Testing a coax cable for open ends - diyAudio
Go Back   Home > Forums > Design & Build > Equipment & Tools

Equipment & Tools From test equipment to hand tools

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 23rd July 2013, 12:40 PM   #1
diyAudio Member
 
triode_al's Avatar
 
Join Date: May 2004
Location: Westland
Blog Entries: 1
Default Testing a coax cable for open ends

On youtube I found this very interesting video showing how to use a scope to measure a cable: for instance for an open end.
Use a scope to measure the length and impedance of coax by w2aew.

The video explains very clearly how you can test a cable under its specific impedance. Misaligned - terminated with a wrong impedance - , there is overshoot. Terminated halfway, you get a bounce from the open not terminated end - and from that you can even measure the distance.

This sounds exotic - it is.

But I also remember that when testing input transformers I got the same step-response. Now I understand that the transformer was not loaded correctly, and not terminated at the end of the winding - hence the signal bounced and returned.
__________________
DAC TDA1541 S1; Kondo post-I/V filter; MC30-Super --> two stage RIAA; Pre: Kaneda ; F5 ; 300B PP ; ESL 57
  Reply With Quote
Old 23rd July 2013, 03:31 PM   #2
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Not exotic, but perhaps not known to audio people who have never done any radio stuff. TDR has been around for a long time!
  Reply With Quote
Old 25th July 2013, 08:20 AM   #3
diyAudio Member
 
triode_al's Avatar
 
Join Date: May 2004
Location: Westland
Blog Entries: 1
When I get around it, I will retest a signal transformer to show the same step-wise response.
  • Just as a hypothesis, is this then a bounce from the end of a unused coil - that is a not properly terminated winding?
I remember that some users of an output transformer with say a 2k3 and a 5k primary and a 8 ohm and 16 ohm secondary have terminated the primary with a small cap and resistor like 10 K and the secondary with similarly the 16 ohm with a 20 - 30 ohm resistor and a 0,1 uF cap.

This definitely reduces ringing. I tried it myself. But is the hypothesis right?

albert
__________________
DAC TDA1541 S1; Kondo post-I/V filter; MC30-Super --> two stage RIAA; Pre: Kaneda ; F5 ; 300B PP ; ESL 57
  Reply With Quote
Old 25th July 2013, 10:33 AM   #4
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
A transformer winding could act like a transmission line, but normally it is better to use a lumped or semi-lumped approximation. Don't confuse transmission line termination with damping an LC resonance; they have similarities but are not the same thing.
  Reply With Quote
Old 25th July 2013, 11:04 PM   #5
diyAudio Member
 
Speedskater's Avatar
 
Join Date: Sep 2002
Location: Lakewood, Ohio
Notice that they are measuring nanoseconds. Grace Hopper "the mother of COBOL," at her seminars, would hand out pieces of wire about 10 inches long. Explaining that an electric signal could travel the length of wire in a second. But she said that she was handing out 'nanoseconds'.

It takes a good high-speed scope to make that measurement.
__________________
Kevin
  Reply With Quote
Old 26th July 2013, 08:43 AM   #6
diyAudio Member
 
triode_al's Avatar
 
Join Date: May 2004
Location: Westland
Blog Entries: 1
Yes I agree, have only a 10 Mhz scope. lowest res is 200 nS . .

Anyway: this is the signal I found on an input?? transformer.

Afb001.jpg

trafo.jpg

See the same step-wise response of the rising and falling edge.
Is this due to wrong (too high) termination?

It shows a transformer that is terminated at 1000 ohms; it is a transformer that does +6,5 dB; primary 3,5 ohms, secondary 26 ohms. It can pass very high frequencies.
  • The lower Fc is 230 Hz terminated at 1000 and 400 ohm;
  • When I terminate it at 100 ohms I have +7,5 dB with an Fc of 73 Hz
  • dropping to 25 ohm I get 30 Hz but now gain drops to + 0,5 dB. Very good square wave.
  • going down further improves the response to about 20 Hz and an upper bw of > 200 KHz. Still 1:1.

The video of measuring a cable in the first post reminded me of this strange transformer. Might it be a pulse transformer?
__________________
DAC TDA1541 S1; Kondo post-I/V filter; MC30-Super --> two stage RIAA; Pre: Kaneda ; F5 ; 300B PP ; ESL 57
  Reply With Quote
Old 26th July 2013, 02:25 PM   #7
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
If you load a transformer too lightly you will see the HF resonance. Load it properly and it will pass a wide range of frequencies. That is what your results show. As I said, nothing to do with transmission lines and TDR.
  Reply With Quote
Old 27th July 2013, 05:26 PM   #8
RNMarsh is offline RNMarsh  United States
diyAudio Member
 
RNMarsh's Avatar
 
Join Date: May 2012
Location: 2457 Cascade Trail; Cool, CA. 95614
FYI -- The old TEK 7000 series oscilloscopes had a wide range of plug-ins available. One is a TDR. I use their model 7S12.

BTW - How does the loading of the transfomer affect the THD thru it or of the driving circuit to the transformer? Any affects there?

Thx-RNMarsh

Last edited by RNMarsh; 27th July 2013 at 05:28 PM.
  Reply With Quote
Old 29th July 2013, 01:36 PM   #9
diyAudio Member
 
triode_al's Avatar
 
Join Date: May 2004
Location: Westland
Blog Entries: 1
I now did the step-down configuration. So input on the 26 ohm as primary; output on the 3 ohms as secondary.
  • I tested the THD, it is about 0,035% - not bad. (Radiometer Copenhagen)
Unloaded output. I have a pot that allows 10 ohm-45 ohms.
But i can't see the distortion itself as the level is too low; connecting the output of my distortionmeter creates [a ground loop] that spoils the input

The whole test setting only works above a certain level.
Input 2,7 V RMS from a fixed frequency pure sine generator of 2765 Hz.
Because the primary also drops when I lower the secondary load, I did not retest it; input drops to where the distortion meter thinks it is too low (below 300 mV in fact).
I also tested square wave in this step down; it exhibits quite some 'dirt' on the rising edge (still that bump half-way) ;
at 400 KHz there is a bump of +3 dB;there is a null (-20 dB) at 660 kHz; a bump of +4 at 1,167 Mhz; again one of +5 at 1,4 MHz;
the system looks so flat that even at 5 Mhz it is 0 dB;
__________________
DAC TDA1541 S1; Kondo post-I/V filter; MC30-Super --> two stage RIAA; Pre: Kaneda ; F5 ; 300B PP ; ESL 57

Last edited by triode_al; 29th July 2013 at 02:06 PM. Reason: correct protocol
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
What happens when a TL is open at both ends heinz1964 Multi-Way 8 6th January 2011 09:33 PM
2 15" on a baffle against ceiling corner (open ends) OTMOPO3OK Subwoofers 2 6th November 2007 11:16 PM
FS: Gold Neutric Male and Female Cable ends PHilgeman Swap Meet 0 7th January 2004 03:03 PM
Best digital coax cable barni Digital Source 5 8th September 2003 09:32 PM


New To Site? Need Help?

All times are GMT. The time now is 02:32 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2