Time Domain Reflectometry

1) Forget non-existant Maxim crap. 2) CMOS gaes can be paralleled to give whatever drive you want. 3) You can use a resistive "tee" for matching. 4) *NO* CMOS parts have picosecond rise, fall or delay times. 5) Try ECL type logic (offset the supplies if need). 6) Or try non-linear parts in the "tee" (eg: saturating L for the input element). 7) Or try a snapoff diode. 8) Or an Esaki diode.

If you are driving a 50R cable, it is a good idea to drive it from a

50R resistive impedance, so that any reflection back from the cable is absorbed in the source termination, rather than reflected back into the cable, to be reflected back again and again.

If you take the small signal output impedance of each 74AC14 to be about 30R, adding 220R in series gives a total output resistance of

250R per gate. Five 250R resistive impedances in parallel is 50R.

If you tried to make do with fewer gates, the gates would run out of current when the signal was close to Vcc or Vss, and their output impedance would be higher when the signal was in these regions.

When you've got enough gates in parallel, adding more won't make any difference to the performance, though the individual devices will run cooler.

-- Bill Sloman, Nijmegen

No. A fast edge will propagate, as a fast edge, for great distanxes along a transmission line. The line has distributed inductance as well as distributed capacitance. Only non-ideal losses (dielectric and resistive losses) soften up a propagating edge. A 1 ns edge can be propagated through 100 feet of good coax.

Tomi's circuit looks fine.

John

You could use an emitter folower with a 50ohm resistor down to ground, a very short high pulse will drive the cable hard, when the pulse returns the transistor will be off and so just see the 50r resistor.

for such a pulse the cable is not like a capacitance, untill the reflection comes back from the other end it looks like a 50r resistor.

Colin =^.^=

What is good coax? My experience is 1ns edges won't go very far in RG-58U. I forget exactly, maybe 10 ft.

TDR also has round-trip, which doubles the distance.

Regards,

Mike Monett

I don't think many people would consider RG-58 good coax. 9913, RG-213 or RG-8X are my favorites for 50 Ohm cable.

Hardline.

John

That will be an asymmetric non-50 ohm impedance, and proper TDR needs a true 50 ohm source; most electrical TDR is done with a step, not a pulse. But you don't need a lot of voltage - 0.25 volts into 50 ohms is standard - so you can pad down a 5-volt swing and get a very good

50 ohms.

John

No fair. Not many people get a chance to see that. Especially a 100 ft run:)

So you agree the ordinary everyday plain run-of-the-mill standard flexible coax that everyone and loves knows won't transfer a 1ns edge very far? Regards,

Mike Monett

Hi,

I've made rather a lot of these generators -- both fast edges, and pulses. Over long (1000 metres) cables I personally prefer pulses. I have attached two small PDF files (am I meant to do that here ??) showing rather old versions of the circuits. The one using the S family TTL has worked for 20+ years, and I've used it to find faults at 750 m down a 75 ohm buried video cable. You need to check a test length -- the velocity factor is never quite right.

The 2n2222 based pulse generator can generate any pulse length you like. The circuit originally had a avalanche transistor -- 2n2369 from memory, and as long as you don't try a SOT23 2n2222 the can type ones work well. Layout is everything.

The circuit came from a Jim Williams app note

Page 21.

A later version is here.

The coax sets the pulse length, and it can be quite long, or even very short.

Some related links.

As some others have commented the attenuation can be fierce -- so 20 volt pulses are good -- that way you might get 10mV back.

Steps work just as well, and are always easier to see. Also easier to get the impedance from.

begin 644 20v.pdf M)5!$1BTQ+C(@#0HEXN//TPT*(`T*-R`P(&]B:@T*/#P-"B],96YG=&@@."`P M(%(-"B]&:6QT97(@+TQ:5T1E8V]D92`-"CX^#0IS=')E86T-"H`,1!`H$X-"!>

M1IB-I+)[O(I(,*Q=S'>[O*10,13=S4#2+;QI91G6+"(+&# M,QC%1K6+I'!!=[R*!ED2ID\\J#1MJ==F\\[N8K#IQ#M)I@:-=3#[KK[P#10,]I MD]3%1I4YCL!`*!IS\\I;QQB!-TD MQ.\\D#;K(6IO'M`IC2/6N2L\\C0A:&*2.G#(05^U(9[0(,JMJ!H6\\DK'*-!Y(3)IBJ5@A)`&*&X,HQE@!R76.)&B* MMSBFDQB47%F1981%R1T@Y(D]BM0=6;JOTN= M6&Z%P+IUG.Q6I5B2V)]RZ[ADT3/T;%'0L-=H/3&;I79_W)B/>V336RVWZC&N MF:K-1Q!F=XT,.BH[16XUYBW!62Z_X5H=,:0*C,7'JRG'\\ME6>*_ M,_X5]5EZ!;3E4;FK*MH"&$\\((6*.\\BT1"KO[0=]+Q30+S'EWG^ M-^A33Z/E/H>/,B]!H+S?`N$,UY)9B'/**7`U@8K-E=(2;^ZP:0TF,2O@?N[*]AYS"OY6#+'>_^?:;XGRO>U1^3\\%@\\>_-TFQ6]$%.SM]A MO/8HIC2/6KRV]#1A:&*2.O*L/A@U89W=+3;@:%O&*QQW(23P:+),_

For various values of very far. People could make 200 GHz oscilloscopes, but there's basically no way to get the signal to them.

The edge gets ugly. On, say, 100 feet of RG58, a steep part is still transmitted, but its amplitude declines with distance and is followed by drool.

_________ _____/ ___/ __/ drool _/ / / / steep / /

-----------

I'll post some scope pics tomorrow maybe.

John

Thanks - that would be interesting.

I'm surprised it would go that far. Your RG-58 must be a lot better than the cheap stuff everyone else gets. Regards,

Mike Monett

16" wide brass waveguide.

--
Service to my country? Been there, Done that, and I\'ve got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

But the OP might be stuck with a particular coax type for his application. Remember, you TDR with the coax you've got, not the one you want (or something to that effect).

--
Paul Hovnanian     mailto:Paul@Hovnanian.com
------------------------------------------------------------------
The world is coming to an end.  Please log off.

For future reference: Your pdfs are not actually attached (which is not allowed here). They are links to a web site, which is the preferred method for posting graphics, binaries and such. Some of us who don't have access to binary newsgroups can still see them.

If you must attach binaries, post to alt.binaries.schematics.electronic and refer to the subject line here. We can all hop over there and try to find them among all the jpegs of J.T.'s grandkids. ;-)

--
Paul Hovnanian     mailto:Paul@Hovnanian.com
------------------------------------------------------------------
The ark was skippered by amateurs, the Titanic was skippered by
professionals.

There's a 50 metre reel of Andrew's LDF5-50A in an adjacent cube, that was used for TDR work.

Regards Ian

Mike Monett wrote in news:Xns990ED7112F58DNoemailadr@208.49.80.251:

Tektronix sold a lot of their 1503 long-range TDRs,measuring up to 50,000 ft of cable,IIRC. phone companies,cable companies.It used a 1/2 sine pulse.

IIRC,the TEK 1502 measured up to a couple of thousand feet,using a fast- rise step pulse.

--
Jim Yanik
jyanik
at
kua.net

I used to own one. Can't remember if it was a 1502 or 1503. The resolution was so poor on long cables that I gave it away to the avionics company that maintained my Piper Malibu in San Jose.

The 1503A used a tunnel diode step generator with a risetime of about 140ps. The B and C versions used a patented step generator designed by a rather famous individual whose name escapes me now.

I tried to use his idea as a step generator in the Binary Sampler, but ended up using a simple ecl logic signal from a 100EP16. A simplified schematic omitting the 50 ohm output pad is shown here:

Regards,

Mike Monett

Regards,

Mike Monett

Antiviral, Antibacterial Silver Solution:

snipped-for-privacy@spsdialup.com/index.htm SPICE Analysis of Crystal Oscillators: snipped-for-privacy@spsdialup.com/spice/xtal/clapp.htm Noise-Rejecting Wideband Sampler: