Mixer as a phase detector for moving object?
For example, 1. we direct 5GHz beam to the object.
2. Object is not moving, reflects 5GHz back.
3. Then we do mixing and measure voltage. 0 Volts = phase of 0 degree
What will happen if object moves?
1. we direct 5GHz beam to the object.
2. Object is not moving, so some xxxHz doppler shift will occur, Then object reflects 5GHz+123Hz back
3. Then we do mixing and measure voltage? 0 Volts = phase of 0 degree? But there is also some phase shift because of doppler? My brain is overheating. I can't understed, please help, tell me what to read about it! =)
I see a couple mistakes in your thinking. For one, you will need a different type of mixer, like and I/Q mixer:
http://www.hittite.com/products/view...view/HMC525LC4
If i take this mixer it gives me if1 and if2. I know such mixers well, can be used to measure speed and distance using fmcw. I do not interested in fmcw. I need mfcw hint. Only when target moves. only good thing is full 360phase doppler frequency for two channel mixer. How do such mfcw (NOT fmcw) sensors get distance? i understand that two switching freqs and wavelength relation works. I cant understand hardware. What do they measure? Voltage of phase detector? But we already used intermediate frequency data in fft to know doppler. Where is the distance analog value? How it combined with traditional double balanced mixer?
The old style Doppler motion detectors use a simple diode as a mixer, and where the output is DC.
In this case I think is hard to find the relation between distance and output voltage. Perhaps you need a pulse modulation/demodulation to get the delay, like in the radar systems.
If I understand what you are saying, you step a frequency source in a number of discrete steps--dwelling at each step for some time period. If you target is relatively close and stationary, there is no beat note generated in the homodyne IF mixer. You instead have a discrete round trip phase shift for each frequency.
If you step in uniform step sizes, you can compute the distance to your target by using a group delay measurement T = (Δ Phase / Δ Frequency), where the phase is in radians, and the frequency is in radians/second, and the T is round trip time in seconds. For each pair of steps...you get another estimate of round-trip time.
To make unambiguous phase measurements, you need an I/Q mixer or some soft of ADC/FFT sampling system. You also need a small enough frequency step size to not have N2Π range ambiguities.
Thanks guys. I found the solution here: http://www.lib.utexas.edu/etd/d/2008...rsond72378.pdf
simultaneously transmits three different frequency tones. I thought it switching freqs.... Maybe it can be so for stationary.
Dear Sirs! I changed my thinking. And stuck in other way. I understand how this work with moving targets, but can't understand how it would be with stationary targets.
Moving target case:
We have Transmitter with two tones of tone1=5GHz and tone2=(5+DF)GHz
We have Reciever 2x double-balanced mixer. From each moving targed we will get doppler IQ for each tone. After complex FFT of IQ_tone_1 and IQ_tone_2 we get two frequencies and two phases. Then we get the things: Frequency -> speed. Phase1-phase2->formulas->distance.
Stationary target case:
We have Transmitter with two freqs of tone1=5GHz and tone2=(5+DF)GHz
We have reciever and I think we will get nothing. Actually we have multiple targets, because it is not ideal case. Then we get some interference of 5GHz with reflected 5GHz and the same for tone 2. So is that true that for stationary targets we can't measure two distances? And only can get one distance in ideal enviroment with IQ-voltage measurement?