EMC Question of the Week: December 14, 2020
Reducing the video bandwidth on an EMI test receiver or spectrum analyzer impacts the measurement in a manner similar to
- reducing the resolution bandwidth
- reducing the signal bandwidth
- adding an attenuator to the input
- averaging over several sweeps
Answer
The best answer is "d". The video bandwidth is essentially a low-pass filter on the detector output. It doesn't change the resolution bandwidth or the signal bandwidth. It essentially smooths rapidly varying amplitudes detected at any given frequency.
The figure shows two measurements of the same 10-MHz square wave. Both measurements are made with a resolution bandwidth of 1 MHz. The upper measurement was made with a video bandwidth of 1 MHz, and the lower measurement was made with a video bandwidth of 10 kHz. Note that the amplitude and bandwidth of the measured harmonics is unchanged. The noise floor, on the other hand, looks very different. Instead of rapidly fluctuating, the noise floor is smooth and relatively consistent from one sweep to the next. The average level has not changed, but it is much easier to distinguish the noise floor from the signal.
Most spectrum analyzers will set the video bandwidth equal to the resolution bandwidth by default. Reducing the video bandwidth can be a useful technique for smoothing the noise floor in situations where the measured signal is relatively constant.
Have a comment or question regarding this solution? We'd like to hear from you. Email us at
