If you cannot obtain a stable reading, the sig­nal-to-noise ratio (often designated S/N or SNR) might be too low, probably less than 6 to 10 dB. Then you should use a filter. Certain conditions call for special solutions like highpass, bandpass or notch filters, but usu­ally the unwanted noise signals have higher frequency than the signal you are interested in. In that case you can utilize the built-in lowpass filters. There are both analog and dig­ital filters, and they can also work together.

Fig. 3-6 The menu choices after selecting FILTER.

Analog Lowpass Filter

The counter has analog LP filters of RC type, one in each of the channels A and B, with a cutoff frequency of approximately 100 kHz, and a signal rejection of 20 dB at 1 MHz.

Accurate frequency measurements of noisy LF signals (up to 200 kHz) can be made when the noise components have significantly higher frequencies than the fundamental signal.

Digital Lowpass Filter

The digital LP filter utilizes the Hold-Off function described below.

With trigger Hold-Off it is possible to insert a deadtime in the input trigger circuit. This means that the input of the counter ignores all hysteresis band crossings by the input signal during a preset time after the first trigger event.

When you set the Hold-Off time to approx. 75% of the cycle time of the signal, erroneous triggering is inhibited around the point where the input signal returns through the hysteresis band. When the signal reaches the trigger point of the next cycle, the set Hold-Off time has elapsed and a new and correct trigger will be initiated. Instead of letting you calculate a suitable Hold-Off time, the counter will do the job for you by converting the filter cutoff frequency you enter via the value input menu below to an equivalent Hold-Off time.

Fig. 3-7 Value input menu for setting the cutoff frequency of the digital filter.

You should be aware of a few limitations to be able to use the digital filter feature effectively and unambiguously. First you must have a rough idea of the frequency to be measured. A cutoff frequency that is too low might give a perfectly stable reading that is too low. In such a case, triggering occurs only on every 2nd, 3rd or 4th cycle. A cutoff frequency that is too

high (>2 times the input frequency) also leads to a stable reading. Here one noise pulse is counted for each half-cycle.

Use an oscilloscope for verification if you are in doubt about the frequency and waveform of your input signal. The cutoff frequency setting range is very wide: 1 Hz - 50 MHz

Fig. 3-8 Digital LP filter operates in the measuring logic, not in the input amplifier.