Measuring
Frequency
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- Measurements - Frequency
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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.
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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.
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First, the instrument
is connected like a voltmeter, and set to measure frequency.
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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).
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Then, the frequency is
computed by dividing the count by the time period, T.
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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.
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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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