Frequency Sweep Sonar System

Fred,

> 
> There's a very good reason that FMCW won't work for sonar although it may 
> work in air.  Volume Reverberation!
> At short range the receiver will be flooded with volume reverberation 
> signal.  So, how to get the nice mixing of target and near-field cross 
> coupling?  Very tough to do.  Likely not possible.
> 
> It is very common for pulsed CW sonar to be reverberation limited at the 
> receiver for short range targets.  After a time the volume reverberation has 
> decayed to a point where the receiver becomes noise-limited.  Of course, 
> this is for a broadband receiver.  But, isn't that what you'd use for FMCW? 
> If the receiver has a filter bank or FFT then you expect noise limited 
> performance except around zero doppler.  If the target isn't moving then 
> that's where it will be and you're back to being reverberation limited.  (By 
> "limited", I mean the "noise" floor in the channel is determined by this 
> component and determines the necessary signal level for detection).
> 

This is really insightful.  I would have never thought of this if I was 
designing a sonar system to be used under the water.  However, the 
system that I am designing is to be used in determining the distance to 
the sea surface.  It is a sonar used to be operated in the air medium.

Nicholas

The sonar system that I am designing is similar to your own, Vladimir.  
I am actually using the FMCW sweep to determine distance to the water 
surface.  It is an air-coupled sonar that is operated in the air medium.

Your solution of using analog mixing seems to be very good.  But I 
wonder (as academic problem) if this could be done completely using DSP.

Nicholas

Nicholas Kinar wrote:

> The sonar system that I am designing is similar to your own, Vladimir.  
> I am actually using the FMCW sweep to determine distance to the water 
> surface.  It is an air-coupled sonar that is operated in the air medium.

The other problem is the high attenuation of the ultrasound in the air.
For the moderately high frequencies (about 40kHz) the max. operating 
distance will be at the order of several meters.

> Your solution of using analog mixing seems to be very good.  But I 
> wonder (as academic problem) if this could be done completely using DSP.

Certainly it can be done with the DSP. However there is no need to use 
the heavy weight solutions for that problem.

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

http://www.abvolt.com

> The other problem is the high attenuation of the ultrasound in the air.
> For the moderately high frequencies (about 40kHz) the max. operating 
> distance will be at the order of several meters.
> 

The system is intended for application over distances on the order of 
several meters, so this is not a problem.  My center frequency is ~30 
kHz.

I have been trying the signal processing with a test system.  I have two 
transducers set up above a large piece of plywood.  The signal is 
produced in MATLAB and converted to analog with a DAC.  The signal is 
sampled at 96 kHz with ADC.  I can get a reflection, but there appears 
to be crosstalk.

Nicholas
  

Most active sonar systems have a gain control mechanism. One such
method is Time Varying Gain (TVG), where the gain is adjusted in a
fixed fashion over time - the signals are compensated later on for the
gain effect. Another alternative is to blank the receive hydrophones
to reduce the received reverberation/transmit leakage.

Either of these mechanisms is going to complicate the sonar circuitry.

Good luck.
Cheers,
Dave

In article <1171553917.740214.109420@m58g2000cwm.googlegroups.com>, 
david@array.ca says...
> Most active sonar systems have a gain control mechanism. One such
> method is Time Varying Gain (TVG), where the gain is adjusted in a
> fixed fashion over time - the signals are compensated later on for the
> gain effect. Another alternative is to blank the receive hydrophones
> to reduce the received reverberation/transmit leakage.
> 
> Either of these mechanisms is going to complicate the sonar circuitry.
> 
> Good luck.
> Cheers,
> Dave
> 
> 


Thanks, Dave.  I'll have to take this into consideration when designing 
the prototype system.  Once again, thank you!

Nicholas

On Feb 15, 6:12 pm, Nicholas Kinar <n.ki...@usask.ca> wrote:
> In article <1171553917.740214.109...@m58g2000cwm.googlegroups.com>,
> d...@array.ca says...
>
> > Most active sonar systems have a gain control mechanism. One such
> > method is Time Varying Gain (TVG), where the gain is adjusted in a
> > fixed fashion over time - the signals are compensated later on for the
> > gain effect. Another alternative is to blank the receive hydrophones
> > to reduce the received reverberation/transmit leakage.
>
> > Either of these mechanisms is going to complicate the sonar circuitry.
>
> > Good luck.
> > Cheers,
> > Dave
>
> Thanks, Dave.  I'll have to take this into consideration when designing
> the prototype system.  Once again, thank you!
>
> Nicholas

CWFM is used in simple radars and sonars because the output power -
can- be kept constant, avoiding the cost of gain varying amplifiers
and allowing lower peak power by measuring returned frequency
difference over a longer time interval. See an example:

http://www.st-andrews.ac.uk/~jcgl/Scots_Guide/RadCom/part15/page2.html

for graphs and discussion. The received signal is mixed with the
transmitted signal to produce the difference frequency where frequency
is proportional to distance. Transmitter crosstalk is DC term out of
the mixer and can be filtered out.

Dale B. Dalrymple
http://dbdimages.com