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How to calculate beat signal frequency from delay?

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tony_lth

tony_lth

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I am curious about the beat signal frequency.
Assuming a simple radar system.
Let's say RF freq is 100M, so its period is 10ns.
LO has the same freq, i.e. 100M, but LO phase is 500ps delayed, that is 1/20 of period.
AFter the mixer, get the beat signal,
1. so how to calculate the beat signal freq from above data?
This is just help me to understand the radar beat signal, not a live system.
2. How this beat signal depend on the phase noise of LO?
Thanks.
 

Hi,

I guess you're talking about CWFM radars where sending sine wave signal and the beating frequency is a function of the distance to the object.
https://en.wikipedia.org/wiki/Continuous-wave_radar

Notice that you can detect multiple objects simultaneously - they will create different beating frequencies. A FFT is the best way to detect the objects and to estimate the distance to them.
 
tony_lth said:
I am curious about the beat signal frequency.
Assuming a simple radar system.
Let's say RF freq is 100M, so its period is 10ns.
LO has the same freq, i.e. 100M, but LO phase is 500ps delayed, that is 1/20 of period.
AFter the mixer, get the beat signal,
1. so how to calculate the beat signal freq from above data?
This is just help me to understand the radar beat signal, not a live system.
2. How this beat signal depend on the phase noise of LO?
Thanks.
Click to expand...

2. Except of very poor LO phase noise, it will not affect the results as the beat frequency is "cleaned" by integration. Range error is defined without any component effect.
 
Hi, luben111,
From your given link, there is no equation for calculate the beat signal, could you give more details or other links for beat signal calculation?
Best,

- - - Updated - - -

Hi, jiripolivka,
Is there any equation to describe how the poor phase noise can mask the beat signal?
I need equation to calculate to assuare that phase noise can't impact on beat signal.
Thanks.
 

**broken link removed**
 
See this article - it has the formula and good explanation
 

Attachments

  • Frequecy Modulated Continuous Wave (FMCW) Radar.pdf
    1.7 MB · Views: 172
The original post is talking about fixed frequency, not FMCW. You don't get a beat frequency by mixing signals of same frequency.
 
Hi FvM

I agree with you - in case of fixed frequency he needs just to check for Doppler radar for beating frequency
https://en.wikipedia.org/wiki/Doppler_radar

But in case of fixed frequency the only one thing he could get from beating frequency is the relative speed of the object. In case of FMCW radars he gets the distances and speeds for multiple objects which is more attractive.

Luben
 
Hi, FvM and luben111 and jiri,
Thank you very much.
Now I have some clear idea about FMCW.
I will think about how to do it.
One more thing about the license:
From China requirements, it seems that FMCW can work at 2400-2483.5MHz or 24-24.25GHz without license approval.
Are these band wide enough for FMCW (83M@2.4GHz and 250M@24GHz)?
And are these band free in other country when using micro-power and short-distance radio equipments?
 

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