How to use a test tone generator with an oscilloscope

The step function is also applied to the input of the digital filter. Cursor measurements show that the spectrum of the impulse function is flat to within 20 mdB over the bandwidth of the filter being tested. It shows a flat frequency spectrum over the 100 MHz band of interest. The next lower trace is the FFT of the impulse. You can reduce the sample rate to a minimum of twice the desired measurement bandwidth and still satisfy the Nyquist Criteria. Note that the derivative math function is sensitive to baseline noise, which you can counter by decreasing the effective sample rate using the oscilloscope's sparse function. Math trace F4, just below the trace of the input signal, differentiates the step input. To make the measurement, apply the input signal to channel 1, shown in the upper left trace. The signal's bandwidth is inversely proportional to the step's transition time. The oscilloscopes вЂњfast edgeвЂќ or вЂњcalвЂќ signal could be used depending on the desired measurement bandwidth. 1, we use a step waveform as the input signal. An FFT function lets you measure the frequency response of a digital filter that's excited by a step-signal source. Figure 1 shows all the steps involved in this measurement.ДЇigure 1. Our first example uses the step function to excite a digital filter, from which we measure frequency response.

This later fact means that the oscilloscope can make self-contained frequency response measurements. Keeping in mind that the impulse function is the derivative of a step function, it is also possible to use a fast edge or step signal from the oscilloscope to do frequency response testing. Swept sine waves, white random noise, and the Dirac impulse function all provide flat responses and all are available in arbitrary waveform generators. This means that the signal spectrum is constant across all frequencies of interest. The simplest type of frequency response measurement requires a signal source with a spectrally вЂњflatвЂќ response in the frequency domain. You can use several types of input excitation signals to measure frequency response: step functions, swept sines waves, white noise, and phase-modulated carriers. The FFT (Fast Fourier Transform), spectrum analyzer options, and similar frequency domain tools, let you measure a circuit's frequency response with an oscilloscope. Oscilloscopes are time-domain instruments, but because they digitize waveforms, oscilloscopes can process time-domain signals into the frequency domain.

How to use a test tone generator with an oscilloscope