
cle, or 36° of the modulation signal, leads to
an error of approx 1.5%.
Select the measurement time:
t
f
measure
modulation
£
´
1
10
To be confident that the captured maximal fre
-
quency really is f
max
, you must select a suffi
-
ciently large number of samples, for instance
n ³ 1000.
AM Signals
The counter can usually measure both the car
-
rier wave frequency and modulation fre
-
quency of AM signals. These measurements
are much like the burst measurements de
-
scribed earlier in this manual.
Carrier Wave Frequency
The carrier wave (CW) is only continuously
present in a narrow amplitude band in the
middle of the signal if the modulation depth is
high. If the sensitivity of the counter is too
low, cycles will be lost, and the measurement
ruined.
To measure the CW frequency:
–
Enter the INPUT A menu.
–
Select a measurement time that gives you
the resolution you want.
–
Turn on Manual trigger.
–
Press Trig level and enter 0 V trigger
level (press the numeric key 0 and
EXIT/OK).
–
Select AC coupling.
–
Select 1x attenuation to get a narrow hys
-
teresis band.
–
If the counter triggers on noise, widen the
hysteresis band with the ‘variable hyster
-
esis’ function, i.e. enter a trigger level
>0 V but <V
P-Pmin
. See Fig. 4-8.
Measuring Functions
4-8 AM Signals
A / 1 0
A
f
< 0 . 0 0 7 B
B
t
M o d u l a t i o n
s i g n a l
Fig. 4-7 Error when determining f
max
Fig. 4-8 Effects of different sensitivity
when measuring the CW Fre-
quency of an AM signal.
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