Gray Pamona Cables are 75 Ohm - Page 1
Forum section: electronics / testgear

author: graybeard
author_rank: Frequent Contributor
timestamp: April 02, 2022, 07:37:18 pm
content: For the last day I have been sorting and TDR testing my collection of test cables. I have several of thesePomona Electronics 2BA-AL-24 dual banana to large alligator clip cables. Since I have a TDR test set running I connected them and found that the coax used in these cables has a characteristic impedance of 75 Ohms.In most cases it does not really matter what the impedance is since they are typically used for DC or low frequency testing. I was just surprised to find they are not 50 Ohms. Thespecification sheetdoes not mention the impedance of the cable, however it does say if the part number starts with 2BC instead of 2BA the cable is RG58C/U which is 50 Ohm cable.

author: TimFox
author_rank: Super Contributor
timestamp: April 02, 2022, 07:54:29 pm
content: For low-frequency use, the 75 ohm cable has lower capacitance per foot than the 50 ohm cable.

author: edpalmer42
author_rank: Super Contributor
timestamp: April 02, 2022, 08:42:16 pm
content: Can you really be sure that it's even coax? It could be unshielded (or shielded) twisted pair that happens to look like 75 ohms. Since neither end is a controlled-impedance connector, anything is possible.

author: pope
author_rank: Frequent Contributor
timestamp: April 02, 2022, 10:10:15 pm
content: Quote from: TimFox on April 02, 2022, 07:54:29 pmExcuse my ignorance but what is considered "low-frequency"? Up to a couple of MHz or more/ less?

author: TimFox
author_rank: Super Contributor
timestamp: April 02, 2022, 10:16:56 pm
content: It depends on the length of the cable. At 1 MHz, the period is 1 us, and in RG-59U the signal will go 200 m in that time."Low frequency" means that the wavelength (in the transmission line, where the speed is slower, typically 0.66 c) is much longer than the length of the cable.Equivalently, where the period (1/frequency) is much longer than the electrical length (in seconds) of the line.Therefore, the reflections at improper terminations can be neglected and the cable looks only like a capacitor.

author: graybeard
author_rank: Frequent Contributor
timestamp: April 02, 2022, 11:01:15 pm
content: Out of curiosity I measured this twisted pair test lead I made and it measures as an impedance of 120 Ohms.I terminated it with a 120 Ohm carbon comp resistor in the ezhook clips and got a flat line.

author: pope
author_rank: Frequent Contributor
timestamp: April 03, 2022, 08:15:23 am
content: Quote from: TimFox on April 02, 2022, 10:16:56 pmThanks. I didn't understand anything but I will read it a few more times

author: tooki
author_rank: Super Contributor
timestamp: April 03, 2022, 10:27:25 am
content: Quote from: edpalmer42 on April 02, 2022, 08:42:16 pmWhy guess? The datasheet linked in the original post clearly states it’s twisted pair.

author: gf
author_rank: Super Contributor
timestamp: April 03, 2022, 10:39:41 am
content: Quote from: pope on April 02, 2022, 10:10:15 pmSee alsohttp://audiosystemsgroup.com/TransLines-LowFreq.pdfAn ohmic frequency-invariant characteristic impedance is just an approximation, assuming an ideal lossless transmission line. This approximation may good enough under certain boundary conditions, but in general, the characteristic impedance of real-world lossy transmission lines is not ohmic, but complex, and frequency-dependent.

author: edpalmer42
author_rank: Super Contributor
timestamp: April 03, 2022, 06:19:28 pm
content: Quote from: tooki on April 03, 2022, 10:27:25 amEven more interesting is that the 2BA version is twisted pair while the 2BC version is RG58C. Did you notice that the twisted pair is actually 18 gauge? They obviously expect that cable to be used for power rather than signal.Ed

author: tooki
author_rank: Super Contributor
timestamp: April 03, 2022, 06:57:45 pm
content: Quote from: graybeard on April 02, 2022, 07:37:18 pmP.S. I actually just noticed that you linked to the wrong product page and datasheet.Here’s the correct datasheet:https://www.pomonaelectronics.com/sites/default/files/d_2BA-AL_2BC-AL_101_1.pdf

author: tooki
author_rank: Super Contributor
timestamp: April 03, 2022, 06:59:06 pm
content: Quote from: edpalmer42 on April 03, 2022, 06:19:28 pmIndeed!

author: graybeard
author_rank: Frequent Contributor
timestamp: April 03, 2022, 07:59:25 pm
content: Quote from: gf on April 03, 2022, 10:39:41 amTreating things that are shorter the 1/20 of a wavelength as a transmission line is not very useful in most cases. Most such situations should be analyzed as lumped circuits rather than using a distributed model.It useful to know the high frequency impedance of lines you would normally treat as lumped to analyze high speed transient cases. For example if a crowbar circuit in a power supply activates the turn-off edge can be fast with many high frequency components.However my motivation for testing the cables was more curiosity than anything else since I had myColby PG1000A pulse generatorandTDS784Dconfigured for TDR testing and it was easy to try things as I was sorting cables.