| Frequency |
Measuring Frequency
Introduction
When you measure frequency, you are often
measuring the frequency of a voltage signal, so the first thing you need to
remember is that you are making a voltage measurement, so that everything that
is important to a voltage measurement will be important when you measure a
frequency. In particular, you need to remember the following.
When you measure a voltage
the two terminals of the voltmeter (in the figure, the red terminal and the
black terminal) are connected to the two points where the voltage appears
that you want to measure. One terminal - say it is the red terminal - will
then be at the same voltage as one of the points, and the other terminal -
the black terminal - will be at the same voltage as the other point. The
meter then responds to the difference between these two voltages. When you measure voltage, the
voltmeter should not disturb the circuit where you are attempting to measure
the voltage. In the circuit above, that disturbance is the current drawn by
the voltmeter. You want that current to be as close to zero as it can
possibly be. That means that you need to have the resistance of the
voltmeter as large as possible. There's more discussion of that effect in
the
lesson on measuring voltage. Ideally, the resistance of a voltmeter
would be infinite.
In most cases, when you measure frequency you take
the above into consideration, and then you adjust the meter to take a frequency
measurement. That's usually just a matter of a adjusting a single control on
the instrument.
If you want to measure frequency, there
are some things to understand about that kind of measurement.
Measuring Frequency
When you measure the frequency of a
voltage signal, the typical instrument will do the following.
First, the instrument is
connected like a voltmeter, and set to measure frequency. When the measurement is
taken, the instrument counts the signal. It might count zero crossings of
the signal, or it might just assume that the signal is a sequence of pulses,
and count the pulses. In either case, the
instrument counts for a predetermined length of time, T
(which you might be able to control). Then, the frequency is
computed by dividing the count by the time period, T.
The computation of
frequency cannot have a resolution better than one count. For example,
if the instrument counts for one second, a count of ten would compute as
10 Hz, and a count of 11 would compute as 11 Hz. You couldn't get a
good measurement of 10.5 Hz, and would always be off by 0.5 Hz. What
you got would depend upon the timing of the count - when it started. The resolution is
probably not a problem if you are interested in a 20KHz signal and the
instrument counts for a second, but you have to be cognizant of what is
taking place.
That's the one thing you need
to be cognizant of when you take a frequency measurement. Remember that and the
instrument won't fool you.
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