stretching a pulse

Isn't that an oxymoron?

Absolutely. A rectangular pulse response is equally unattainable.

-- Bill Sloman, Nijmegen

Why would the output amplitude be unchanged? That's your assumption, not mine. COE is meaningless here anyhow, since the network, even if passive, even if the output amplitude is unchanged, could impedance scale. Voltage is not energy.

By your reasoning, any lowpass filter (which my stretcher is) is nonlinear. And a coaxial cable must be nonlinear. Only a Brickenbox network is linear.

I guess you have your own working definition of "nonlinear", which isn't the one they taught us in engineering school.

We thought a network was linear if the output scales exactly with the amplitude of the input.

John

The propagation delay of light in air (or vacuum) is about 1nsec per foot. The propagation delay in the dielelctric of coaxial cable is slower, at about 1.5nsec per foot. You can get 1.1mm OD teflon dielecric minature coax, and three feet of that can be coiled into a reasonably compact bundle (and I've used that as a delay line).

Twisted pair is going to have a mixed dielectric and experiment would seem to be called for. Twisted transformer wire - with enamel insulation - makes a remarkably compact transmission line, but I've no idea of the propagation delay, beyond that it too has a mixed dielectric (air plus enamel).

-- Bill Sloman, Nijmegen

What about this?

in----+-----delay line------------+ | | | sum -----integrate----out | | +-----------(-1)------------+

Its impulse response is a rectangular pulse and it's linear.

John

How about a short piece of transmission line open on the far end. It will reflect back with twice the delay and add to the original pulse.

That would double the pulse length.

tm

--- news://freenews.netfront.net/ - complaints: snipped-for-privacy@netfront.net ---

nd

If the pulses are always the same height and polarity, there is no need for such complexity.

That almost works.

I think this works:

input----50r-----+-------integrator----out | | | | | 50r transmission | line | | gnd

It has the ideal transfer function, impulse in and rectangular pulse out.

John

If the pulses were always the same height, I wouldn't have to measure them.

John

But I think you want the line open so the reflected pulse will be positive. Shouldn't take much to find out.

tm

--- news://freenews.netfront.net/ - complaints: snipped-for-privacy@netfront.net ---

d

an

Hz

ns

tty

t and

r

ate

With all the signal torture you have suggested, you won't be measuring anything like the original pulses.

Sample the pulse source at a rate fast enough to detect the shortest one and store the samples in a ring buffer of sufficient size to store the longest one . When a pulse is detected, store the contents of the buffer in a more permanent location and analyze them at your leisure.

g

Come to think of it, buried strip-line in a printed circuit board is a non-dispersive transmission line. You'd need a four layer board in which to bury the strip-line - and six layers would allow to stack two layers.

Figuring on 0.004 inch tracks and track spacing, and an 0.004 inch wide ground finger between adjacent tracks, the structure is only

0.016 inches wide, and you could get five feet of strip-line into a square inch of board space (half a square inch with a six layer board). It's difficult to get higher than 50R track impedance in buried strip-line - the tracks start to get very narrow - but ti might ber worth looking at.

-- Bill Sloman, Nijmegen

-integrate----out

Not with a real delay line. They always degrade the rise and fall times to some extent.

-- Bill Sloman, Nijmegen

I'll be measuring the pulse energy, which is what matters. A simple passive pulse stretcher, if I can find one, greatly reduces the cost of downstream electronics.

With 2 ns pulses, the sample rate would be 5 GHz or so, with maybe 10 bits of ADC resolution. That's 50 gigabits of data per second. The "leisure" I have for analysis is hundreds of nanoseconds.

John

Yes. That would make the fall time of the impulse-provoked rectangular pulse slower, but it still winds up at zero.

Since I want a flattish pulse to analyze, and recovery to baseline before the next one hits, that might do.

John

----integrate----out

I'm sure that it would. A pulse with fast - but finite - rise and fall times is all that you need (provided that the transition times aren't too long) and that is compatible with real components. And I'm pretty sure that around a square inch of buried strip-line would serve as a more than adequate delay line to do the job.

-- Bill Sloman, Nijmegen

too

ould

ssian

0 MHz

les

e 1 ns

pretty

get and

at

of

y

lter

ediate

s

tty

Uh, energy needs to be conserved. Your stetcher is so vaguely defined, it is hard to judge. Now if you say the amplitude is lower, then so be it. It would have helped if you thought of that sooner.

When the hell did I say lowpass filters are not linear? Uh, nowhere, thank you very much.

-integrate----out

A rectangular pulse requires infinite bandwidth, hence not obtainable. One assumes your signal will have finite rise and fall times.

Are we to assume the input pulse is wider than the delay of the delay line? So the sum output goes from zero to minus in to zero to plus in to zero. Integration yields a triangle.

Read again...

Likewise, its frequency response has 1/4-wave nulls, since its frequency response is sinc. Hmm, remove the integrator and you remove the 1/x dependency; it becomes a proper comb filter. They never mentioned that in class.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms