3iMaJ,
I am not trying to measure any Doppler shift. I am looking for an object within a zone but there are lots of reflections from false targets--lots of metal in the area. I would like to take a background reading and create a window around the background. If there is a significant change within my zone of interest, then I know I have an object within the target zone that is not part of the 'noise'. I would like the circuit to be as small and simple as possible.
Edit:
I thought I was responding to 3iMaj, but his message just disappeared when I posted my reply [I just noticed that it was on page 1]. In response to some of the other comments, I am not going to transmit the data. The PIC will analyze the waveform to discern the presence of an object from noise within a target zone.
I actually have two different applications. The first is a pulse-echo application with lots of false targets. The second has a separate movable transmitter and the receiver needs to return an accurate time of flight that is consistent from location to location. I.E., it will always trigger on the same pulse# within the transmitted 40 KHz burst. In that case, I think I might look for the first decayed pulse which should indicate the end of the transmitted 40 KHz burst (unless it continues to ring at near-full amplitude for an inconsistent number of pulses). My concern with simply triggering is that the attenuation may be different from location to location and I would get inaccurate results using a leading pulse for a trigger (while the transmitter starts to resonate and builds up amplitude). I am not worried here about false targets because the first arrival will be the shortest distance. For this application, I want as accurate a distance measurement as possible. Each pulse means almost 1 cm difference in distance.
DSG