Resolving Phase Ambiguity in Pulsed Doppler Ultrasound

[BobP]

Hi.
I understand the principle behind Pulsed doppler ultrasound. If the echo from a transmit pulse is sampled (gated) during a narrow fixed time period, the echo will correspond to a particular depth. What I do not understand is how we can extract phase information by repeating the process of transmit-echo-transmit-echo over and over again.

Here is why (I am going to use an analogy to explain my problem).
If we have a door that is moving away from us then if we transmit-receive-transmit-receive... then each time we transmit our signal the door will be slightly further away so it will take longer to receive the reflection. Hence we get a phase shift.

If however we have a stack of thousands of doors flowing along a line and we measure the reflection from a distance, d, from our transmitter (as we do with pulsed ultrasound) we will be receiving the reflection from a different door each time (we are not following a single door) hence there should be no phase shift as the reflection will occur at the same time, each time.

Is my problem clear?
Thanks for your help

 

[tech99]

Hi.
I understand the principle behind Pulsed doppler ultrasound. If the echo from a transmit pulse is sampled (gated) during a narrow fixed time period, the echo will correspond to a particular depth. What I do not understand is how we can extract phase information by repeating the process of transmit-echo-transmit-echo over and over again.

Here is why (I am going to use an analogy to explain my problem).
If we have a door that is moving away from us then if we transmit-receive-transmit-receive... then each time we transmit our signal the door will be slightly further away so it will take longer to receive the reflection. Hence we get a phase shift.

If however we have a stack of thousands of doors flowing along a line and we measure the reflection from a distance, d, from our transmitter (as we do with pulsed ultrasound) we will be receiving the reflection from a different door each time (we are not following a single door) hence there should be no phase shift as the reflection will occur at the same time, each time.

Is my problem clear?
Thanks for your help

If a Doppler radar uses frequency to find velocity there is no ambiguity. If the radar measures phase, it can find distance, but there will be an ambiguity if the velocity creates a phase change of more than one cycle for each pulse. So it is for stationary or slow moving targets.