Why frequency measurement has such a big impact on other parameters

Introduction

When engineers are testing, they often encounter phenomena such as test data jumps and efficiency abnormalities. From experience, this has a lot to do with the frequency measurement of the signal. This article analyzes the importance of frequency measurement, hoping to help everyone better solve daily testing problems.

First, let's look at why the measurement of frequency has such a big impact on other parameters.

1. Selection of synchronization source

Engineers who have used power analyzers will definitely remember that when setting up the instrument, there is a setting option called "synchronous source". This option includes the voltage and current of each test channel, and engineers can choose it independently. The selection of this option has little effect on DC signal testing, but it will have a great impact on AC signal testing. The reason is that if the interval of the AC signal measurement data is not synchronized with the signal cycle, it is equivalent to the test data being a non-integer cycle, and the calculated result will also be inaccurate.

The period of the signal detected and calculated by the power analyzer is determined by the synchronization source, so choosing an accurate synchronization source is critical to the test results. The principle of selecting a synchronization source is to choose a signal as close to a sine wave as possible. For example, for the power grid power frequency, we generally choose voltage as the synchronization source. For the PWM signal output by the motor drive, we can choose current as the synchronization source.

2. Selection of PLL Source

In addition to the fact that the synchronous source signal has a great influence on the measurement data, there is another very critical source when we set up harmonic analysis - the PLL source. We can first look at the method of harmonic measurement. You can refer to "Understanding the Harmonic Measurement Method in One Article". The commonly used harmonic analysis uses the synchronous sampling method, which can ensure that there will be no spectrum leakage and ensure the accuracy of harmonic measurement. For example, the IEC6100-4-7 standard stipulates the sampling principle of 10 times the base frequency. The basis of the synchronous sampling method is the selection of the PLL source.

Above we have analyzed the influence of synchronization source and PLL source on the measured data and harmonics. So what is the relationship between these two "sources" and signal frequency? The answer is that the relationship is very large. The synchronization source ensures that the instrument performs technology according to the signal period, and the PLL source ensures that the measurement period is an integer multiple of the measured signal period during harmonic analysis. Here we can see that the accuracy of the signal period is the basic requirement for the "source", and the measurement of the signal period is actually the measurement of the signal frequency.

3. Ensure accurate frequency measurement

At this point in the article, I believe everyone understands why accurate frequency measurement is so important to the measured data results. Then let's talk about how to ensure the accuracy of frequency measurement.

The most critical point to ensure accurate frequency measurement is the selection of the signal range. If the range selection is inappropriate, for example, the amplitude of the input signal is less than 10% of the set range, the frequency measurement circuit will not be triggered due to the signal amplitude being too low , resulting in no measurement results or incorrect measurement values, making it impossible to accurately measure the frequency. Therefore, choosing the appropriate range is the first step to accurate frequency measurement.

Secondly, if the input signal contains a large interference signal, the frequency measurement circuit will be triggered incorrectly, resulting in measurement errors. In order to ensure the integrity of the measurement signal and the accuracy of the test frequency, the frequency filter in the setting can be turned on to eliminate the influence of the interference signal on the frequency measurement.

When you encounter unstable data and efficiency during power analyzer testing, you may want to follow this article to rule out whether the problem is caused by inaccurate frequency measurement. Often, you will get unexpected surprises!