Ti millimeter wave radar learning (I)

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TI Millimeter wave radar learning （ One ）

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Preface

Starting today, I will update the article on millimeter wave radar synchronously , Now follow TI The official video of , You can see it by yourself , All pictures in this article are also taken from TI Official video , Links are as follows http://training.eeworld.com.cn/TI/video/10162

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FMCW

FMCW: Frequency-modulated continuous wave
effect ： Measure the distance between the objects in front 、 Speed and angle of arrival .

How does radar estimate the distance between it and objects ？

FMCW The core of radar is the signal of linear frequency modulation pulse .
LFM pulse is a sine wave whose frequency increases linearly with time

One FMCW The radar has a single TX And single RX The antenna .
working principle ： The synthesizer generates a linear frequency modulation pulse .TX Send out the linear pulse , Reflect back after meeting an object ,RX Receive reflected pulses , When RX and TX When the signals are mixed , The generated signal is called IF（IF Indicates if signal ）

When is the mixer ？

Mixer has two inputs and one output , Input sine wave to the two input ports of the mixer , The sine wave output by the mixer has the properties of these two sine waves
1. Instantaneous output frequency = Sine wave 1 Input frequency - Sine wave 2 Input frequency
2. The starting position = Sine wave 1 phase - Sine wave 2 phase

When there is only one object in front of the radar , The received signal is the delay of the input signal , Same shape .
namely IF The frequency does not change , stay f-t There is a horizontal line in the image .

t For round-trip delay ,t=2d/c.
Be careful ： When using ADC when , Make sure IF The signal is only in RX Only when the antenna receives the reflected signal , That is, only after time t Then receive the sample （ What I received before must be clutter ）, And it can only last until TX Before disappearing .

Because the frequency modulation time is very short , about 300m distance , And pulse time Tc Only for 40 Microsecond radar , be t/Tc=5%

FMCW Fourier changes in

Fourier transform in FMCW It's very important , We use it to estimate the distance 、 Speed 、 angle .

Fourier transform transforms time domain signals into frequency domain , Fourier transform can refer to this easy to understand article , Fourier transform .

If your so-called God is a human shape , I don't think so . If you ask if there is a creator , I think so . Because the structure of the whole world is not accidental .—— Yang Zhenning

As mentioned above, there is an object in front of the radar , When there are multiple objects in front of the radar , The situation is as follows .

The farther the distance , Corresponding IF The bigger it is , The following figure is the corresponding of three objects IF spectrum .

When two sine waves are very close （ When the reflected signals of two objects are very close ）, We can extend IF The length of the signal is used to expand the observation window of the sine wave , By extending the time of LFM pulse, we can expand IF.
Thus we can see that , Larger bandwidth may have better range resolution , This is the same as our perception , The longer the test takes , The more you can distinguish the distance between objects .

S=B/T（S It's the slope ,Tc Is the time width of radar frequency modulation ,B Is the frequency width of radar coverage ）
The distance between the two is Δd The object
Δf=S2Δd/c（Δf Is the frequency interval between two objects ）
Δf>1/Tc
From the above two formulas
Δd>c/2B What does this formula mean ： The distance between two objects in front of the radar is greater than c/2B

Therefore, it can be estimated that the distance between two objects is d=c/2B
It can be seen that the range resolution only depends on the bandwidth .
Like to see , Theoretically, two bandwidths B The same radar , Their range resolution should be the same , But actually , The radar with longer pulse duration , Have longer IF Signal to observation window , Consider the properties of Fourier transform , We will choose radar A.

Radar processes signals
In most radars , Radar will process the signal , First, low-pass filtering , then ADC Go digital , Then it is sent to the appropriate processor , for example DSP,DSP First, Fourier transform , To estimate the distance of objects , Then carry out other treatment , Used to estimate speed and angle of arrival .

When digitizing the target , We need to know the target bandwidth , Easy to set low-pass filter and ADC Sampling rate .

The cut-off frequency of the low-pass filter needs to be higher than max frequency , And ADC Sampling rate should be >= The max frequency

When two radars enter , From the above formula ,A And the slope of that is B Twice as many , that B Of f(IFmax) The value of is also A Twice as many , In this way dmax With the same value ,A Only half IF bandwidth , That is, a smaller sampling rate , however B Only half the sampling time is needed .

dmax=Fs*c/s(Fs yes ADC Sampling rate )

For RF bandwidth and IF Bandwidth we need to distinguish clearly , Range resolution depends on RF bandwidth ,IF Bandwidth allows the radar to see a wider range .

summary

I will study and record later FFT（ Fast Fourier transform ） Such as knowledge , Thank you very much for watching .