Scope FFT and waveform math functions take on RF measurements

WisdomAugust

To avoid aliasing in the digitizing process, sampling must occur at a rate at least

twice the frequency of any appreciable frequencies present in the signal under test.

In this example, a 1.2-GHz sampling rate would be required. Clearly, if the scope is

sampling at its maximum 5-GS/s rate, that is more than sufficient. However, it will

be shown later that for certain scope time-base settings the sample rate (and

bandwidth) will decrease.

So what kind of quality is there in the FFT measurement made on the 600-MHz sine

wave? Referring back to the oscilloscope FFT measurement in Figure 1, notice the

main single frequency spike with a related measurement marker showing around a

600-MHz frequency and 0-dBm power. That matches expectations, but the FFT

response looks very wide for a single frequency input signal.

WisdomAugust

It’s important to understand how the oscilloscope sampling characteristics play

into the quality of this FFT measurement. The oscilloscope analog bandwidth,

sample rate, memory depth, and related time capture period all can have a

profound effect on the measurement result. This effect is heavily influenced by

the characteristics of the signal under test and how those signal characteristics

are related to the oscilloscope capture performance.

For example, in this simple illustration of measuring a single-tone 600-MHz sine-

wave signal and wanting to see the basic spectral characteristics of that signal,

the oscilloscope has to have enough analog bandwidth to minimally attenuate the

amplitude of the signal. Since this oscilloscope has a maximum 1-GHz analog

bandwidth, there is plenty of oscilloscope bandwidth to measure the 600-MHz

tone.

WisdomAugust

An FFT spectral view also is helpful when looking at more complex, wide spectral signals to verify if the proper modulation is happening. Time-gated FFTs further
evaluate spectral components of a signal. Math functions such as a frequency trend
can quickly verify whether a classic modulation scheme is happening properly, like
a linear frequency modulation across pulses in a stream. This article will explore a
number of these examples and look at practical considerations for the measurements.



FFT measurement with an input sine wave

An oscilloscope that has a 1-GHz analog bandwidth and up to a 5-GS/s sample rate will

be used for measurements. These are both important specifications that will tie into

what kinds of measurement applications are possible. The first example measurement

is the capture of a 600-MHz, 632-mV (p-p), 0-dBm, 1-mW sine-wave signal into 50 ?

(orange) and resultant FFT (white) as shown in Figure 1.

Figure 1. Time-domain capture at 1 ns/div and FFT display from a 600- MHz sine-wave input