Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!iuvax!silver!commgrp
From: commgrp@silver.bacs.indiana.edu
Newsgroups: sci.electronics
Subject: Re: Hardware Correlators
Message-ID: <7200044@silver>
Date: 5 Dec 89 15:41:00 GMT
References: <1048@baird.cs.strath.ac.uk>
Organization: Indiana University CSCI, Bloomington
Lines: 52
Nf-ID: #R:baird.cs.strath.ac.uk:-104800:silver:7200044:000:1933
Nf-From: silver.bacs.indiana.edu!commgrp    Dec  5 10:41:00 1989




dcthomso@cs.strath.ac.uk (Duncan C Thomson IE87) writes:

>I am doing a final year project which will require correlation of an 
>(ultransonic) signal with a reference signal.  I am currently trying 
>to decide whether to do this in software or in hardware.

>Does anyone know of any hardware correlators?
>How fast are they?
>How much do they cost?
>Who manufactures them?
>Who supplies them (preferably in the UK?)
>Thanks in advance,
> -Duncan


One type of correlator can be made as follows:

       __     __     __         __     |\
in >--|__|---|__|---|__|- ... -|__|----|  \
           |      |____________________|   >----- out
           |___________________________|  / 
                                       |/

The input is an AC-coupled detected pulse train (e.g., a series of 
ultrasonic pulses converted to dc by a receiver).  Each box is an 
analog delay-line IC (Radio Sh** used to sell two-channel analog-
delay chips made by EG&G). The delay lines are clocked at n times the 
input pulse repetition frequency (prf), where n is the number of 
stages per delay line (typically 256).  The outputs of all the delays 
are summed in an op amp.

Repetitive signals marching through the delay line at the right prf 
are added; other signals and noise average to zero.  I don't remember 
the equations; I think signal/noise ratio improves as the square root 
of the number of delay-line taps (summer inputs).

The effect is that of a very narrow-band filter. The output is dc 
voltage.  Response time is slow, as it takes a while to fill the whole 
delay train with signal.

This interesting circuit was used in an inductive audio-frequency 
mine-rescue beacon receiver manufactured by General Instruments.  
Faster versions can be built with other types of delay line; some old 
radar receivers used acoustic delay-lines consisting of tubes filled 
with mercury.

--

Frank Reid   W9MKV    reid@gold.bacs.indiana.edu