Data AcQuisition And Real-Time Analysis
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Frequency Counter Update Rate (Fast/Slow)

Controls: Options Menu >> Frequency Counter >> Fast/Slow
Macro: FcountRate

The Frequency Counter functions can be set to update the displayed measurement at 10 (Fast) or 1 (Slow) readings per second.

You can change the Slow rate via the Fcount Slow control in the Run Preferences dialog of the Edit Menu.

A small asterisk is shown to the right of the Slow button on alternating counter updates. This assures you that the counter is in fact updating, even if the reading is unchanged due to a very stable frequency.

You may want to select Slow if the reading is changing too fast to follow easily. This can happen when using the Total or SpecTot modes when events are occurring at a rapid rate, or in the frequency (Hertz) or period (msec) modes when the frequency source is not steady.

The Slow update rate results in higher accuracy, but unlike a conventional benchtop counter the update rate does not directly control the resolution. A conventional counter simply counts the number of triggers in the update interval, and reports that value with an appropriate decimal place. To read with 1 Hz resolution, such a counter must actually count the number of cycles (triggers) in one second; at 10 updates per second, the resolution would be no better than 10 Hz.

Instead, Daqarta counts the total number of samples that elapse during the total number of triggers in the update interval. Since the sample rate is known and is very accurate, the signal frequency can be computed with similar accuracy after only a short time. This is analogous to setting Period mode on a conventional counter and counting the number of time-base cycles between input triggers, then computing the reciprocal. Here Daqarta takes care of all computations, and allows an arbitrary number of time-base cycles (samples) with an arbitrary number of input triggers between computations.

Benchtop counters typically use a time base of 1 MHz or more, so they can obtain period measurements with a resolution of microseconds or better. Daqarta is constrained to use the sample rate of the sound card, which is typically only 48 kHz. This would seem to limit Daqarta to a resolution that is more than 20 times worse than the benchtop unit. However, Daqarta has access to more information about the signal than the benchtop unit does, since it knows the amplitudes of the samples before and after the trigger event. This allows interpolation of the trigger time to give much higher resolution.

For example, suppose the trigger level is set to 10% of full scale, but the actual incoming samples don't fall exactly on this value; one sample falls on 9% and the next on 11%. In this case the 10% value must have arrived exactly midway between these two samples. Since sample amplitudes are known with high resolution, a large improvement in timing resolution can be obtained by interpolation.

The fact that the waveform may not be perfectly linear between samples is usually not an issue, especially when you set the trigger threshold to the most linear portion of a waveform, near the zero crossing. In addition, at low signal frequencies the linearity assumption is very good because the signal is changing slowly between signals; at higher frequencies where there is greater change, the linearity assumption is poorer... but there are also proportionately more signal cycles per update so the overall resolution is maintained. The frequency resolution on the Slow range is typically better than 0.001 Hz up to several thousand Hz, then starts to deteriorate. At 20 kHz the resolution is still better than 0.5 Hz.

Macro Notes:

Use FcountRate=Fast or FcountRate=0 to set the Fast rate, or FcountRate=Slow or FcountRate=1 to set the Slow rate. FcountRate=x toggles between the two.

See also Frequency Counter


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