Digital Frequency Counters
We are surrounded by systems working entirely off of radio signals.
Radio frequency has applications in mobile phones, radios, Bluetooth,
Wi-Fi, and 5G applications that power cloud computing, automotive
technology, and the Internet of Things (IoT). Industries such
as electronics, telecommunication, defense, industrial transportation,
manufacturing, IT, and aerospace regularly rely on robust devices
that transmit, receive, and analyze a slew of signals.
This makes the need for highly accurate digital frequency counters
important. Digital frequency counters help test electronics by
accurately measuring repetitive square and pulse signals, the
time between edges of digital signals, and even act as timers.
While selecting a digital frequency counter for test applications,
it is important to look at parameters such as accuracy, resolution,
and interval timers.
Types
of Digital Frequency Counters
Selecting the type of digital frequency counter relevant to your
application is crucial. Most frequency counters accumulate a
set of events happening over a short period to carry out the
analysis. Different types of counters include:
- Universal counters - As the name suggests, they measure
a range of parameters such as frequency, time interval, oscillation,
and pulses per second.
- RF frequency counters - These specialized test devices
help measure precise frequencies up to 3 GHz and beyond.
- Microwave frequency counters - They help measure frequencies
in the microwave range i.e. 40 GHz and beyond.
- Time interval analyzers - These are specifically designed
to measure time intervals. It can continuously measure the period,
pulse width, and time interval of signals that vary with time.
- Modulation domain analyzers - These are designed to
show relative measurements of modulation quantities such as phase
vs. time, frequency vs. time, and time interval vs. time.
All frequency counters can be segregated into two broad categories
- direct counting frequency counters and reciprocal counting
frequency counters.
- Direct
counting frequency counters - These devices simply count cycles of signal
over a period known as gate time and send the results to the
counter's display unit.
* Pros - Simple and inexpensive.
* Cons - The counter's resolution is fixed in hertz and
the accuracy may be lower than a reciprocal frequency counter.
The speed of analysis may be slower.
Reciprocal
counting frequency counters - These devices measure and reciprocate the input
signal's period to find the frequency. This helps the counter's
resolution appear as a full number on digit displays.
* Pros - Faster and higher resolution measurements with
a minimum amount of time.
* Cons - Comparatively expensive than direct counting
frequency counters.
RF Frequency Counter Applications
Radio Frequency (RF) counters are widely used in the mobile telecom
domain as it is driven by RF and microwave engineering. Its testing
applications include measurement of the following in RF range:
- Frequency of the pulse produced by a square wave generator.
- Frequencies of the outgoing signal at the transmitter and incoming
signal at the receiver.
- Data transmissions due to clock pulses.
- High power data transmitters and oscillators.
- Devices with steady repetitive signals.
Counter Accuracy vs. Resolution & Errors
Counters are easy to operate. You simply plug in the signal input
and the display will show the frequency and other parameters.
Though it is important to pay attention to how you set up the
counter to get the best results. Test engineers often equate
higher resolution of display reading to greater accuracy.
But that isn't necessarily true.
Resolution is essentially the smallest variation that a digital
counter can detect. A higher number of digits signifies higher
resolution. However, it can only be accurate if other errors
do not disrupt the counter's measuring capability. The measurement
disruptors can be classified into random and systematic errors.
Random errors include:
- Quantization error - There can be a minor non-coherence between
the input signal and internal clock frequency of the counter.
This can cause a +/- 1 count ambiguity in the least significant
digit.
- Trigger error - Any noise in the input channels of the counter
or input signal can cause a trigger error in the measurements.
- Timebase error - Digital counter measurement can be affected
by any difference between the actual time base oscillator frequency
and its nominal frequency.
Systematic errors can creep in due to external factors that may
not be in control of the operator. It can cause biases in the
measurement system that deviates the readings away from actual
frequency. Factors that cause systematic errors include aging,
temperature variations, and line voltage variations.
Systematic and random errors are inter-dependent. For example,
Counter A with a high resolution but serious voltage variation
can give inaccurate results. On the other hand, Counter B with
smaller resolution and low systematic error can give better results.
Tips for Making Accurate Measurements Using Frequency Counters
Although there could always be minor variations in the output
of a counter, the following steps can help avoid major errors:
- Choosing the right timebase - Timebase is the reference against
which the counter measures the input signal. The better the timebase,
the more accurate measurements will be. The quartz crystals vibrate
at frequencies that are influenced by ambient temperatures. Following
timebase technologies can help resolve this thermal behavior:
* Room Temperature Crystal Oscillator (RTXO) - This timebase
does not depend on temperature compensation or control. They
are designed to have a minimum effect on frequency over a range
of temperatures - typically between 0 and 50 degrees Celsius.
* Temperature Compensated Crystal Oscillator (TCXO) - This method
adds external components that counter thermal variations and
obtain a stable frequency.
* Oven Controlled Crystal Oscillator (OCXO) - With this method,
the crystal oscillator is housed inside an oven that holds the
temperature stable thus giving an accurate timebase.
- Use the best arming mode - A typical digital counter has four
arming modes - automatic, external, time, and digits. While automatic
arming mode is the fastest, it is also the most inaccurate. You
can improve the resolution by increasing gate time with external,
digit, or time arming modes.
- Monitor trigger level timing error - Resolution, accuracy,
the fidelity of the input amplifier, slew rate of the input signal,
and width of the input hysteresis band are some of the factors
while making timing measurements such as time interval, pulse
width, rise time, fall time, phase, and duty cycle. To get better
measurements, trigger at the offset value of the sine wave or
square wave signal.
- Lock all timebases to a single clock - Using independent time
bases can introduce skew and jitter.
- Setting the counter to a known state - After sending a reset
command, it is advisable to add a wait or delay of 1 second to
the program for the instrument to come back to a steady-state.
- Avoid noise triggers - Noisy signals can result in the counter
triggering to spurious zero crossings causing inaccurate measurements.
High-quality counters allow you to adjust the band to avoid unwanted
triggering.
- Improve low-frequency measurements - Some of the steps you
can follow to improve the measurements are:
* Utilize low pass filter
* Use manual triggering
* Use DC coupling
* Decrease the counter's sensitivity
* Monitor the status register.
- Use
limit testing
- The readings shown by a counter can be 10 - 12 digits long,
and they can change every second. One easy way to interpret the
readings easily is to set a limit beyond which the counter will
stop taking the readings.
Conclusion
With any measuring instrument, getting accurate readings is the
key to testing and development activities. If signal measurement
and analysis are crucial in your line of work, you must stay
in the know about the latest in test equipment development. You
should also enable access to the best-in-budget frequency counters
out there for your teams.
While digital frequency counters are simple instruments, engineers
can arm themselves with additional knowledge on how to use the
device and get accurate results results by referring to the tips
mentioned above.
