spectum analyzer display of 2 targets in fmcw example

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Philip
Philip 2014-12-29
Hello, I am modifying the fmcw example for 2 targets, I have made the modifications all the to the end of the sweep.
I have included all the parameters for the second target, ie speed, distance and so on. Here is the section I am having difficult with. Starting at the spec. analyzer, the code reads
hspec = dsp.SpectrumAnalyzer('SampleRate',fs,...
'PlotAsTwoSidedSpectrum',true,...
'Title','Spectrum for received and dechirped signal',...
'ShowLegend',true);
Nsweep = 64;
% assign array for second target,
start sweep ie move radar and targets, tx, propogate signal and rx signal.
2 signal are propogated
xt = step(hchannel,xt,radar_pos,tgt_pos,...
radar_vel,tgt_vel);
and xt_2nd = step(hchannel,xt,radar_pos,tgt_pos_2nd,...
radar_vel,tgt_vel_2nd);
% 2 signals 'xt' and 'xt_2nd' are rx, ,
% both signals 'xt' and 'xt_2nd' are dechirp to xd and xd_2nd
% visualize signal
step (hspec, [xt xd and xd_2nd])
% both signals are buffered
xr(:,m) = xd;
xr_2nd(:,m) = xd_2nd;
% end of sweep.
Question: I can see 3 channesl being displayed in the spectrum analyzer. However the dechirp signal for the second target
is coincident with the dechirp signal of th first target. I have played with different settings of the speed, distance for the second target as well as range resolution for the system overall to no avail. The second target is placed at 25 meters, different
speed (much faster) and same or smaller rcs. I am probably not applying the way a 2 target simulation that should be done
or the way the dechirp signal should be handled. I am ignoring multi-path/multi-bounce issues and doing a straight reception of
2 received signals from 1 source of transmission, arriving at different times with different doppler shifts after dechirp.
Not getting the expected shift in the SA display, both chirps are at the same frequency shift. Puzzled ?? :(

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