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Create a Variety of Frequency Sweeps, Including Stepped Sweep Using Frequency Modulation

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Many function/arbitrary waveform generators provide a sweep capability - the ability to smoothly change from a specified start frequency to the specified stop frequency. The rate at which this change is made can usually be either linear or logarithmic, and setting the stop frequency either above or below the start frequency can vary the direction of the sweep.

However, some applications require a variation of frequency sweep. A common need is a stepped sweep, moving from the start frequency by a set incremental frequency until the stop frequency is reached. A stepped sweep, as well as more complex pattern sweeps, can be created using frequency modulation.

A linear sweep can be seen as frequency modulation with a ramp waveform. A positive ramp is used when the stop frequency is greater than the start frequency; otherwise a negative ramp is used. Frequency modulation uses carrier frequency (center) and deviation rather than start and stop frequency, requiring some "translation" between sweep and modulation parameters.

First, set up your basic waveform same as you would have for a sweep, by selecting the wave shape, amplitude, and offset. Next, enter the carrier frequency as the frequency of the waveform.

Carrier (center) = (start frequency + stop frequency) / 2

Turn on frequency modulation and set your source to internal (rather than external). Set your start and stop frequencies through your deviation:

Deviation = |start frequency - stop frequency| / 2

The modulating frequency is equal to your sweep rate.

Modulating Frequency = 1/ sweep time

Now select the type of sweep. An upward linear sweep is a positive ramp. A downward linear sweep is a negative ramp. A triangle wave will ramp the frequency in both directions. An upward logarithmic sweep is an exponential.

Using the arbitrary waveform features of your generator, you can add more choices to your sweep library. A stepped sweep is created with a staircase waveform, the number of steps equaling the number of discrete frequencies desired.

Figure 1

The arbitrary waveform shown in figure 1 can be used to create a stepped sweep. This particular waveform would output 9 discrete frequencies from the start to stop frequency. Adjustments to the width of these steps will change the relative dwell time (duration) of each frequency. Adjustments to the height of these steps will modify which frequencies are output. A pattern sweep can be created by independently adjusting the dwell and frequency for each step. A good example of creating a pattern sweep is the generation of music. For more details see the technical note: Create Music using Frequency Modulation