Spread spectrum time domain reflectometry - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: November 14, 2021, 09:28:32 pm
content: Hi,When you’re doing Spread Spectrum Time domain reflectometry, and you want to inject frequencies from 93kHz to 24MHz into a cable (to see where it has a fault)…..what do you use to inject those frequencies into the cable?It could be electrical cable, data cable, coaxial cable or shielded power distribution cable.Why is the range of SSTDR longer in “ shielded power distribution cable”.

author: pqass
author_rank: Super Contributor
timestamp: November 14, 2021, 10:35:24 pm
content: I'm not sure about SS TDR but according to w2aew it's the fast edge that's important.He uses a 74AC14 (because it has a fast 2ns edge) as a 6KHz pulse generator with 50 ohm resistor in-series into the cable under testWRT TDR range, he said in another video, that TDR will only work on cable with a parallel return path (not sure how he put it). ie. coax, twisted-pair, romex, etc. I assume more of the signal is lost in the unshielded cable therefore less range as it is harder to see the return signal (having less amplitude).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 14, 2021, 10:40:33 pm
content: Sounds more like testing the line with a sweep and watching for peaks/dips, than actual (sharp risetime) TDR. Same underlying mechanism, but tested in the frequency domain.What's so particular about 93kHz? Is this something you were reading about?Tim

author: tggzzz
author_rank: Super Contributor
timestamp: November 14, 2021, 11:10:09 pm
content: Quote from: Faringdon on November 14, 2021, 09:28:32 pmWhat type of spread spectrum?Why not just use a connector that matches whatever is on the cable?QuoteBecause your reference (which is hidden from us) says so.

author: Bud
author_rank: Super Contributor
timestamp: November 15, 2021, 01:08:23 am
content: Quote from: Faringdon on November 14, 2021, 09:28:32 pmWe do not do spread spectrum time domain reflectometry.

author: ch_scr
author_rank: Frequent Contributor
timestamp: November 15, 2021, 07:35:02 am
content: Quote from: T3sl4co1l on November 14, 2021, 10:40:33 pmSSTDR is quote:A new and patented form of TDRhttps://www.livewireinnovation.com/sstdr/Its benefits compared to normal TDR seem specialized tolive wire circuits- the spread spectrum part allows to differentiate between the measurement pulses and whatever is going on on the wire otherwise.I'd say it's a adaption of TDR for safety and security to continously monitor live wires, (hopefully) without interfering and without beeing interfered by normal use.

author: Faringdon
author_rank: Super Contributor
timestamp: November 15, 2021, 08:19:48 am
content: QuoteThanks yes, the product just said its range was over those frequencies.

author: tggzzz
author_rank: Super Contributor
timestamp: November 15, 2021, 09:03:06 am
content: Quote from: ch_scr on November 15, 2021, 07:35:02 amAnd that answers Faringdon's question: use a capacitor.Proposing to use these on aircraft in flight makes me shudder (myemphasis)):The only way to locate these “small” faults, therefore, is tolocate them when they are(intermittently) nearly open or shortcircuits. This requires a system that can continuously test thewires while they are live (and potentially in flight). STDR andSSTDR have been shown to be effective for location of inter-mittent faults on both analog (400 Hz) and digital (MilStd 1553)systems.

author: David Hess
author_rank: Super Contributor
timestamp: November 15, 2021, 07:46:34 pm
content: TDR can be done with an impulse, an edge, a swept sine, or noise. I do not know about now, but in the past there have been some time domain instruments which could use all of those methods. VNAs which support TDR obviously use the swept sine method. I do not remember what advantage if any the noise method has.

author: coppercone2
author_rank: Super Contributor
timestamp: November 16, 2021, 12:23:30 am
content: the 93KHz is in a FCC band for cable locating IIRC, that method of cable location might also skip some frequencies as to avoid causing trouble for electrical equipment (band skip capability)