Scope FFT and waveform math functions take on RF measurements

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The FFT measurement of the peak value amplitude and frequency of the spike

shows that the RF pulse begins with a carrier frequency around 300 MHz. If

the time-gate window is moved to the center of the RF pulse, the frequency

is seen to be around 600 MHz. And it is 900 MHz at the end of the RF pulse.

This appears to be a linear frequency-modulated chirp as desired.

Frequency measurement and measurement trend math function

In some cases, a measurement trend math function can give a helpful view of

the frequency chirp profile. The oscilloscope is able to display up to 1,000

measurements in a trend format. In a similar signal example, a 600-ns-wide

pulse train, repeating every 20 μs, needs to be verified. The FFT function now is

turned off, and purely time-domain measurements are made.

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First, the acquisition mode of the oscilloscope is changed from Normal capture to High Resolution capture mode. Second, a frequency measurement is selected from the list of possible measurements,
by pressing the Measure button. A middle threshold for carrier zero crossing detection is set to 30 mV
given that the swing of the carrier signal is from around -316 mV to +316 mV (1-mW signal, 0 dBm into
50 ?). Then the Math key is pressed, and a math function called measurement trend is chosen. Markers
are assigned to have their source be the math function result. An interesting view of frequency
measurements taken across the RF pulse can be seen in Figure 6.


Figure 6. Measurement trend math function on frequency measurements across the pulse

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Clearly, the pulse carrier is shifting in a linear fashion across the pulse, from left

to right, as designed. Notice that the linear ramp display is not going across the

entire width of the RF pulse. This is because the 1,000 measurement limit in the

trend calculation has been reached. It is important that a portion of the pulse FM

function can be seen, and it is linear. For the frequency measurements across the

pulse to have enough precision, it was imperative that the High Resolution

acquisition mode was selected.

Summary

FFTs in oscilloscopes are a valuable tool to give a frequency-domain view of a signal.

This can ultimately be done with very wide bandwidth, enabling measurements not

possible with a narrower band vector signal analyzer. Example FFT measurements

were able to verify that a linear FM chirp signal was shifting the carrier frequency

as it should. There also was a place for other math functions, namely the measurement

trend function. In this example, such a calculation allowed for a very simple verification

of a linear FM chirp.