Zero Response and DC Measurements

What is zero response?

When viewing frequency spans that start at 0 Hz (or very close to 0 Hz), a spectral line is usually visible at the extreme left of a spectrum analyzer's display. This is called zero response or LO feedthrough. In an FFT spectrum analyzer, zero response is caused by residual dc that originates in the analyzer's own input amplifiers. Zero response gives the illusion of a dc offset, even if the input signal has no dc component—and this occurs even if the analyzer has an ac-coupled input.

In the Keysight 89600 VSA software, some degree of zero response is always present in the 0 Hz bin (sometimes called the dc bin). The residual dc that causes this offset may leak into the first several bins as well. If you don't want to see any zero response on the VSA's display, simply start the frequency span several bins above 0 Hz.

Can spectrum analyzers measure DC?

Most spectrum analyzers are not intended to measure dc. However, analyzers such as the Keysight 89600 VSA can measure very low frequencies. The 89600 VSA can, in fact, measure dc, but not without including a dc offset of its own that can contribute to (or obscure) a dc offset in the input signal. As we mentioned, this internal offset is caused by residual dc that originates in the measurement hardware's input amplifiers. Thus, measurement performance at dc is not specified.