www.piclist.com/techref/io/atod.htm?key=analog

BY : Olin Lathrop email (remove spam text)

> I know what digital delay lines are (and how they can be built cheaply

from

> slow logic gates), but I am not familiar with analogue delay lines. What's

> inside these black boxes (literally, they are black and square) I know

that

> RC/RL circuits introduce phase shifts, is this how they work. I've noticed

> that one such device is represented on the schematic with a tapped

inductor

> symbol. It's hard to imagine why they are so hard to get hold of.

The R/L type of analog delay line is an attempt to approximate a

transmission line with discrete components. Think of an L in series,

followed by a C accross, followed by another L in series, followed by

another C accross... Now do the usual math hand waving of making an infinte

number of these, but making each infinitely small (in other words, you

picture the integral instead of a finite summation). That's a transmission

line. The total L and C over a fixed length defines the transmission line

impedance. It also defines the propagation speed. Series R also enters

into this, but can be ignored for the kinds of purposes we are talking about

here.

A rough rule of thumb is that the propagation speed of realizable

transmission lines is about 1/2 the speed of light in vacuum. Therefore,

your 400nS of delay can be achieved by about 200 feet of coax. Some delay

lines are actually built this way. From you description, however, you have

a box far too small to cram 200 feet of transmission line into. In that

case they approximated the transmission line with some number of L C L C L C

... This type of approximation is valid up to some frequency limit. Above

that frequency you will get varying delay and large attenuation.

Note that when I said "transmission line" above I was using it in the signal

processing sense. A "transmission line" for delivering 60Hz power to your

local substation is a totally different thing.

*****************************************************************

Olin Lathrop, embedded systems consultant in Devens Massachusetts

(978) 772-3129, olinEraseMEEraseMEembedinc.com, http://www.embedinc.com

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