For constant frequency LO, does doppler phase changes when reciever moves?

[Terminator3]

When our reciever moves, all receiveng signals be shefted "left" or "right" on the spectrum depending on moving direction to or from the transmitter. That shift would depend on speed. The same situation if transmitter moves, or them both moving. I hope i understand it right.

So the problem here i can't understand is: how would be phase affected? If i compare pure CW sine on both sides, i am sure that phase difference between transmitter and reciever would change from 0deg to 360 deg each, for example, 20-50mm (depending on wavelength).
Does it mean, that when transmitter moves, not only frequency shifted by Fd (doppler frequency), but also phase changes according to position? Assumme constant speed, phase change is constant increment to real sine phase, and i think equal to frequency change. So is that phase change already "included" in Fd (doppler shift), or it somehow exists in receiving sine?

 

[jiripolivka]

When our reciever moves, all receiveng signals be shefted "left" or "right" on the spectrum depending on moving direction to or from the transmitter. That shift would depend on speed. The same situation if transmitter moves, or them both moving. I hope i understand it right.

So the problem here i can't understand is: how would be phase affected? If i compare pure CW sine on both sides, i am sure that phase difference between transmitter and reciever would change from 0deg to 360 deg each, for example, 20-50mm (depending on wavelength).
Does it mean, that when transmitter moves, not only frequency shifted by Fd (doppler frequency), but also phase changes according to position? Assumme constant speed, phase change is constant increment to real sine phase, and i think equal to frequency change. So is that phase change already "included" in Fd (doppler shift), or it somehow exists in receiving sine?

Yes, the phase varies, too. You can imagine the Doppler effect by visualizing a sinusoid (the signal) laying on the ground. If your receiver is stationary, it picks up the signal at one point. When the receiver moves toward the source, it picks up the signal faster due to this movement, and reverse.

There are Doppler sensors on the market with a Gunn oscillator as the transmitter, and two mixer diodes in the waveguide, separated by one-quarter wavelength. By comparing the phase of two mixers, one can determine the movement direction, too.

You can visualize the phase as a rotating vector. Due to Doppler effect, the vector rotates faster than "normal", or stationary state, if the receiver moves with respect to the source.

 

[biff44]

assumption: the frequency sources in transmitter and receiver are rock solid in frequency...ie G forces or shocks do not make the frequency move.

If TX and RX have a relative velocity between them, then Doppler theory predicts a shift in frequency depending on velocity and the transmit frequency.

If the vector velocity is a constant, then the frequency shift is a constant. A constant frequency difference is the same as saying a ramping phase shift.

 

[jiripolivka]

assumption: the frequency sources in transmitter and receiver are rock solid in frequency...ie G forces or shocks do not make the frequency move.

If TX and RX have a relative velocity between them, then Doppler theory predicts a shift in frequency depending on velocity and the transmit frequency.

If the vector velocity is a constant, then the frequency shift is a constant. A constant frequency difference is the same as saying a ramping phase shift.

Maybe you are right. Then the phase alone can be a measure of acceleration. The problem I can see is that the phase is a relative quantity requiring a reference. What is the reference in a moving system?

 

[Terminator3]

I can't understand this:
1) frequency is already have doppler shift Fd
2) then we have some phase "ramp" because of movement (phase=phase+k*time)
But this phase ramp "k" is equal to some frequency shift Fsome.
Does this mean that final frequency shift would be Fd+Fsome? I think it may be wrong.

Suggestion for frequency reference: passive RFID chip using incoming RF wave with certain frequency from moving object. As RFID chip does not produce any LO, it uses outside LO with some phase.

 

[jiripolivka]

I can't understand this:
1) frequency is already have doppler shift Fd
2) then we have some phase "ramp" because of movement (phase=phase+k*time)
But this phase ramp "k" is equal to some frequency shift Fsome.
Does this mean that final frequency shift would be Fd+Fsome? I think it may be wrong.

Suggestion for frequency reference: passive RFID chip using incoming RF wave with certain frequency from moving object. As RFID chip does not produce any LO, it uses outside LO with some phase.

I have designed a number of Doppler radars at mmwaves, from 8 to 94 GHz. Mostly they consisted of a Gunn or VCO , a coupler to feed the mixer, and a circulator to connect mixer RF port and oscillator main output to one antenna.
In such systems a moving tArget like a car return the transmitted signal with a delay proportional to distance, and a frequency shift proportional to target speed from or to the radar. Nobody ever was interested in any phase relations.
I saw Plessey Doppler sensors with two mixer diodes and simple electronics evaluating the phase shift of a return. If the phase shift between the two outputs goes positive, a target approaches, and vice versa. In a transmitter-receiver system I understand ow to evaluate frequency shift but how to evaluate phase ? Between what? A phase is a quantity referred to a reference. Where can I find any in such system?

 

[Terminator3]

I thought that moving object would generate frequency shift additional to doppler shift. After reading about FSK distance measurement it becomed more clear. Starting from two switching/simultaneous frequency it is possible to measure distance to the moving object. And the smaller frequency shift, the longer distances we can measure until phase difference wrap around. And, as jiripolivka already said, it is possible because there is a reference exists. So (maybe) actually moving object does not produce any additional frequency shift, only doppler shift. For example, if i get signal analyzer and measure sinusoid of frequency F+Fd in space, no matter how fast i switch between measuring points in space, frequency still would be F+Fd, not F+Fd+Fsome..... And only phase would differ in different points of space. And i did not get what biff44 said about ramp phase shift is the same thing as doppler frequency first time i read it, now it seems more clear.

 

[biff44]

Maybe you are right. Then the phase alone can be a measure of acceleration. The problem I can see is that the phase is a relative quantity requiring a reference. What is the reference in a moving system?

that is actually not true, there are methods to extract the absolute phase info.

and remember, phase is the time integral of frequency, and vice versa