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작성자 키트 작성일2017-08-30 16:31 조회1,359회 댓글0건

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6 GHz frequency modulated radar

2014-12-02

In Electronics.

2093095301_1uGjrdoF_thumb.ready.jpg

Finished radar board without antennas.

Introduction

I've for some time now wanted to do more RF design. Although I have taken some RF design courses, I haven't actually made a single RF design before. But you can't learn without doing and inspired by the MIT coffee can radar designed by Gregory Charvat, I figured that building a radar should be a doable project that would offer some challenge while also having some real world use.

The simplest radar is a continuous wave Doppler radar, which continuously transmit a constant frequency signal. This signal reflects from a moving target and Doppler shift causes reflected signal to change frequency. This reflected signal is then received and mixed with the transmitted signal. Mixer product is the difference of the frequencies which is proportional to the speed of the target. This kind of radar is very simple to make, in fact there are even some children's toys. Unfortunately it can't detect the range of the target and isn't that exiting.

A little more sophisticated radar which can detect also the range can be made by modulating the frequency of the transmitted signal. This kind of radar is called continuous wave frequency modulated radar (FMCW radar).

Radar transmits a chirp, which increases linearly with the frequency. This chirp is then radiated with the antenna, it reflects from the target and is received by the receiving antenna. On the reception side received and undelayed copy transmitted chirps go to the mixer. Because received chirp reflected from the target, but copy of the transmitted chirp is undelayed there is delay between the signals. This causes mixer output to have a low frequency signal which frequency depends on the distance to the target. When there are several targets the output signal is sum of different frequencies and the distances to the targets can be recovered with Fourier transform.

2093095301_OH0SwfCN_block.png

FMCW radar block diagram

Glossary of the terms in the block diagram:

  • MCU - Microcontroller
  • VCO - Voltage controller oscillator
  • PA - Power amplifier
  • LNA - Low noise amplifier
  • ADC - Analog to digital converter
  • FFT - Fast Fourier transform

Transmitted signal is: f(t)=sin(2π(f0+f1trampt+f0)t) and the received signal is a delayed copy of it, f(ttd). Mixer multiplies these signals, outputting two frequencies which are the sum and difference of the multiplied signals. These can be solved using the trigonometric identity: sinϕsinθ=12(cos(ϕθ)cos(ϕ+θ)). Sum frequency is too high to be detected and is filtered out. Frequency of the difference term gives: fdiff=td(f

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