F Max; F Min - Fluke PM6690 Operator's Manual

may remain uncompensated for, and lead to a
measuring result that is too high or too low.
W o r s t C a s e M e a s u r i n g T i m e
D u r a t i o n , w h e r e
f = f
m e a n
f
{
m a x
D f
m a x
f
0
f
m i n
Fig. 4-6 Frequency modulation
In the worst case, exactly half a modulation
cycle would be uncompensated for, giving a
maximum uncertainty of:
- = ±
f f
0 mean
t
measuring
For very accurate measurements of the carrier
wave frequency f , measure on the
0
unmodulated signal if it is accessible.
n
Modulation Frequencies
above 1 kHz
–
Turn off SINGLE.
–
Set a long measurement time that is an
even multiple of the inverse of the modu-
lation frequency.
You will obtain a good approximation when
you select a long measurement time, for in-
stance 10 s, and when the modulation fre-
quency is high, above 1000 Hz.
n
Low Modulation Frequencies
Press SETTINGS ® STAT and make the
No. of samples parameter as large as possi-
ble considering the maximum allowed mea-
surement time. Press STAT/PLOT and let the
0
D
f
max
´ ´ p
f
mod ulation
counter calculate the mean value of the sam-
ples.
You will usually get good results with 0.1 s
measurement time per sample and more than
30 samples (n ³ 30). You can try out the opti-
mal combination of sample size and measure-
ment time for specific cases. It depends on the
and Df
actual f .
0 max
Here the sampling frequency of the measure-
ment (1/measurement time) is asynchronous
with the modulation frequency. This leads to
individual measurement results which are ran-
domly higher and lower than f
cally averaged value of the frequency f
approaches f when the number of averaged
0
samples is sufficiently large.
When the counter measures instantaneous fre-
quency values (when you select a very short
measurement time), the RMS measurement
uncertainty of the measured value of f
1
- = ± ´ D
f f
0 mean
2 n
where n is the number of averaged samples of
f.
f
max
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 f
f
min
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 f
Frequency Modulated Signals 4-7
Measuring Functions
. The statisti-
0
mean
is:
0
f
max
.
max
.
min