Whether you use your frequency counter on the bench or in an automated test system, you want your measurements to be as accurate as possible. There’s a key component in your frequency counter that dictates its accuracy: the timebase oscillator.
Measurement accuracy in frequency counters begins with the timebase because it establishes the reference against which your input signal is measured. The better the timebase, the better your measurements can be. I said ‘can be’ because you need to do scheduled maintenance of your frequency counter. To maximize your counter’s accuracy, you need to calibrate it. Here’s why:
The frequency at which quartz crystals vibrate is heavily influenced by ambient temperature.
There are three main categories of timebase technologies, divided up based on how they address thermal behavior: room temperature crystal oscillators (RTCO), oven controlled crystal oscillators (OCXO) and temperature compensated crystal oscillators (TCXO).
Standard Timebase – A Room Temperature Crystal Oscillator
The first category is the standard timebase, also known as room temperature crystal oscillator (RTCO). A standard timebase doesn’t employ any kind of temperature compensation or control. While this has the advantage of being inexpensive, it also gives the largest frequency errors. The curve in Figure 1 shows the thermal behavior of a typical crystal. As the ambient temperature varies, the frequency output can change by 5 parts per million (ppm) or more. This works out to ± 5 Hz on a 1 MHz signal, so it can be a significant factor in your measurements.
Figure 1. The frequency output of an unprotected crystal can vary widely in response to ambient temperature. Putting the crystal in a controlled thermal environment (an oven) helps maintain a stable output frequency.
The obvious solution to this temperature-induced variance is to control the temperature, which leads us to our next type of oscillator.
Oven-controlled Crystal Oscillators (OCXO)
For an OCXO, the crystal oscillator is housed in an oven that holds its temperature at a specific point in the thermal response curve. Surrounding the crystal with temperature-control circuitry gives it better timebase stability. Typical errors are as small as 0.0025 ppm (±0.0025 Hz on a 1 MHz signal). Additionally, oven-controlled timebases also help minimize the effects of crystal aging. This means you don’t have to calibrate your frequency counter as often.
OCXOs are very accurate but more expensive and have a bigger footprint than other timebase options. So, many engineers opt for temperature-compensation circuitry instead of temperature-controlled circuitry.
Temperature Compensated Crystal Oscillators (TCXO)
Temperature compensated crystal oscillators (TCXOs) are designed to account for temperature changes instead of trying to hold a fixed temperature. One method of compensating for frequency changes due to temperature variation is to add external components with complementary thermal responses.
This approach can stabilize the thermal behavior enough to reduce timebase errors by an order of magnitude relative to RTXO (approximately 1 ppm, ±1 Hz on a 1 MHz signal).
Choose a Timebase That’s Right for You
Now you have learned that different types of timebases bring a different response to a counter.
The quality of your frequency counter’s timebase will affect your measurement accuracy. Depending on the accuracy that you need, you can choose the right one.
It’s also worth pointing out that the timebase does not need to be housed within the frequency counter. You can connect a precision source or house-standard external source to the counter to improve measurement accuracy. You should also note that, no matter what timebase you select, leaving your timebase powered up will provide the most accurate results.
To learn more about frequency counter measurements, download the 10 Hints for Getting the Most from Your Frequency Counter application note.