Spectrograms are graphs which display signal power as a function of both time and frequency on a single plot. One axis represents time and another, frequency.

Colors or grayscale values are used to indicate magnitude. Recent enhancements to the mathematics which are used to produce spectrograms have greatly enhanced the image quality. This article uses the software package Spectro VSA from Constant Wave to create high resolution images of various wireless signals. What these images reveal can lead to a new understanding of wireless signals.

Bluetooth

Let’s start with Bluetooth 1.0. Figure 1 shows a spectrogram for Bluetooth. The horizontal axis is frequency running from 1.5 MHz below the carrier on the left, to 1.5 MHz above the carrier on the right. The carrier frequency is at 0 Hz, in the middle of the frequency axis. The vertical axis is time running from the bottom to the top. The bottom left corner of the plot is minimum time, minimum frequency. The colors represent the magnitude of the signal at a specific frequency and time.