Fluke PM6690 - F Max; F Min

may remain uncompensated for, and lead to a
measuring result that is too high or too low.
In the worst case, exactly half a modulation
cycle would be uncompensated for, giving a
maximum uncertainty of:
ff
f
tf
mean
max
measuring ulation
0
-=±
´´
D
mod
p
For very accurate measurements of the carrier
wave frequency f
0
, measure on the
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
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
actual f
0
and Df
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
0
. The statisti
-
cally averaged value of the frequency f
mean
approaches f
0
when the number of averaged
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
0
is:
ff
n
f
mean0
1
2
-=±´D
max
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
max
.
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
min
.
Frequency Modulated Signals 4-7
Measuring Functions
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
m e a n
= f
0
f
m a x
f
0
f
m i n
D f
m a x
{
M o d u l a t i o n s i g n a l
f
m o d u l a t i o n
Fig. 4-6 Frequency modulation