Understanding of Doppler Shift

[hamid159]

Hi Guys,
One thing is confusing me. Let say I have a sine signal of frequency 'Fc' and I transmit it. It comes back bouncing from a moving object (Lets assume human's heart). I am assuming environment free of noise for the time. Then Would the coming signal be sin(2*pi*Fn*t + theta) where Fn = Fc + Fd ?
Where theta is phase delay and Fd is change in frequency.
Right?
Fd (Change in frequency) would be variable or constant?
Or If I consider noise what would be approximate receiving signal?

Any help would be appreciated.
Thanks!

 

[AMK1971]

Doppler is dependent on the relative velocity (rate of change of distance between the two objects) so it will not be a constant.

but if you use a higher frequency and reduce the observation time significantly (say in microseconds) the Doppler could be taken as constant for that observation period.

With noise, the signal received will be sin(2*pi*Fn*t + theta)+N0 where N0 is noise.

 

[D.A.(Tony)Stewart]

THe relative shift in carrier frequency is equal to the ratio of relative velocity to the propagation speed in the medium. i.e ultrasound is slower than light so more sensitive. Thus the bandwidth and SNR of your signal affects the result. THermal noise and EMI are dominant factors.

 

[hamid159]

Doppler is dependent on the relative velocity (rate of change of distance between the two objects) so it will not be a constant.

Let consider, rate of change in velocity constant. That is the rate with which distance is changing is constant (probably will) then Won't Fn be (Fc + 1.5) in one direction and (Fc - 1.5) in another (considering the change in frequency is 1.5 Hz in one side)?
What would exactly be the signal at receiving end?
Thanks!

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Thanks SunnySkyguy.
So, the receiving frequency would not be constant?
If I subtract the doppler shifted signal (Fn) from original signal (Fc), Can I get the doppler shift (Fd)? Assuming no phase delay and noise.

 

[Warpspeed]

I assume you will be using ultrasound for this.

Your received frequency will swing either side of the transmitted frequency as the direction of heart motion changes.
The problem will be that the heart is not a simple single reflective moving plane, but moves in three dimensions, and all four heart chambers (and respective valves) are doing very different and complex motions together at any instant in time.

So I would expect the overall reflected wave will be a confused jumble of many frequency components, some will be additive, and others cancel.

I am sure you could define overall heart rate from that mess, but I doubt if you could resolve much more detail than that.

 

[hamid159]

The problem will be that the heart is not a simple single reflective moving plane

So, it will be a jumble of frequencies between (Fc + 1.5) and (Fc - 1.5). All the frequencies will lie in this range. Right?
What I want is just to detect whether a man is alive under the rubble. I think it will give me at least detection signal to detect alive human. Isn't it?

 

[Warpspeed]

How exactly do you plan to feed and receive ultrasound into a buried victim from a distance ?

Medical ultrasound always requires direct intimate contact to the skin with an ultrasound coupling jelly applied, otherwise all the sound just reflects off the skin surface.

It all works on reflections caused by changing transmission velocity through various tissue density, and the biggest most reflective surface of all is the air/skin interface.
Sorry, but no air allowed between your transducer and the patients skin.

If you can actually see and touch the patient just light finger pressure on any exposed skin will quickly tell you if blood circulation is still functional.
The white area quickly goes back to pink.

What you really need for finding buried disaster victims is a dog !

 

[FvM]

I'm not sure if manid159 is elaborating an ultrasonic project in parallel, the original project was dealing with radio waves. https://www.edaboard.com/threads/345704/#post1475511

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The original thread also briefly explains the project background.

 

[hamid159]

I'm not sure if manid159 is elaborating an ultrasonic project in parallel, the original project was dealing with radio waves.

Yes FvM. I am still using radio waves. I wanted to confirm here about the general understanding of doppler shift.

 

[FvM]

I believe, the expectable back scattered spectrum has been already explained in your previous thread, for the case of a 1.5 Hz heart rate:

what you get is (...) the spectrum of a 1.5 Hz phase modulation, with sidebands at multiples of 1.5 Hz and a distribution depending on the (probably low) modulation index.

Put in an expected displacement, translate it to phase shift and get the modulation spectrum.

 

[hamid159]

Put in an expected displacement, translate it to phase shift and get the modulation spectrum.

Can we get change in signal due to doppler shift (only Fd) by subtracting receiving signal with original one? Assuming that I have equalized the phase of both signals by some means.

 

[Warpspeed]

This has already been done.
https://www.wired.co.uk/news/archive/2013-09/11/heartbeat-rubble-sensor

Its probably derived from the ubiquitous 10 Ghz microwave motion detectors used in security systems which detect human movement quite well at a distance.

The spooks and law enforcement also have these systems for detecting the presence of people lurking within buildings.

 

[FvM]

Can we get change in signal due to doppler shift (only Fd) by subtracting receiving signal with original one?

Substraction isn't the right way. The modulation is mixed down to baseband and separated from the demodulated carrier by a high-pass. A quadrature mixer is preferred because it allows to detect frequency modulated signals specifically.

 

[Warpspeed]

The commercial 10Ghz motion detectors I have seen have two cavities.
A transmit cavity with a Gunn diode oscillator, and a receive cavity with a pickup link and a simple detector diode.

The diode detects bulk reflected radiation at the carrier frequency as dc, and any doppler return appears as a faint low frequency ripple on top of that.
Ac couple that to a high gain amplifier with an active low pass filter (a few hertz) and you have it.

The front of commercial motion detectors usually has a primitive cast metal horn antenna held on by two screws.
A much larger narrower horn should increase the sensitivity, and make it much more directional.

Pictures I have seen of the law enforcement version (FBI) are a quite small hand held box with a much larger horn antenna.