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I've wondered for a long time how phase information is captured for wireless signals. It's used in applications like QAM for decoding symbols and radar beam forming to to locate targets, but I've never been clear on how we can get phase information from voltages measured as a time series of real numbers.

Some signals from medical applications like MEG and EEG do not have phase information. What is the difference between such signals and radio signals? Does it have to do with being able to make certain assumptions about the nature of the signals being sinusoidal?

Moppy
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    Some radars use I/Q mixers during receive. In this case there would be two mixers and two ADC's. So these basically generate a series of complex numbers (not real valued numbers), so each sample, in effect, has a phase value. Some radars use signal processing (I think it is a Hilbert transform) to convert the real valued series to a complex series. Once it is a complex series, each complex sample has a specific phase value. – user57037 May 19 '17 at 03:55
  • I'm not at all sure what you might be talking about in your question. – Andy aka May 19 '17 at 11:15
  • I think this is what you're after: https://en.wikipedia.org/wiki/Carrier_recovery Once you have the carrier, you can compare the signal vs the carrier to determine the phase difference. Obviously, sending a strong carrier to the receiver is the easiest way of recovering the carrier. – horta May 19 '17 at 19:54
  • In radar signal processing the receiver surely knows, what the transmitter sends (in CW radar) or sended (in pulsed radar). So, there's no lack of the reference. If the radar uses a complex modulated signal, the receiver surely knows also the pure non-modulated signals which are or were used to form the transmitted signal. BTW. The beamforming in the antenna is based on phase differences even in the oldest style antennas that have a single horn radiator and a parabolic reflector –  May 19 '17 at 20:40
  • @user287001, in a heterodyne (CW) radar, you typically feed the mixer output to a LPF then sample. I have worked with these radars a bit. So you end up with a series of digital samples. Where does phase emerge from a sequence of digital samples? It is in this case that you convert the series to the analytic signal, which is a sequence of complex numbers, and has phase. With a typical pulsed radar, I believe you would use an I/Q mixer. So you would be directly sampling complex numbers from the get-go. You would increment frequency, pulse, record I and Q, then repeat. – user57037 May 21 '17 at 02:44
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    @mkeith You're right about what a pulsed radar "typically" does. The radar will do an I and Q demodulation and sample each branch. From here you can get your phase and frequency information for Doppler/MTI processing. – Envidia May 23 '17 at 17:10

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Radar beam forming is performed by feeding an array of small aperture antennas at different phases of the RF carrier. The phase shifting can be accomplished by digital processing, lumped circuits, spacial positioning, or delay lines formed from various electrical lengths of transmission lines. Phase shifting is the basis of nearly every multi element antenna. Phase shifting is also generally reciprocal for receiving and transmitting on the same antenna.

So generally we are not interested in the exact phase of an RF signal but rather its relative phase to another path of the same signal. This extends beyond antennas to include selective fading, multipath distortions, standing waves and other topics.

QAM is a different beast. The QAM signal modulates a carrier frequency. So the phase of the carrier is not of interest for decoding.

Radio signals propagate due to the accelerating nature of the sinusoidal waveform and because they are composed of an electric and magnetic field at right angles to one another.

Glenn W9IQ
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  • In QAM, the Quadrature must be identified separately from the Inphase – analogsystemsrf May 19 '17 at 19:16
  • +1 for elaborative answer, one side comment: with in wavelength limit, one can extract also the absolute phase. But as @Glenn mentioned for beamforming we are more concerned about relative phase between antenna elements rather than absolute phase. By the way, the absolute phase of radar signal can provide you with the range of target within wavelength limit (which is very very small in case of mmWave RADAR) – Zeeshan Jul 28 '17 at 06:03
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Its right in the equation for QAM

$$r(t) =s(t)\cos{i2\pi f_0t}$$ Where

$$s(t) = Re{[I(t)+iQ(t)]\exp^{i2\pi f_0t}}$$

So you get

$$r(t) = I(t)\cos(2\pi f_0t)\cos(2\pi f_0t) - Q(t)\sin(2\pi f_0t)\cos(2\pi f_0t) $$

So your not really measuring the phase of the modulated signal but check for orthognality against the carrier. That means you have to know what your modulation frequency is and then you can find the properties of the amplitude and phase. The values of phase are orthognal to one another which makes them easier to detect.

Voltage Spike
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