Difference between 50 ohm adapater and 50 ohm terminator? - Page 1
Forum section: electronics / beginners

author: Copernicus
author_rank: Newbie
timestamp: July 26, 2017, 08:45:46 pm
content: Hi everyone,I'm looking to get some 50 ohm terminators to have for the lab after watching Dave's video here:I'm looking at these two parts here, one of the parts is under the "adapters" category and one is under the "terminators" category. Both have an impedance of 50 ohms. Is there a difference?Terminator:https://www.digikey.ca/products/en/connectors-interconnects/coaxial-connectors-rf-terminators/382?k=&pkeyword=&pv60=92&FV=ffe0017e%2C11400b9&mnonly=0&ColumnSort=0&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=25Adapter:https://www.digikey.ca/product-detail/en/amphenol-rf-division/112667/ACX1475-ND/1989924Are the prices on these things usually so high?Thanks!

author: edpalmer42
author_rank: Super Contributor
timestamp: July 26, 2017, 09:12:04 pm
content: A little vague on the descriptions aren't they?The adapter is just a male BNC on one end and a female BNC on the other. The '50 ohm' refers to the impedance. If you measured the resistance between the center and outer conductor, you'd see an open circuit. What's the point? It's a bit unusual to see such an adapter in the BNC family, but it's quite common on higher frequency devices where it's known as a 'connector saver'. You put one on a device that uses a small, somewhat delicate SMA connector (or a higher frequency version like 3.5 mm, 2.92 mm, etc.) so that if it becomes damaged you just change the connector saver rather than have to change the connector on your device. That could cost thousands to have the device repaired and recalibrated.The terminator is also known as a feed-thru or feed-through terminator. As shown on the datasheet, there is a 50 ohm resistor between the center and outer conductors. As Dave discussed in the video, it's used to terminate the cable when connecting to a device with a high input impedance. It's most often used with a scope because most other devices already have 50 ohm inputs. Some scopes have the option built in.The cost of these terminators depends on the performance and the brand. If you search the forum you'll find a couple of threads that discuss some cheaper versions that are available online. Whether the cheaper versions are acceptable will depend on what frequencies you work with. Below 100 MHz, the cheaper versions should be fine for all but the highest precision work.Ed

author: bson
author_rank: Unknown
timestamp: July 26, 2017, 09:13:23 pm
content: A terminator has a 50ohm resistance across the two leads, forming a 50ohm load. Not sure what an "adapter" would be, but I'd be leery of buying it as a terminator without additional information. It's most likely just an inline terminator, but it could conceivably also be some special item to turn a very low impedance source into a 50ohm source so would contain a series source impedance, not a load shunt. (Or as, pointed out while I typed a connector saver. But I wouldn't buy it for that either without specifics.)

author: WastelandTek
author_rank: Frequent Contributor
timestamp: July 26, 2017, 09:15:34 pm
content: the only possible use I can see for the "adapter" is to save wear and tear on an equipment port that is plugged/unplugged very frequently, otherwise it seems uselessyes, terminations seem really overpriced for what they are, try a local hamfest

author: bson
author_rank: Unknown
timestamp: July 26, 2017, 09:19:51 pm
content: $20 doesn't seem that expensive for anything BNC with a 4GHz spec.

author: WastelandTek
author_rank: Frequent Contributor
timestamp: July 26, 2017, 09:24:44 pm
content: I'm seeing it at $63 in singles with a 500MHz spec, and I frequently see the HP and Tek ones on ebay for $100 or more

author: ebastler
author_rank: Super Contributor
timestamp: July 26, 2017, 09:27:37 pm
content: Quote from: bson on July 26, 2017, 09:19:51 pmBut that isnota 50 Ohm terminator, just a male-to-female BNC "adapter". No resistor installed in that one, it seems. (There is certainly no power specification on it.)

author: ebastler
author_rank: Super Contributor
timestamp: July 26, 2017, 09:29:21 pm
content: Quote from: WastelandTek on July 26, 2017, 09:15:34 pmOr you could build your own, from standard BNC plugs and jacks. (Don't use the $20 Amphenol plugs...)https://www.eevblog.com/forum/projects/home-made-feed-through-terminator-for-bnc/

author: bson
author_rank: Unknown
timestamp: July 26, 2017, 09:32:16 pm
content: Quote from: bson on July 26, 2017, 09:19:51 pmI take that back. Mini-Circuits will sell a 40GHz terminator for $8.25. Meh.

author: ebastler
author_rank: Super Contributor
timestamp: July 26, 2017, 09:43:16 pm
content: Quote from: bson on July 26, 2017, 09:32:16 pmBut they don't have any terminators in the pass-through style, do they?(For whatever reason those seem to be significantly more expensive than the "end cap" style.)

author: edpalmer42
author_rank: Super Contributor
timestamp: July 26, 2017, 09:49:35 pm
content: Quote from: bson on July 26, 2017, 09:32:16 pmWhich model is that? The only 40 GHz termination I see is the ANNE-50K+ which uses a 2.92 mm connector and costs $145.95. Of course, that isn't a feed-thru terminator. In fact, I don't see any feed-thru terminators on their web site.Ed

author: helius
author_rank: Super Contributor
timestamp: July 26, 2017, 10:05:38 pm
content: 50-ohm feedthrough terminators are useful even if your scope has a built-in 50-ohm termination. The reason is power dissipation: on virtually every scope, the amplitude of the signal must be < 5Vp or the built-in termination resistor will overheat. A feedthrough terminator can dissipate more heat, so it can be used with hotter signals.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 26, 2017, 10:14:39 pm
content: Cost is due to low quantity of production, design NRE, and paper trail.Military, test equipment and industrial sectors don't mind, because they either need that paper trail, or they buy a few, one time, and that's it.Tim

author: WastelandTek
author_rank: Frequent Contributor
timestamp: July 26, 2017, 10:19:19 pm
content: Quote from: ebastler on July 26, 2017, 09:29:21 pmnow we're talking

author: Brumby
author_rank: Unknown
timestamp: July 27, 2017, 02:44:51 pm
content: I would expect a 50 ohm adapter would, by definition, be a piece of hardware that could be inserted into a signal path that would maintain the geometry of a 50 ohm transmission line to avoid the nasties you would get with impedance differences.How much of an issue is this in reality?

author: mc172
author_rank: Frequent Contributor
timestamp: July 27, 2017, 02:58:38 pm
content: Quote from: Brumby on July 27, 2017, 02:44:51 pmIt's the bane of my life.

author: alm
author_rank: Super Contributor
timestamp: July 27, 2017, 03:33:02 pm
content: The adapter is just a short 50 Ohm transmission line with BNC connectors on both end. It only adapts the connector, not the impedance. Like a BNC to SMA adapter, except it adapts BNC to itself. Can be useful to physically extend a BNC connector.You might use one if you were trying to use a scope probe with pins around the BNC connector on a scope without a cut-out for those pins (e.g. some passive Agilent/Keysight/Tektronix probes). Or when attaching a power sensor to a crowded instrument front panel (except that would be an N female to N male adapter). For connector with low mating cycles, like SMA and other high-frequency connectors, you might want to use them on frequently used connections as not to wear out the jacks on the equipment. In this capacity they are called connector savers.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 27, 2017, 03:34:12 pm
content: BNC isn't really 50 ohm. It's merely short enough not to matter at frequencies where it's been used historically.Every connector and adapter is a step change in impedance, but this only causes problems in the GHz.This is why SMA and others are universal for high frequency applications: they maintain impedance, or at least have less mismatch over shorter lengths.Tim