Making low frequency measurements with a Frequency Counter

Measuring low frequency signals can be a bit touchy, but with a few tips you can begin making better measurements.  The first step is to use dc coupling.  Dc coupling removes a blocking capacitor, which is normally in series with the input signal. Generally a frequency counter would be ac coupled, which limits the amplitude of low frequency signals.

Low frequency measurements are particularly susceptible to false triggers caused by spurious noise.   A modern frequency counter has a built-in high frequency filter to help eliminate high frequency noise and should be enabled for low frequency measurements.  For example, the Agilent 53131A, 53132A, and the 53181A frequency counter has a 100 kHz low-pass filter that you can switch into the signal path. This reduces the chance of triggering on harmonics and high-frequency noise.

A low frequency signal may have a low slew rate--meaning the signal is slow to change states. The lower the slew rate the harder it is to create a repeatable trigger. Decreasing the counters sensitivity will help. In order for a counter to successfully trigger, the signal will need to pass through a lower and an upper threshold. The trigger band, the delta between the upper and lower threshold is determined by the counter's sensitivity. Decreasing the counter sensitivity will increase the difference between the upper and lower threshold, widening the trigger band.

When a counter is set to auto triggering, it estimates the peak to-peak level of the signal and computes the midpoint to establish a trigger level. While this approach generally leads to good results, it can cause trouble with low-frequency signals. The problem occurs when the auto trigger algorithm completes before the signal transition between its minimum and maximum values. The trigger will be set based upon a portion of the waveform, rather than a trigger level based on an approximate average of the minimum and maximum values of multiple cycles.  The solution is to turn off auto trigger and set the trigger level manually.

Be patient. A low frequency measurement can take time to complete. If you are controlling the counter from a computer, you may want to check the status register before requesting a reading. The counter will continue to make a measurement until it receives a second valid trigger condition, indicating the end of the measurement. If the input signal becomes disconnected the counter will wait indefinitely for the measurement to complete. If you request a measurement (MEAS?) the computer will be stuck waiting until the counter measurement finishes before responding to the query. To avoid this, start the measurement (INIT) and then check the status register to be certain a measurement has completed before requesting the reading (FETCH?).

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