The next part of this project was to understand what exactly is a FMCW Radar?

In a Frequency-Modulated Continuous Wave (FMCW) Radar, a signal is transmitted in which its frequency increases with time, this is known as "chirp"

“Chirpiness” is the rate of change of the instantaneous frequency.

The same concept applies and the signal is reflected off a target and comes back delayed.

Both signals are compared against each other the transmitted and received. This is done using a mixer. The difference between the two is called the “beat frequency”

This is done because frequency is changing with time, and the two frequencies the one transmitted and received are slightly different. This beat frequency lets you compute range.

The closer the target is the lower the beat frequency, and the farther the target is, the higher the beat frequency.

What do we do with this?

Fast Fourier Transform (FFT) transfers this data from the time domain into the frequency domain. This is done on the radar. The beat frequency is sampled, runs an FFT, and each FFT bin corresponds to a distance.

This produces a range of spectrum:

Peaks = targets

Peak position = distance

With motion as we discussed before, this gives rise to the Doppler effect. When the target is moving the beat frequency shifts slightly, this gives the velocity. Doing so and we process these multiple chirps as the radar is doing, the distance is derived and so the velocity from phase change over time.

I/Q signal preserve direction and phase.

This is what the radar outputs:

I (in-phase)

Q (quadrature)

In a radar system they are 90 degrees out from each other. These are fundamental components that are used to capture amplitude, phase and frequency of the radar echo.

In-phase (I) signal

This represents the amplitude of the received signal that is in phase with the original transmitted carrier signal. Convection is a cosine wave.

Quadrature (Q) signal

This represents the amplitude of the received signal that is 90 degrees (π / 2) radians out of phase with the original carrier signal (by convection, sine wave)

Together I and Q signals come together and allow the radars digital signal processor to treat it as a complex number.

A FMCW radar turns time delay into frequency, then frequency into distance.

An FMCW radar transmits a frequency-swept signal, compares the received echo with the current transmit signal, and converts the time delay of the echo into a measurable low-frequency tone. The frequency of this tone gives distance, and changes in its phase over time give velocity.

Once this base knowledge has been established we can utilise off the shelf components and are able to start trialling and building a basic radar before proceeding to hardware development. As much as it is really exciting to start drawing out a schematic and such, it is important to validate first and know what you are actually going to design.