EEVblog Electronics Community Forum
Electronics => RF, Microwave, Ham Radio => Topic started by: Ben321 on August 06, 2017, 05:53:55 am
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This usually is a technique applied to radar, but it also can be used with non-electromagnetic waves (mainly ultrasound, such as for hospital ultrasound imaging). I'm trying to figure out just who's phase what is being shifted (which signal is being delayed). Usually there's a carrier wave (one signal), being amplitude modulated by a pulse (another signal). So lets say you had microwave carrier wave with a frequency of 1GHz, and it was being amplitude modulated by a rectangular pulse waveform with an on time of 0.1ms (0.0001s) and an off time of 100ms (0.1s). If I wanted to steer the beam, which signal would I apply the delay to? Would I apply it to the carrier wave (a sinusoidal wave), or to the pulse wave (repeating rectangular pulse)?
Or would I apply the delay to both waveforms simultaneously, to make sure that the phase of the carrier wave relative to the start of the pulse was always the same?
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It is applied to your final signal you want to steer. Imagine generating non-steered signal, apply whatever modulation you like. Then emit the same same signal from multiple emitters with their phase shifted.
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The phase adjustment is applied to the carrier either before or after amplitude modulation.
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I'm not sure about ultrasound, but in radio location this will not work. Radars use modulation to increase distance resolution (often BPSK with modulation according to M-sequnces or Barker codes). If you apply modulation after the phase shift, then when you receive the signal back, you have only one option - to shift the whole thing, which will mean that modulation will not be in-phase at the summing stage.