How to test frequency and what is the test accuracy?

The following describes how the two measurement methods are implemented and their respective advantages. InfiniiVision Oscilloscopes The following describes how the two measurement methods are implemented and their respective advantages.

Software method: After the oscilloscope tests the waveform, it will be displayed on the screen. The signal cycle is analyzed by displaying the amplitude of the waveform, and then the inverse is calculated to obtain the frequency value. As shown in the figure below:

Figure 1 This method uses the test item in the oscilloscope's button function to test. It requires that the waveform to be tested can clearly display at least 2 cycles of the waveform on the screen, otherwise it cannot be tested. If too many waveforms are displayed, it cannot be tested accurately.

The figure below shows the difference between the 10 MHz square wave output by the standard signal source tested by the 3000X series oscilloscope using software analysis.

By adjusting the horizontal time base of the oscilloscope, you can control the number of waveforms displayed on the screen:

Figure 2 shows a 2-cycle square wave on the screen. The frequency can be tested normally. The measurement display is: 10.00 MHz.

Figure 3: The screen shows a square wave with one cycle. It cannot be analyzed correctly. The measurement shows: no edge.

Figure 4 shows a square wave with N cycles on the screen. The waveforms are superimposed and cannot be distinguished. The measurement shows: no edge.

Figure 2, 2 cycle waveform

Figure 3, 1 cycle waveform

Figure 4, N-cycle waveform

As shown in the figure above, the measurement results can be displayed in 4 digits at most.

The test accuracy of the software method is calculated using the Δ time accuracy formula. Taking the 3000X oscilloscope as an example, the formula is as follows:

ΔTime Accuracy = (Using Cursors) ± (Timebase Accuracy x Reading) ± (0.0016 x Screen Width) ± 100 ps

Since (frequency = 1/period), the period accuracy can be measured first and then the frequency accuracy can be derived.

Hardware method: Do not change any connection, directly use the oscilloscope analog channel to test the signal. Use the test principle of direct counting method of traditional frequency counter (such as Keysight Technology's discontinued 53131A) to collect and test the signal.

This function of the InfiniVision-X series oscilloscopes can achieve a maximum resolution of 5-10 bits (the maximum resolution of different sub-series of InfiniVision-X oscilloscopes is different, such as the resolution of the 3000X series is 3-8 bits).

The frequency measurement by hardware is not limited by the number of cycles of the waveform displayed on the screen. As shown in Figure 5, the same 10MHz square wave can no longer be distinguished on the test screen of the 3000X oscilloscope, but the 9.9999936MHz signal can still be tested. The display resolution reaches 8 bits, but from the principle of direct counting, the higher the resolution, the longer the gate time and the slower the measurement time.

Figure 5 shows the waveform of N cycles. The hardware method can test a more accurate frequency value.

The test accuracy of the hardware method can refer to the calculation accuracy of the traditional frequency meter counter, which mainly consists of three parts:

Accuracy = time base uncertainty + system uncertainty + random uncertainty

Since the influence of systematic uncertainty and random uncertainty is small, when calculating the accuracy of InfiniiVision-X series oscilloscopes, only the time base uncertainty, that is, the uncertainty of the oscilloscope's own time base, is considered. Figure 6 shows a comparison of InfiniiVision-X series oscilloscopes. The red box specifically points out the different time base accuracy indicators of these sub-series oscilloscopes:

Figure 6

If higher measurement accuracy is required, the 4000X and 6000X series oscilloscopes can also use an external timebase reference to achieve higher frequency measurement accuracy.

Previously, the function of testing the frequency by hardware mode of InfiniiVision-X series oscilloscope was integrated into the DVM option. Now DVM has become a standard option. In addition to frequency testing, it can also test the period and counter, and even has the function of hardware voltmeter. If the InfiniiVision oscilloscope purchased earlier does not have this function, you can obtain it by upgrading the latest firmware version.

Visit the oscilloscope webpage, search for the oscilloscope model, visit technical support, and obtain the latest firmware version upgrade information.

Comparison between software and hardware methods:

From the perspective of test accuracy, the hardware method has a much higher measurement accuracy than the software method, but the test speed is slower at high resolution. It is very suitable for testing products such as crystal oscillators that output stable periodic waveforms.

The software method can realize fast testing and statistically analyze the measurement results. At the same time, the window analysis function can be used on some sub-series oscilloscopes to perform frequency measurement on specific waveforms in complex waveforms.