70 Performing Basic Measurements
5.9 Frequency Measurements (918D or 818E Detectors)
The Frequency Measurement in the 1936/2936 Series is a background task.
It runs all the time and the user can access it via the Statistics screen
(see Section 4.7). Because of this advanced way of measuring the
frequency, the user has the convenience to “take a look” at the signal
frequency as needed, just by pressing a key. By doing so, the user does not
need to change the current optical power measurement in progress, just to
know what the repetition rate is. Moreover, as explained in Section 4.7,
when using the Statistics screen, the user has an instant view of the input
signal regarding Frequency, instant measured value, Minimum, Maximum,
Mean, Standard Deviation, Signal Range.
The 1936/2936 Optical Meters have the capability of measuring the
modulation frequency, or repetition rate, of the incoming optical signal with
frequencies as low as 0.5 Hz on most ranges and up to 200 kHz. Below
0.5 Hz the measurement error will increase, and in some cases the
measurement will freeze. Frequencies below 0.5 Hz cannot be measured.
Refer to the Specifications table (Section 2.6) for the actual frequency limits
depending on range and mode of operation.
In order to perform an accurate frequency measurement, the AC signal
amplitude must be higher than 5 % of the full-scale of the range the meter is
set on. For example, if the DC value of the incoming signal is 1.25 V and the
modulating AC signal amplitude is 50 mV, the frequency measurement error
might increase or the measurement might freeze. This is due to the fact that,
for a DC signal of 1.25 V, the meter sets the range at 2.5 V and the AC
amplitude of 50 mV is 2% of the full-scale range of 2.5 V. The frequency
measurement is based on the transitions the system “sees” in the input signal.
If there is high noise in the input signal, the measurement might be
inadvertently affected because the noise transitions would appear as being
valid transitions. Also, as the input signal transitions become “softer”, i.e. the
modulation signal is not a square but a sine wave, as the frequency decreases
below 50 Hz, the system might consider that the input signal is DC and the
measurement stops.
The frequency measurement is always available and a value is displayed at
all times independent of the Mode. This being the case, it is up to the user to
assess whether the frequency measurement makes sense or not for that
particular measurement.
For example: in CW Continuous Mode, with the 5 Hz filter on, the AC signal
will be negligible. However, the meter might see some small transitions
coming from noise and will display a value for the frequency. Obviously this
displayed value should be ignored for this case.
Below the specified minimum frequency value the measurement might skip
or even freeze. This is normal, because the input signal approaches a
frequency that can be perceived as DC by the system.
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