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My understanding of Pulse Doppler Radar is the system measures Time Of Flight for the pulse.

My confusion is that during the down conversion process prior to ADC the lowpass filter causes some time delay in addition to the time of flight. In principle the delay should be known beforehand but I'm sure it's not exact and in general the delay is not completely constant even among the band of interest.

How is this delay dealt with?

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  • $\begingroup$ So the analog filter is nonlinear phase and the group delay is not constant... is that the problem ? $\endgroup$ – Fat32 Dec 11 '19 at 2:44
  • $\begingroup$ Yes, as it impacts the time the pulse spends going through the lowpass filter prior to being sampled. TOF measurements are based upon knowing how long it takes the pulse took to travel to calculate range. $\endgroup$ – FourierFlux Dec 11 '19 at 3:21
  • $\begingroup$ Sorry for my lack of radar terminology, but If it's not a moving target, you can simply callibrate the filter delay by measuring it against a known distance. For a moving target, the pulse shape changes (Doppler shift) then the exact delay will depend on the target velocity, but I still guess the minimum and maximum delays can yield some bounds on the delay estimation... $\endgroup$ – Fat32 Dec 11 '19 at 3:37
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It is dealt by calibration which requires having a good characterization of the filter, as Stanley mentioned.

You asked about isolation:

There are different implementations of how saturation is avoided during transmission in pulse-Doppler radar, however they virtually all involve physically isolating the receiver. The time while transmitting is your effective pulse width.

There's the additional time of filtering the received pulse. Here is where the filter must be taken into account for measuring range: the pulse width and this extra processing time from the filter determine the minimum measurable range, called the blind range. Many times the filter processing time is much less than your pulse width and does not contribute to the blind range significantly. This is less the case when you have slower processors and shorter pulse widths, where the filtering processing time can be well within the order of your pulse with.

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  • $\begingroup$ The blind time is related to relevant time length of the pulse right? So the receiver has to be turned off until the main part of the transmitted pulse has been radiated. I assumed an adaptive element/subtraction would work but you would probably saturate your rx amplifer. $\endgroup$ – FourierFlux Dec 13 '19 at 19:41
  • $\begingroup$ Yes, the blind time is essentially your pulse width since the receiver is isolated until the pulse is radiated as you mentioned. Can you elaborate on using an adaptive element? Why do you think that would work? $\endgroup$ – Envidia Dec 13 '19 at 19:49
  • $\begingroup$ I was thinking you could somehow remove the pulse either in the analog domain(like invert and sum prior to amplification) or through DSP but there are dynamic range limits which may prevent this. I'm confused on how Continuous Wave radar work though since they never shut off their transmitter(or at least that's how I understand them). $\endgroup$ – FourierFlux Dec 13 '19 at 22:02
  • $\begingroup$ Maybe I'm not understanding at what point you want to do this "cancelling". Is this on transmission or reception? Also, for FMCW, there is still some isolation but it is not as good as it is in pulse-Doppler systems. Pulsed-Doppler systems can afford to implement good isolation at the cost of having that blind range. FMCW radars accept some feed-through into the receiver but sacrifice dynamic range to remove the blind range limitation. $\endgroup$ – Envidia Dec 13 '19 at 22:22
  • $\begingroup$ Instead of shutting off the receiver you feed through the transmitted signal to the receiver and do some operation to cancel out the immediate pulse or wave which is picked up at the antenna. This could be done on both the pulse and CW radars. (Wikipedia mentions something about this). If this isn't done how would the FMCW radars even work? I see fairly cheap marine ones, etc which aren't pulse based yet the transmitter and receiver are located on the same device which seems like it would saturate the receiver(so obviously something is done). $\endgroup$ – FourierFlux Dec 13 '19 at 22:30
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The delay is dealt with by knowing it. It is calculated and measured and any residual error is minuscule compared with any other uncertainty.

It’s dealt with design and calibration.

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  • $\begingroup$ Ok, I thought there might be something else that is done. Additionally how is the initial pulse stopped from saturating the receiver? Assuming the transmitter/receiver is from the same antenna or they are close it seems it would always be saturated by this pulse. I could see an adaptive element being used to cancel out the initial pulse, maybe. Is there another way? $\endgroup$ – FourierFlux Dec 11 '19 at 4:54
  • $\begingroup$ @FourierFlux Saturation is typically avoided by using an isolation switch $\endgroup$ – user28715 Dec 11 '19 at 5:00

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