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10 Hints For Getting The Most From Your Frequency Counter - how to measure hertz frequency


10 Hints For Getting The Most From Your Frequency Counter-how to measure hertz frequency

10 Hints For Getting The Most From Your Frequency Counter
Our thanks to Agilent Technologies for allowing us to reprint the following article.
Counters can be a be deceptive when other errors push the counter's
plug and play resolving ability away from the actual frequency. In other
instrument and words, it's possible for a counter to give you a very
seem fairly simple accurate reading of an incorrect frequency.
from the outside.
You connect a Random and systematic errors both determine a
signal to the input, counter's accuracy. Random errors are the source of
and a digital resolution uncertainties and include:
readout tells you the frequency or some other ? Quantization error
parameter. However, to achieve the best results, When a counter makes a measurement, a ?1
whether that means speed or quality, attention to how count ambiguity can exist in the least significant
you set up the counter measurement is important. digit. This can occur because of the non-
Choosing the Best Counter coherence between the internal clock frequency
and the input signal.
Selecting which counter will best meet your needs is the
first step. There are several related products that ? Trigger error
perform a variety of tasks at various frequencies: Noise spikes can be triggered by noise on the
input signal or noise from the input channels of
? Universal counters the counter.
Both frequency and time interval measurements, ? Timebase error
as well as a number of related parameters. Any error resulting from the difference between
? RF frequency counters the actual time base oscillator frequency and its
Precise frequency measurements, up to 3 GHz nominal frequency is directly translated into
and beyond. measurement error.
? Microwave frequency counters Systematic errors are biases in the measurement
Precise frequency measurements, up to 40 GHz system that push its readings away from the actual
and beyond. frequency of the signal. This group includes effects on
? Time interval analyzers
the time base crystal such as aging, temperature and
Optimized for precision time interval
line voltage variations.
measurements. Compare the two counters in Figure 1. Counter A has
? Modulation domain analyzers
good resolution but a serious systematic error, so its
Designed to show modulation quantities, such
displayed result in most cases will be less accurate than
as frequency versus time, phase versus time,
those of Counter B, which has poorer resolution but a
and time interval versus time.
smaller systematic bias error.
1: Recognize the difference between resolution and
accuracy
Assuming a large number of digits equates to a very
accurate measurement may not be correct. It is a
common mistake to equate resolution and accuracy.
They are related, but different concepts.
The resolution of a counter is the smallest change it can
detect in closely spaced frequencies. All other things Figure 1. Simplified view of resolution vs. accuracy. Systematic
being equal (such as measurement time and product errors related to the timebase "push" the displayed frequency
cost), more digits are better--but the digits you see on away from the actual frequency. The random errors create a range
the display need to be supported by accuracy. Digits can
of frequencies inside of which the counter can't distinguish
different signals.
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2: Understand counter measurement methods
Frequency counters fall into two basic types: direct
counting and reciprocal counting. Understanding the
effects of the two different approaches will help you
choose the best counter for your needs and use it
correctly.
Direct counters simply count cycles of the signal for a
known period - the gate time. The resulting count is sent
directly to the counter's readout for display.
This method is simple and inexpensive, but it means that
the direct counter's resolution is fixed in Hertz. For Figure 2. The number of digits displayed by a direct counter vs.
example, with a 1 second gate time, the lowest frequency (for a 1 second gate time)
frequency the counter can detect is 1 Hz (since 1 cycle
of the signal in 1 second is 1 Hz, by definition). Thus, if
you are measuring a 10 Hz signal, the best resolution
you can expect for a 1 second gate time is 1 Hz, or 2
digits in the display. For a 1 kHz signal and a 1 second
gate, you get 4 digits. For a 100 kHz signal, 6 digits, and
so on. Figure 2 illustrates this relationship. Also note that
a direct counter's gate times are selectable only as
multiples and sub-multiples of 1 second, which could
limit your measurement flexibility.
Reciprocal counters, in contrast, measure the input
signal's period, then reciprocate it to get frequency.
Given the measurement architecture involved, the
resulting resolution is fixed in the number of digits Figure 3. Comparing resolution for direct and reciprocal counters
displayed (not Hertz) for a given gate time.
(for a 1 second gate time)
In other words, a reciprocal counter will always display
the same number of digits of resolution regardless of the
input frequency. Note that you'll see the resolution of a
reciprocal counter specified in terms of the number of
digits for a particular gate time, such as "10 digits per
second."
You can determine whether a counter is direct or
reciprocal by looking at the frequency resolution
specification. If it specifies resolution in Hertz, it's a
direct counter. If it specifies resolution in digits-per-
second, it's a reciprocal counter.
The counter industry has standardized on measuring Figure 4. Here are the gate times needed to yield various
relative to a 1 second gate time. Figure 3 compares the resolutions with a 10 digits/second reciprocal counter
resolution of direct and reciprocal counters. In the lower The choice comes down to cost versus performance. If
frequency spectrum, reciprocal counters have a your resolution requirements are flexible and you aren't
substantial advantage over direct counters. As an too concerned with speed, a direct counter can be an
example, at 1 kHz, a direct counter gives a resolution of economical choice. A reciprocal counter is required for
1 Hz (4 digits). A 10 digit/second reciprocal counter the fastest, highest resolution measurements.
gives a resolution of 1 Hz (10 digits).
If precision resolution is not a priority, the reciprocal 3: Choose the appropriate timebase
counter still offers a significant speed advantage: the The measurement accuracy of a frequency counter is
reciprocal counter will give 1 mHz resolution in 1 ms, strongly dependent on the stability of its timebase. The
while a direct counter needs a full second to give you timebase establishes the reference against which the
just 1 Hz resolution (Figure 4). Reciprocal counters also input signal is measured. The better the timebase, the
offer continuously adjustable gate times (not just decade better your measurements can be. The frequency at
steps), so you can get the resolution you need in the which quartz crystals vibrate is heavily influenced by
minimum amount of time. ambient temperature, and timebase technologies fall into
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three categories based on the way they address this 1. Time-invariant counter performance factors
thermal behavior: (such as the temperature stability of the
? Room Temperature Crystal Oscillator (RTXO) timebase as discussed in Hint #3)
This type of timebase does not utilize any 2. Time-variant counter performance factors
temperature compensation or control. These (such as the aging rate of the timebase)
types of oscillators have been manufactured for 3. The input signal clarity and level of noise
minimum frequency change over a range of
temperature- typically between 0?C and 50?C. 4. Counter set-up and configuration
This is accomplished through the proper choice
of the crystal cut during the manufacturing Item #2 is where calibration plays a role. Although
process. A high quality RTXO would vary by counters are electronic instruments measuring electrical
about 2.5 parts per million (ppm) over that signals, the quartz crystal that is the heart of every
temperature range. This works out to ?2.5 Hz on counter's time-base is a mechanical device. Since it is a
a 1 MHz signal, so it can be a significant factor mechanical device, the crystal is susceptible to physical
in your measurements. It carries the advantage disturbances that can change the frequency at which it
of being inexpensive, but results in large vibrates which ultimately affects the counter's accuracy.
frequency errors. The cumulative effect of these various disturbances is
known as crystal aging, and it is this aging that you are
? Temperature Compensated Crystal Oscillator compensating for when you calibrate the counter.
(TCXO) Aging is a factor that is fairly easy to predict and easy to
One method of compensating for frequency compensate for through calibration. You can determine if
changes due to temperature variation is through calibration is required by looking at the aging rate
externally added components that have specification in your counter's data sheet. For example:
complementary thermal responses to obtain a If the aging rate is 4 x 10-8 per day and it has been 300
more stable frequency. This approach can days since calibration, aging will add a timebase error of
stabilize the thermal behavior enough to reduce 1.2 x 10-5 into the overall accuracy calculation. If this
timebase errors by an order of magnitude uncertainty (?12 Hz on a 1 MHz signal), in addition to the
relative to RTXO (approximately 1 ppm (?1 Hz other inherent errors reviewed earlier is acceptable for
on a 1 MHz signal)). your measurements, calibration is not required.
? Oven Controlled Crystal Oscillator (OCXO) Otherwise, calibrate.
In this technique, the crystal oscillator is housed 5: Make the most accurate measurements
in an oven which holds its temperature at a
specific point in the thermal response curve. The ? Select the best arming mode
result is much better timebase stability, with If you want to make quick measurements, using
typical errors as small as 0.0025 ppm (?0.0025 your frequency counter s automatic arming
Hz on a 1 MHz signal). Additionally, oven- mode is a simple way. However, of the four
controlled timebases also help with the effects of typical arming modes (automatic, external, time,
crystal aging, which means you don't have to and digits), automatic mode is the least
take your counter out of service for calibration as accurate. You can improve resolution and
often. systematic uncertainty (both elements of
4: Schedule calibration to match performance measurement error, as discussed in Hint #2) by
demands increasing gate time with either the external,
time, or digits arming modes.
The frequency at which you calibrate your counter ? Use the best timebase available and calibrate
depends on several factors: frequently
? The type of timebase The quality of the timebase and how often you
? The conditions the counter is subjected to during
calibrate will affect your measurement accuracy.
measurement
For most applications, you can make a tradeoff
between accuracy, timebase quality, and
? How much accuracy you need from the calibration period. If you purchase a higher-
measurement quality timebase, you can lengthen the time
between calibrations. If you calibrate more
Calibration can be a complex issue and is directly frequently, you may be able to meet your
related to counter accuracy in general. The quality of the accuracy requirements with a less-costly
measurement you see on the display depends on four timebase. The timebase does not need to be
factors: housed within the frequency counter. You can
use a precision source or a house standard
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Corporate Phone: 610-825-4990 ? Sales: 800-832-4866 or 610-941-2400
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Which instrument can be used for measuring frequency in hertz? frequency meter, device for measuring the repetitions per unit of time (customarily, a second) of a complete electromagnetic waveform. Various types of frequency meters are used. Many are instruments of the deflection type, ordinarily used for measuring low frequencies but capable of being used for frequencies as high as 900 Hz.