Another TDR circuit and its impedance resistor network which does not look right - Page 1
Forum section: electronics / beginners

author: facosta
author_rank: Newbie
timestamp: March 15, 2018, 08:06:51 am
content: Comming across an interesting TDR schematic and PCB designed and/or published by g4usp @www.qls.net/g4usphas an impedance matching network which has got me puzzled. To me, the 50 ohms output impedance is made up of eight 100 ohms resistors in parallel. Wouldn't the output impedance be, in this case, 12.5 ohms instead of the claimed 50 ohms? If this portion of the circuit is wrong all the other outputs will be out of wack anyway!If you are curious enough to go to the parts list, the voltage regulator shown would also "fry" the 74 IC. Wouldn't it?Thanks for your comments on this post.

author: capt bullshot
author_rank: Super Contributor
timestamp: March 15, 2018, 08:21:10 am
content: I guess, one assumes an output impedance (internal ON resistance) of the AC14 inverters of about 100 ... 150 Ohm here. This is a quite common assumption and should be proven on a particular prototype in the desired application. So it looks at least plausible.The 'AC14 is a 5V chip, the 3pin Vreg isn't specified in the schematic, an 7805 would be right. Some overvoltate won't fry the chip, many of these CMOS chips will run on 7 or 8 Volt for a prolonged time.

author: tggzzz
author_rank: Super Contributor
timestamp: March 15, 2018, 09:17:44 am
content: I use three sets of (74lvc1g14 + 143 ohms in series). The LVC o/p is ~7ohms, so overall the output impedance is 50ohms. That is easy to veriify by observing the waveforms into an open circuit and a known 50ohm load. Note some "50ohm" cables are actually 52ohms.You need very good decoupling since the risetime is <<1ns; watch out for dielectrics where applied DC voltage reduced the capacitance by 80%.I wouldn't use a 2g or 3g device, since all the current will go through the same ground leads, and at these currents/frequencies ground bounce can't be ignored.

author: facosta
author_rank: Newbie
timestamp: March 15, 2018, 10:14:03 am
content: Thanks for your responses. Granted the AC14 gate output Z has to be taken into consideration. Alan, w2aew, uses fives gates each one with a 220 ohms series resistor. So adding 30 ohms to 220 ohms and parallel 5 times gives the correct impedance of 50 ohms. In this particular design 30 ohms + (100 ohms // 100 ohms) // 5 times gives an overall Z of 20 ohms. Still way too low to drive a 50 ohms.The regulator used in the original design is a 7812. In my opinion, I would not like to power the IC with that regulator.My current goal is to test wiring integrity in industrial vehicles where wiring insulation has been compromised by poor splicing/tapping practice. Most wiring is PVC double insulated Fig 8 with a presumable Z of around 100 ohms but I do not have a clue of the Z of single insulated wiring bundle in harnesses.An avalanche type TDR design is probably better for this exercise, as a pulse may work better than a step one for such the short distances involved in the above exercise but I'm leaving that for "plan B"

author: tggzzz
author_rank: Super Contributor
timestamp: March 15, 2018, 12:22:54 pm
content: Quote from: facosta on March 15, 2018, 10:14:03 amThe nominal impedance is unimportant, since a TDR detectschangesin impedance.A pulse and a step are equivalent; the distance resolution is determined by the transition time. I have a TDR with a step output that resolves ~1cm impedance variations with a max range of (IIRC) 100m or so.Assess how any voltage in the car might affect the tester. The TDR mentioned above can be damaged by the residual static charge in a cable, let alone a DC voltage.I don't know anything about modern car electrics, but consider whether differential impedance is important. The impedance will vary for many reasons, e.g. what other conductors are nearby.