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Re: Time Domain Reflectometer (was Re: internet outages)




On Sunday 23 December 2018 22:20:30 rhkramer@xxxxxxxxx wrote:

> On Sunday, December 23, 2018 09:03:15 PM Gene Heskett wrote:
> > On Sunday 23 December 2018 20:27:37 rhkramer@xxxxxxxxx wrote:
> > > On Sunday, December 23, 2018 07:33:38 PM Doug wrote:
> > > > RE: Time Domain Reflectometer:
> > > >
> > > > Theoretically, you can build your own with a fast pulse
> > > > generator and an oscilloscope. The trick is, you need a REALLY
> > > > FAST oscilloscope!  The pulse generator is easy, just a couple
> > > > of transistors, maybe a diode. The  circuit is probably in every
> > > > edition of the Radio Amateur's Handbook. The scope is expensive.
> > > > If you don't have at least a 1GHz digital sampling scope, don't
> > > > bother!
> > > >
> > > > --doug, WA2SAY, retired RF engineer
> > >
> > > Hmm, with CPU clocks hitting 4 GHz, I wonder how expensive an ADC
> > > converter to work at corresponding speeds would be?  (Just an idle
> > > question ;-)
> >
> > If you have to ask, you can't afford it. Bring outrageous sums of
> > money This is typically done by every trick tek knows about fast
> > analogue circuitry, in the best units with long vertical deflection
> > plates in a custom made crt with teeny delay lines between the
> > sections of the plates so the signal is virtually traveling toward
> > the screen at the same speed as the electron beam is traveling. And
> > its moving fast enough at 22,000 volts, relativity can and does get
> > in the way. Such scopes put the plates so close to the beam that the
> > beam is intercepted by striking the plates at just a hair over 4 cm
> > high, 2cm from the horizontal center line. The only one of those I
> > ever saw was in the early 1980's or so at the NAB show in Vegas, and
> > it was well into a 5 digit asking price then.
>
> Yeah, but I wasn't thinking about actually using a CRT to display the
> signal in real time, but, instead, collect samples (at maybe 4 GHz??),
> store them, and then display them as a static display.
>
> ???

Thats essentially what our high bandwidth scopes do today, $400 gets you 
a 1ghz sampler with an effective bandwidth of 200 mhz. I've got one, the 
nice thing is that because it is digital, a once a second glitch in a 42 
megabaud seriel data train stands out like a sore thumb because its not 
limited to the screen writing speed of an analogue scope. So its just as 
bright as the main signal that doesn't have the glitch. The operating 
software is buggier than a 10 day old road kill in August though. I have 
both. My analogue Hitachi is actually good to around 200 mhz.

Cheers, Gene Heskett
-- 
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