Is it possible to build two-frequency distance measurement without swithcing?

For measurement of distance to moving objects FSK is widely used, where VCO constantly switches between two frequencies F1 and F2. (difference between F1 and F2 is about 1MHz) After calculating doppler shift and phase for F1 and F2, phase difference gives distance information. As i understand, it is possible, because when using single VCO it have same phase "starting point" for two frequencies.

Few years ago i've seen a paper where two old unmodulated CW radars used to measure distance. Today I searched for it, but no luck with that. I want to understand the setup of measurement equipment was used there.

As i remember, there was two CW radars with horn antennas. Radar frequency tuned so frequency difference was about 1MHz.

What i cant understand today, how their's setup was working? Isn't it necessary also measure phase difference between LO of each radars too in such case? Because in case of switching FSK VCO phase is still have single "starting point". But two separate radars would have separate oscillators, and their phase is not locked to any reference phase. And after mixing, doppler shift phases in each radar would also depend on their's RF phase?

I must confess that I don't even know what "old unmodulated CW radar" is. Classical radar is pulsed. The only fixed frequency CW applications I'm aware of are traffic speed control (and target lighting for surface-to-air missiles).

Theoretically, you can determine the phase shift of reflected wave for two frequencies separately, both oscillators don't need to be synchronized. The unambiguity distance range is of course limited depending on the frequency difference.

The practical problem is that the parasitic reflections and T/R crosstalk must be exactly known. Any change in it, or any additional target reflection would ruin the result. So it's effectively useless, I assume.

both oscillators don't need to be synchronized.

Click to expand...

I want understand why they do not need to be synchronized..
For example, we have two radars.
In mixer we have: sin α · sin β=(cos(α-β) - cos(α+β))/2
We filter out cos(α+β) and get doppler frequency IF: cos(α-β).

Assume for first radar we have oscillator phase shift X after powering on. For the second radar oscillator phase shift is Y.
As +x phase shift contained in both radiated α and reflected β signals, then
α-β+x-x = α-β.
And the same for second radar: α2-β2+y-y = α2-β2.
Am i right?

Is it feasible to make distance measurement to modulated scatter RFID this way?
Assuming I have one RFID wich sends data (modulated scatter)
Two RFID readers 1 and 2 placed in one enclosure, frequency difference between reader's LO is 2..3MHz
Then if

both oscillators don't need to be synchronized.

Click to expand...

it is possible to calculate distance to RFID scatter by comparing received signal phase shift between RFID reader 1 and 2.
As a bonus, we can measure distance to moving objects, as they provide doppler shift.

The practical problem is that the parasitic reflections and T/R crosstalk must be exactly known.

Click to expand...

But there are many movement sensors selling all over the world, and from datasheet i see it uses very simple VCO, and FMCW or FSK can be implemented by voltage on Vtune pins. Distance measurement application notes says that complex FFT of result for F1 and F2 is enough to get phase shift for each scatter.

Terminator3 said:

Is it feasible to make distance measurement to modulated scatter RFID this way?
Assuming I have one RFID wich sends data (modulated scatter)
Two RFID readers 1 and 2 placed in one enclosure, frequency difference between reader's LO is 2..3MHz
Then if
it is possible to calculate distance to RFID scatter by comparing received signal phase shift between RFID reader 1 and 2.
As a bonus, we can measure distance to moving objects, as they provide doppler shift.

But there are many movement sensors selling all over the world, and from datasheet i see it uses very simple VCO, and FMCW or FSK can be implemented by voltage on Vtune pins. Distance measurement application notes says that complex FFT of result for F1 and F2 is enough to get phase shift for each scatter.

Click to expand...

To obtain the target distance by "radar", there are two basic methods: pulsing the signal and determining the distance by pulse delay, or, using FM-CW method where the CW signal frequency is swept, so the delayed reflection multiplied by original signal generates the beat frequency proportional to target distance.
There are combinations of the above methods, using two frequencies instead of swept frequency is possible for a distance "window", to generate a beat frequency from reflected echo.

Read Skolnik's Radar book for details