Fluke PM6690 - Frequency Modulated Signals; Carrier Wave Frequency F

not start erroneously during the Burst Off du
-
ration or inside the burst.
n
Possible Errors
Before the measurement has been synchro-
nized with the burst signal, the first measure-
ment(s) could start accidentally during the
presence of a burst. If this would happen and
if the remaining burst duration is shorter than
the set measurement time, the readout of the
first measurement will be wrong. However,
after this first measurement, a properly set
start-arming sync delay time will synchronize
the next measurements.
In manually operated applications, this is not a
problem. In automated test systems where the
result of a single measurement sample must be
reliable, at least two measurements must be
made, the first to synchronize the measure
-
ment and the second from which the measure
-
ment result can be read out.
Frequency
Modulated Signals
A frequency modulated signal is a carrier
wave signal (CW frequency = f
0
) that changes
in frequency to values higher and lower than
the frequency f
0
. It is the modulation signal
that changes the frequency of the carrier
wave.
The counter can measure:
f
0
= Carrier frequency.
f
max
= Maximum frequency.
f
min
= Minimum frequency.
Df = Frequency swing = f
max
–f
0
.
Carrier Wave Frequency f
0
To determine the carrier wave frequency, mea-
sure f
mean
which is a close approximation of
f
0
.
Press STAT/PLOT to get an overview of all
the statistical parameters.
Select the measurement time so that the coun
-
ter measures an integral number of modulation
periods. This way the positive frequency devi
-
ations will compensate the negative deviations
during the measurement.
Example: If the modulation frequency is
50 Hz, the measurement time 200 ms will
make the counter measure 10 complete modu
-
lation cycles.
If the modulation is non-continuous, like a
voice signal, it is not possible to fully com
-
pensate positive deviations with negative
deviations. Here, part of a modulation swing
Measuring Functions
4-6 Frequency Modulated Signals
M e a s u r e
G a t e T i m e
S y n c - d e l a y
Fig. 4-5 Measuring the frequency of the
carrier wave signal in a burst.