Author Topic: Difference between 50 ohm adapater and 50 ohm terminator?  (Read 1290 times)

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Offline Copernicus

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Difference between 50 ohm adapater and 50 ohm terminator?
« on: July 27, 2017, 06:45:46 am »
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=25

Adapter:
https://www.digikey.ca/product-detail/en/amphenol-rf-division/112667/ACX1475-ND/1989924

Are the prices on these things usually so high?  :-\

Thanks!
 

Offline edpalmer42

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Re: Difference between 50 ohm adapater and 50 ohm terminator?
« Reply #1 on: July 27, 2017, 07:12:04 am »
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
 

Offline bson

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Re: Difference between 50 ohm adapater and 50 ohm terminator?
« Reply #2 on: July 27, 2017, 07:13:23 am »
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.)

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Offline WastelandTek

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Re: Difference between 50 ohm adapater and 50 ohm terminator?
« Reply #3 on: July 27, 2017, 07:15:34 am »
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 useless

yes, terminations seem really overpriced for what they are, try a local hamfest
I'm new here, but I tend to be pretty gregarious, so if I'm out of my lane please call me out.
 

Offline bson

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Re: Difference between 50 ohm adapater and 50 ohm terminator?
« Reply #4 on: July 27, 2017, 07:19:51 am »
$20 doesn't seem that expensive for anything BNC with a 4GHz spec.
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Offline WastelandTek

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Re: Difference between 50 ohm adapater and 50 ohm terminator?
« Reply #5 on: July 27, 2017, 07:24:44 am »
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
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Offline ebastler

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Re: Difference between 50 ohm adapater and 50 ohm terminator?
« Reply #6 on: July 27, 2017, 07:27:37 am »
$20 doesn't seem that expensive for anything BNC with a 4GHz spec.

But that is not a 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.)
 

Offline ebastler

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Re: Difference between 50 ohm adapater and 50 ohm terminator?
« Reply #7 on: July 27, 2017, 07:29:21 am »
yes, terminations seem really overpriced for what they are, try a local hamfest

Or you could build your own, from standard BNC plugs and jacks. (Don't use the $20 Amphenol plugs... ;))
http://www.eevblog.com/forum/projects/home-made-feed-through-terminator-for-bnc/
 

Offline bson

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Re: Difference between 50 ohm adapater and 50 ohm terminator?
« Reply #8 on: July 27, 2017, 07:32:16 am »
$20 doesn't seem that expensive for anything BNC with a 4GHz spec.
I take that back.  Mini-Circuits will sell a 40GHz terminator for $8.25.  Meh.
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Offline ebastler

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Re: Difference between 50 ohm adapater and 50 ohm terminator?
« Reply #9 on: July 27, 2017, 07:43:16 am »
Mini-Circuits will sell a 40GHz terminator for $8.25.  Meh.

But 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.)
 

Offline edpalmer42

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Re: Difference between 50 ohm adapater and 50 ohm terminator?
« Reply #10 on: July 27, 2017, 07:49:35 am »
$20 doesn't seem that expensive for anything BNC with a 4GHz spec.
I take that back.  Mini-Circuits will sell a 40GHz terminator for $8.25.  Meh.

Which 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
 

Offline helius

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Re: Difference between 50 ohm adapater and 50 ohm terminator?
« Reply #11 on: July 27, 2017, 08:05:38 am »
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.
 

Offline T3sl4co1l

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Re: Difference between 50 ohm adapater and 50 ohm terminator?
« Reply #12 on: July 27, 2017, 08:14:39 am »
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
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Electronic Design, from Concept to Layout.
Need engineering assistance? Drop me a message!
 

Offline WastelandTek

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Re: Difference between 50 ohm adapater and 50 ohm terminator?
« Reply #13 on: July 27, 2017, 08:19:19 am »


Or you could build your own, from standard BNC plugs and jacks. (Don't use the $20 Amphenol plugs... ;))
http://www.eevblog.com/forum/projects/home-made-feed-through-terminator-for-bnc/

now we're talking  :-+
I'm new here, but I tend to be pretty gregarious, so if I'm out of my lane please call me out.
 

Online Brumby

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Re: Difference between 50 ohm adapater and 50 ohm terminator?
« Reply #14 on: July 28, 2017, 12:44:51 am »
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?
 

Offline mc172

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Re: Difference between 50 ohm adapater and 50 ohm terminator?
« Reply #15 on: July 28, 2017, 12:58:38 am »
How much of an issue is this in reality?

It's the bane of my life.
 

Offline alm

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Re: Difference between 50 ohm adapater and 50 ohm terminator?
« Reply #16 on: July 28, 2017, 01:33:02 am »
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 :P. 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.
 

Offline T3sl4co1l

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Re: Difference between 50 ohm adapater and 50 ohm terminator?
« Reply #17 on: July 28, 2017, 01:34:12 am »
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
Seven Transistor Labs, LLC
Electronic Design, from Concept to Layout.
Need engineering assistance? Drop me a message!
 


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