Trouble making my low end TDR work, Help.

What's the timebase setting on your scope? With a 12ft coax, the reflection gets back after about 40ns.

Jeroen Belleman

Open versus a short should give two very different reflections. GH

If he's using a slow scope, a capacitor will make a slow "step", an exponential actually. He could try 1 uF, for a 50 us time constant.

I use TDR to measure small Cs and Ls and ESRs, and see semiconductor wire bonds and junction capacitances.

(Can't reply to original post for some reason.)

--

John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 

  Claude Bernard

Ahm, please don't hit me for saying, but you did not accidentally have the bandwidth limit engaged on the scope channel?

Won't work, it has to be a cable.

--
Regards, Joerg 

http://www.analogconsultants.com/

Time was my problem, After Jeroen's post I new time was my error, I first used delayed sweep to expand the edge. That found the step. Then I triggered properly and just used a 0.2 us time base to see the step. it saw 50 ohms for abut 320ft and then the open. Joerg, worth asking, but, I fixed that at the start. Thanks Guys.

The time interval will be different for different types of coax, though. He needs to know what the transmission coefficient for his particular coax is.

Good! I once became very frustrated because a delayed-trigger setup just didn't show what I was expecting. Until I found out that the little plastic (!) delayed-trigger clutch inside the otherwise very expensive Tektronix scope had quietly broken off.

--
Regards, Joerg 

http://www.analogconsultants.com/

A 485, right?

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

No, AFAIR it was a Tektronix 2465. To my surprise there was a guy in Greece who made new knobs and other mechanical parts for these. Shipping to the US was pricey but afterwards the scope felt great. That was before 3D printing, he had made molds for all that.

Whenever a client doesn't have a fast analog scope but needs one (and they usually do) I recommend these. Just not a version with a digital meter built in because those tend to overheat.

--
Regards, Joerg 

http://www.analogconsultants.com/

One of these?

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

That's a coincidence! That is the scope I'm using for the TDR. Except it's a Tektronix 2465CTS model. Mikek

Speaking of broken knobs, I have a Tek 7904 with a 7a26 (400 MHz dual vertical) and the volts/div knob is broken - 3/4 of the knob is missing. Still usable, but I'd like to fix it.

I've searched, but not found any print files to 3D print a new knob. Any hints on where to look?

It will need disassembly, as the calibration knob is also gone and the end of that shaft is broken off.

Clifford Heath

The time interval will be different for different lengths of coax. Long coa x cable with a lossy dielectric will kill the high frequency components tha t make the visible step, but that would be the only issue. Cursitor Doom se ems to be a rather low-end hobbyist.

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Bill Sloman, Sydney

Most coaxial cables have insulator dielectric constants K in the 2.2 sort of range, so the velocity is c/sqrt(2.2) ish, around 2/3 the speed of light, a little higher for foamed dielectrics.

Fun extremes are air lines (K=1, v=c) and coaxial ceramic resonators, K in the thousands.

--

John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 

  Claude Bernard

I have a 2467 (350 MHz, microchannel plate CRT) that I use occasionally. It's a beautiful thing, but life has passed it by except when its gloriously precise triggering is needed.

At IBM my daily driver was a 2465B, but these days I generally use DSOs--TDS94C or TDS784A.

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

That is, incidentally, an equivalent-time-sampling oscillograph.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

[...]

The 2465 is among the best analog scopes money can buy these days. I don't know what'll happen when they aren't readily available on EBay anymore one day.

--
Regards, Joerg 

http://www.analogconsultants.com/

Maybe ask guys that sell knob sets for other Tektronix series? They might know the market.

It's been too long ago and I can't find the Greek guy anymore who sold us the 2465 knob set.

--
Regards, Joerg 

http://www.analogconsultants.com/

If you have a matching knob or the pieces to glue the busted knob back together you can make a polyurethane rubber mold from that, and then cast as many polyurethane plastic copies as you want. My favorite source is

I've used their Reoflex stuff to make a few small molds, and cast rigid parts from their Smooth0Cast resin. I'd suggest a 30A or 40A rubber for the mold and a 70D hardness resin for the knob. They have color pigments for the resin so you can make a nice red knob :-), and lots of great tutorial videos on their site. Email or call them for advice. They have always been very helpful and knowledgeable when I've needed it. Not counting time spent down the rabbit hole of watching their videos :-), it should take you maybe an hour to make and pour your first mold and after an overnight cure and a few minutes for demolding and cleaning you should be able to get your first knob cast in less than 30 minutes. Biggest downside is that their small test kits are about $30 each, so $60ish plus mold release for one knob but you will have plenty of material left over.

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Regards, 
Carl Ijames