Steve Hardy wrote:

> > Using a pseudo random pattern will give better results with the
> > correlation though.
> >
> > This will mean that you can get away without the A/D, and that you
> > should be able to lock onto the carrier phase itself. Using the
> > longest code you can given your 10Mhz clockrate, the correlation
> > should reject most noise sources.
> >
> > If you decide to give it a try, please let us know how it turned out.
>
> This may work with other transducer types, however these units strongly
> resonate at 40KHz and nothing else.  Their bandwidth is very limited
> (+/- 100Hz) and thus it would be very difficult to make use of any
> finer level modulation of the signal, since the 200Hz bandwidth translates
> to a very poor distance resolution.  Also, the Tx is very close to the
> Rx owing to physical constrains - the Rx picks up the initial transmission
> which is much higher intensity than the reflected pulse.  This would
> tend to swamp the desired signal (unless an appropriate delay line and
> subtractor was used to cancel this source).
>
> I have actually used something similar to your suggestion, but in the
> field of IR communication.  In this case the transmitter sent out a
> 32-bit pattern (4 of the bits being '1', the rest zero) repetitively.
> The receiver could tune into the pattern being transmitted, even if other
> transmitters were in the vicinity (so long as the other transmitters
> used a different 32-bit code).
>
> Regards,
> SJH
> Canberra, Australia


Hello Steve .

I think there are another solution to measure distance with ultrasonic
Tx/Rx and PIC .

Define ACF ( AutoCorrellation Function ) as:
---
          M-1
Bn(X1,X2)=Sum X[i]*X[i-n] ;
          i=0

where M size of X vector , n is delay(shift distance) ,
X1[0..M-1] , X2[0..M-1] , X1[i=>M] always =0 , X2[i=>M] always =0 too .
---

Let X[]= (0,0,0,1,1,1,0,0,0) ; M=9

n=0 , B0 = 0*0 + 0*0 + 0*0 + 1*1 + 1*1 + 1*1 + 0*0 + 0*0 + 0*0 = 3 ;
n=1 , B1 = 0*0 + 0*0 + 0*1 + 1*1 + 1*1 + 1*0 + 0*0 + 0*0 + 0*0 = 2 ;
n=2 , B2 = 1 ;
n=3 , B3 = 0 ;
n>3 , Bn = 0 ;

At 50 .. 60 years Barker codes was discovered . I've hear only about
Barker codes with following positions quantity: 2,3,4,5,7,11,13

2: It's trivial case ;)
3: (1,1,-1)                             ACF=(3,0,-1)
4: (1,1,1,-1) or (1,1,-1,1)             ACF=(4,1,0,-1) or (4,-1,0,1)
5: (1,1,1,-1,1)                         ACF=(5,0,1,0,1)
7: (1,1,1,-1,-1,1,-1)                   ACF=(7,0,-1,0,-1,0,-1)
11:(1,1,1,-1,-1,-1,1,-1,-1,1,-1)        ACF=(11,0,-1,0,-1,0,-1,0,-1,0,-1)
13:(1,1,1,1,1,-1,-1,1,1,-1,1,-1,1)      ACF=(13,0,1,0,1,0,1,0,1,0,1,0,1)

Size of Burker code result to measure precision.

Send_Packet(Barker[k],0,0,0,0,0,...)
Recv_Packet(x,x,x,...,x,x,Barker[k])

So you send/receive signal with Barker modulation and search maximum of
ACF(Send_Packet,Recv_Packet). If you know n (in Bn ACF function) you can
measure distance .

WBR Dmitry.

P.S. Any additional information see at textbook about radiolocation .