In real life, Q is low enough and c high enough that the measurement is fast by human standards.
It is interesting how losses increase with frequency and length, but the null frequency decreases with cable length, and those sort of compensate.
-- John Larkin Highland Technology, Inc Science teaches us to doubt. Claude Bernard
Right. I think I'm forgetting about all the 'pieces' of the FT for times that are longer than the 'speed of light' length of the coax... Brain fart on my part, sorry.
George H.
I just did a TDR on the 260ft wire I have on the ground. I have the VF figured at 63%*. The first step time was 0.675us, Doing the calculations that results in 127.58 ft, I measured it 126 meters. Not to bad.
Mikek
Retracting the above, I forgot I added the ferrite and had reduced the VF to 59%. Redoing some testing. Mikek
The FT applies at a single point along the coax. In the absence of loss, dispersion, and radiation from the open end, then assuming a perfect match at the TX end, the FT at each point on the cable is the sum of the TX pulse spectrum times (exp(-2 pi f x/v) + exp(-2 pi f(2L-x)/v).
Apart from phase shift, that corresponds to a series of nulls at
f_N = N v/(2 (L-x)),
where N is an integer. That is, the nulls at any point x are at frequencies where the distance from x to the open end is an integer number of half-wavelengths.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
~Best solution so far.....
99GBP isn't $25 though.
You might like this video,
Mikek
If you could get down to sub-mill-inch, a slidewire with a bead could implement the kind of digital-readout length scale that decorates a bunch of machine tooling. You could sell a lot of those; extra credit for an analog output suitable for servo controls (no digitizer delay wanted).
It's *so* easy to omit some vital criteria when you're doing these kind of calculations, I find. My usual downfall is treating all capacitive quantities as negative and completely overlooking those times when they should be positive. But if it wasn't that, it'd be something else as every step is perilous and fraught with incalculable risk. :-)
Someone's already posted that link further up the thread. It is certainly well worth a look for anyone who needs to knock up something simple in a hurry. There are others out there too, like the guy who posts as "Mr Carlsons's Lab" - but his design in a bit more involved and includes a power supply for it.
Maybe Sonnet Lite or some such could model microstrip meander lines.
Somebody here might try that.
There are probably papers, too.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
This won't help with length, but will help with finding the impedance of the coax. >
Mikek
A TDR will measure the electrical length within inches depending on resolution. Baby Bird sez: RG-58 Specifications [Search domain
Do the math.
At 100kHz and 30ft, you won't get much in the way of transmission line effects, but you can measure the capacitance of the open-ended cable and the inductance of the shorted-end cable, and infer the impedance and propagation velocity from that: Z = sqrt(L/C), and V = 1/sqrt(LC).
You could then work out its length from the cross-section geometry. E.g., knowing that the inductance per unit length is L_0=mu_0/2pi * ln(D/d), where D is the inner diameter of the screen and d the diameter of the centre conductor, the overall length would be L/L_0, and the delay t_pd=L/(V L_0). This is very inaccurate however. It's even worse if you choose to compare with C_0, because you don't know the dieelectric constant of the insulator with any useful precision. You're better off using a yardstick.
I haven't seen the video, too tedious.
Jeroen Belleman
Circa 1996, wanting to make some fast edges, I tested a number of NPN transistors I had in inventory. I found many (most?) would avalanche, but most of them were disappointingly slow. A 2n2222a, IIRC, yielded ~1.5-2ns risetimes. 2n2369 was the fastest.
At the time I was testing through-hole parts; SMD would probably help. I'm not sure what geometry is optimal -- perhaps an RF transistor would be faster. (ISTR an MRF571 was perversely awful.) But avalanche mode operation was finicky, part-sensitive, quarrelsome, and not all that fast, so I wound up using 74AC instead -- fast, with no fuss.
I'd use avalanche pulsers today for high-current outputs, maybe. Otherwise, logic is cleaner, faster and friendlier, IME.
Cheers, James Arthur
I think that older-fab parts tended to avalanche better than new ones. Specifically, epitaxial transistors don't avalanche well.
Zetex told me that their avalanche transistors were fabbed in Russia, which I suspect is on an older line.
Early sampling scopes used an avalanche transistor to drive the sampling diodes. Tek SD14 did that and typically got 2 GHz bandwidth.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Actually 7S14.
That was the one that used mercury batteries to back-bias the sampling diodes. Dumb, dumb.
-- John Larkin Highland Technology, Inc Science teaches us to doubt. Claude Bernard
I used a 7S14 making those measurements. :-)
Poking on Digikey, I see 74LVC and 74AUC have tpd's < 2ns into 50pF. And SiGe parts with 50ps edges.
Surely some LVDS or optical SERDES-type parts' edges should scream, too. Lots to choose from these days!
Cheers, James
Some of the Tiny gates have edge rates well below 1 ns.
There are several 1 ns prop delay Tiny parts, gates and flops. NC7SV86, NC7SV08, NC7SV74. The prices are absurd.
Some LVDS line receivers act like RRIO comparators with 2 ns prop delays and 700 ps rise/fall, for 40 cents.
The GigaComm parts are really fast but fairly expensive.
I'm planning to use a 10 gbps SFP module to make a fast optical square wave to test my new 1 GHz o/e converter.
So many toys.
-- John Larkin Highland Technology, Inc Science teaches us to doubt. Claude Bernard
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