Olin Lathrop wrote:
>
> > 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.


I just looked at an old delay line removed from an oscilloscope. It is a
coil wrapped around about a 4 feet long polyethelene core. Actually it
is 2 coils wrapped around the core at an angle to each other. Why 2
coils? Does the capacitance between the 2 coils act as the distributed
capacitance? This delay line may be used in the oscilloscope
differentially?

I know -- more questions than answers.


-- Rich


>
> 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, olin@embedinc.com, http://www.embedinc.com
>
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