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What Is a Sweep Generator?

Geisha A. Legazpi
Geisha A. Legazpi

A sweep generator is an electronic test generator that creates a periodic sawtooth waveform intended to modify the output of a second signal generator, which is usually a radio frequency (RF) generator. The periodic sawtooth waveform rises to a peak level, then rapidly drops to a low level then repeats the process. A sweep generator output may be used to control the frequency output of a signal generator to generate a sweep-frequency output. In general, a sweep generator allows a testing setup to almost simultaneously measure the response of devices within a span of frequencies or a frequency range.

Sweep generators are used for testing the frequency response over a range of frequencies. Whenever RF circuits or electrical circuits have a specific frequency response, sweep generators can generate the test signal that will cover the specific frequency range. The basic sine wave or sinusoidal wave is a periodically time-changing voltage that cycles smoothly from zero to positive peak, then into zero, then negative peak, and back into zero. A complete cycle will have two zero points and two peak points, which are positive and negative.

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Man with a drill

Frequency domain description is used when the device tested is meant to operate over a certain range of frequencies. The frequency range of a device can be evaluated by a sweep generator and a spectrum analyzer, the corresponding test equipment. This has a two-dimensional screen showing both vertical power levels and a horizontal frequency reading. When monitoring a signal with the spectrum analyzer, the vertical axis is still amplitude, in watts (W), but the horizontal axis is frequency, in hertz (Hz) instead of time units.

In an amplifier with transistor circuits, inductors, and capacitors, there is an expected gain or net amplification over a range of frequencies. For an RF power amplifier, the gain is observed by measuring the input to the amplifier. If the output is 1 W while the input is 0.01 W, there is a gain of 100. Given that the center frequency is actually 150 megahertz (MHz), a usual next parameter in question is bandwidth, which is the range of frequencies below and above the center where the gain is half of the gain at center frequency. This is also referred to as “–3dB” points, where –3dB refers to half power gain.

If the lower –3dB point is 148 MHz and the upper –3dB point is 152 MHz, the power amplifier is referred to as having a frequency range of 148–152 MHz. The sweep generator can then be set so that its lower frequency is 148 MHz. The upper level of the sweep generator causes 152 MHz at the signal generator output. If the sawtooth generator inside the sweep generator is turned off, the output of the signal generator will stop sweeping the 148–152 MHz range. Without the sweep generator, the signal generator output will be at fixed frequency of about 150 MHz.

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