50 ohm bnc adapter needed - Page 1
Forum section: electronics / beginners

author: ez24
author_rank: Super Contributor
timestamp: May 20, 2015, 07:01:07 am
content: HiIt seems I need some sort of 50 ohm adapter to use between my Rigol 1054 and my Wavetek 145.Looking on Amazon herehttp://www.amazon.com/s/ref=nb_sb_noss_2?url=search-alias%3Delectronics&field-keywords=50+ohm+bnc+adapter&rh=n%3A172282%2Ck%3A50+ohm+bnc+adapterthere are 1236 hits.I will spend tomorrow trying to understand why I need one, but in the mean time can someone recommend to me what I need via Amazon. From some videos I have seen it looks like I should get a T type. Is this right?thanks

author: tggzzz
author_rank: Super Contributor
timestamp: May 20, 2015, 07:38:48 am
content: This will help you:http://www.catb.org/esr/faqs/smart-questions.html#beprecise

author: vaualbus
author_rank: Frequent Contributor
timestamp: May 20, 2015, 07:40:11 am
content: if both instruments have 50ohm impedence you don't need it, though i thing that low spec scope don't have 50ohm mode.

author: G7PSK
author_rank: Super Contributor
timestamp: May 20, 2015, 07:51:29 am
content: You need one of these.http://www.ebay.co.uk/itm/Pomona-4119-50-Ohm-BNC-Feed-Thru-Termination-round-/171795673132?hash=item27ffd1f02c

author: tautech
author_rank: Super Contributor
timestamp: May 20, 2015, 08:02:03 am
content: Quote from: ez24 on May 20, 2015, 07:01:07 amPPL use the Tee type because they are much easier to acquire and cheaper than the likes of Tek feed-through terminators.The T's do come in handy at times, I have a Tee and 2 Tek 50 Ohm feed thru's, 10:1 and 1:1.If your DSO had 50 Ohm inputs you wouldn't need one.

author: sdg
author_rank: Regular Contributor
timestamp: May 20, 2015, 08:03:39 am
content: Quote from: ez24 on May 20, 2015, 07:01:07 amMay I suggest that you take the 'why' route first ?It will immensely help you understand the hows and the whats...

author: LukeW
author_rank: Frequent Contributor
timestamp: May 20, 2015, 11:29:34 am
content: Assuming that you're really in a situation where putting a 50-ohm terminator on the cable coming into your oscilloscope will have the intended effect that you think it will have, it's much more common (and cheaper) to find a T-piece and a 50 ohm terminator with an ordinary BNC plug on one end, and just stick the T-piece on the scope input, the terminator on one side and your input cable on the other side.Feed-through terminators do exist, but the price difference is 13 dB.http://www.digikey.com/product-detail/en/27-9008/367-1183-ND/3830268http://www.digikey.com/product-detail/en/4119-50/501-1036-ND/603345

author: Asmyldof
author_rank: Regular Contributor
timestamp: May 20, 2015, 11:38:10 am
content: Quote from: LukeW on May 20, 2015, 11:29:34 amYou get 10 Asmyldof-bonus-points for expressing the price difference in dBs. Although had you said "approximately" it could have been 15

author: alsetalokin4017
author_rank: Super Contributor
timestamp: May 20, 2015, 12:28:32 pm
content: Well... you _could_ just buy one of the more reasonably priced 50-ohm through-terminator adapters from... er.... a Rigol dealer.http://www.tequipment.net/RigolADAPTOR-50-OHM.htmlThat way you won't have a T and a straight terminator taking up space in the crowded area of the input BNC jacks on your scope.Note that the photo is of the similar-looking Attenuator (which costs a bit more) but the price and specs are for the 50 ohm thru-terminator.

author: TimFox
author_rank: Super Contributor
timestamp: May 20, 2015, 12:29:08 pm
content: When converting price ratios to dB, always use the factor 10, since money is power.

author: ez24
author_rank: Super Contributor
timestamp: May 20, 2015, 05:35:33 pm
content: QuoteI read Art of Electronics 2nd ed last night and there was a little in there about this, in a general sort of way. Impedance match so the signal does not reflect?What about these :http://www.amazon.com/Eightwood-BNC-Adapter-Female-Female-Female-Pack/dp/B00WFZRFK6/ref=sr_1_11?s=electronics&ie=UTF8&qid=1432141952&sr=1-11&keywords=50+ohm+bnc+adapter+impedance+teeYesterday I looked at several Ts but they all had a freq limit of 4 Hz. This morning I figure this is a typo and should have been 4 GHz because the above ones say 4 GHz and they look the same as the 4 Hz models.If no good I will work on a $100 order from TE to get free shipping and get the Rigol one. Any suggestions on any other oscope accessories for a beginner that TE would sell (unfortunately they do not sell new brains).ps I cannot find anything in the manual for the Rigol DZ1054Z about 50 ohms.thanks

author: LukeW
author_rank: Frequent Contributor
timestamp: May 20, 2015, 06:00:37 pm
content: There's a nice thread here about the selection of terminators in very-high-speed applications.https://www.eevblog.com/forum/testgear/importance-of-good-terminators-for-rf-pulse-applications/

author: ez24
author_rank: Super Contributor
timestamp: May 20, 2015, 07:34:56 pm
content: Quote from: LukeW on May 20, 2015, 06:00:37 pmNICE thanks - lots to learn here

author: Electro Fan
author_rank: Super Contributor
timestamp: May 21, 2015, 12:47:44 am
content: The main take away is that if your Wavetek has a 50 ohm output and your scope does not have a 50 ohm input you should get a 50 ohm feed through terminator for the scope. Without it your scope won't give accurate readings. You can buy a Rigol, or a Pomona, or maybe a used Tektronix on eBay, or one of many available on Amazon. When you get one, try taking some readings with and without the terminator and you will see the impact of properly vs. improperly matched impedances.Example:http://www.ebay.com/itm/2-Pomona-4119-BNC-50-ohm-feed-thru-termiations-very-good-condition-/201353190694?hash=item2ee195dd26

author: Fsck
author_rank: Super Contributor
timestamp: May 21, 2015, 12:56:24 am
content: you could contact franky for buying feedthrough terminators. I bought 2 from him before and they work well.

author: ez24
author_rank: Super Contributor
timestamp: May 21, 2015, 01:14:37 am
content: thanks allI have ordered some and placed bids on some Pomona ones, so I will have some cheap ones to compare with good onesOnce I get them I will test the Wavetek again, also someone suggested that the TTL output may go to the max freq.tks

author: miguelvp
author_rank: Super Contributor
timestamp: May 21, 2015, 01:20:57 am
content: I got mine from Wallco Industrial Electronics:Calrad 75-614 BNC Dust Cap w/ Chain ($0.85 each), for when I don't have probes hooked into the scope, watch for the chain since it would be grounded if the scope is powered up, you can always remove the chain, but I don't let my boards that close to the scope so no need.http://www.wallcoinc.com/Calrad_75_614_BNC_Dust_Cap_w_Chain_p/wal22-75-614.htmCalrad 75-540 "T" Style Adapter w/2 BNC Females to 1 BNC Male ($1.50 each), they do have a 75 ohm version for over $3 but not sure if that would fit the scope's bnc (Calrad 75-690 if you want to get those instead):http://www.wallcoinc.com/Calrad_75_540_T_Style_Adapter_w_2_BNC_Females_t_p/wal22-75-540.htmPan Pacific BNC-7864 BNC TERMINATOR JACK, 50 OHM 1% 1/2 W ($1.15 each):http://www.wallcoinc.com/Pan_Pacific_BNC_7864_BNC_TERMINATOR_JACK_50_OHM_p/wal78-bnc-7864.htmPan Pacific BNC-7862 BNC TERMINATOR, 75 OHM 1% 1/2 W ($1.66 each), for analog video signals:http://www.wallcoinc.com/Pan_Pacific_BNC_7862_BNC_TERMINATOR_75_OHM_1_1_p/wal78-bnc-7862.htmDon't recall how long it took them to ship but I probably used the cheaper (fedex ground) because I was in no hurry. I think ordered about 3 of each, maybe 4.Edit: They do have a lot of other things, like all kinds of bnc adapters, switches, knobs, tools, well almost everything you can think off.Oh maybe you want to get some coax cable as well (search for "coax cable rg58" or "coax cable rg174" since they don't seem to carry rg142 which is best since it's double braided)

author: Electro Fan
author_rank: Super Contributor
timestamp: May 21, 2015, 01:28:04 am
content: Be careful when using a T BNC connector. Some are just T connectors, others actually provide 50 Ohm termination.All the photos below were made with a 4 Volt peak to peak signal from the 50 Ohm output of a signal generator.In the first photo are a proper 50 Ohm T terminator, a regular T connector, and an end cap.In the second you can see the proper 50 Ohm T terminator is producing a 4 Volt peak to peak signal (which is what was output at 50 Ohms from a signal generator) even though one end of the T is left open.In the third photo is the T connector with one end open - the T does not provide 50 Ohm termination by itself.In the fourth photo is the same T connector with the end capped - it now provides 50 Ohm termination.In the sixth photo is a 50 Ohm Tektronix Feed Through Terminator.In the sixth photo is the 4 Volt Peak to Peak signal being fed directly into the 1M Ohm input of the scope (without 50 Ohm terminator).

author: rs20
author_rank: Super Contributor
timestamp: May 21, 2015, 01:37:22 am
content: Quote from: Electro Fan on May 21, 2015, 01:28:04 amWhat happens if you terminate (read: connect anything with a proper 50 ohm impedance) to the other end of the T? If the answer is "it magically converters into a normal T", what is this sorcery? If the answer is "you shouldn't do that, it's already terminated just right", what is the point of it being in a T shape with a plug you can't do anything with?

author: Electro Fan
author_rank: Super Contributor
timestamp: May 21, 2015, 02:38:12 am
content: Quote from: rs20 on May 21, 2015, 01:37:22 amRS, not sure if I understand your question.The point I was trying to make in my post is that there are different types of T connectors/terminators. This was in response to recommendations in this thread for the OP to consider using a T as a 50 Ohm terminator.The T in the second photo will provide a 50 Ohm termination using just one side of the T (with no need for a cap or a second channel or a second piece of equipment to be terminated); it will also provide a 50 Ohm termination if a second channel or a second piece of equipment is terminated on the second side of the T. This type of terminator offers flexibility for someone who needs a 50 Ohm terminator as it will work with 1 or 2 channels or pieces of equipment - and with no need for a cap if just one piece of equipment needs to be terminated at 50 Ohms. It is a convenient alternative to a straight/feed through terminator that also offers the utility of a T connector.The T in the third (and fourth) photo will provide a 50 ohm termination if a cap is used or if the second side of the T is connected to another piece of test equipment (or to a second channel such as a 2nd scope channel with a 50 Ohm input). As long as a user has a cap or a second channel or a second piece of equipment to terminate then this type of T connector can be a good alternative to a straight/feed through terminator.Net, net: I just was trying to help the OP understand the differences between the T connector and the T terminator and reminding the OP that if he gets a T connector that the T connector by itself won't provide a 50 Ohm termination (it needs a cap or something connected to the second side). In other words, it might make sense for the OP to consider a straight/feed through terminator rather than a T connector.Having said all that, it might be good to have both a straight/feed through terminator plus a T connector with a cap (and/or a T terminator) to address various requirements.Hope that helps clarify things. EF- To further clarify, what I have referred to as a "proper 50 Ohm terminator" (or a "T terminator" as opposed to a "T connector") is labeled as a "Auto Term 50 [Omega symbol]" on the T terminator.

author: rs20
author_rank: Super Contributor
timestamp: May 21, 2015, 03:08:41 am
content: Quote from: Electro Fan on May 21, 2015, 02:38:12 amYes, and a very good job done there. I was just partially hijacking this thread to personally try to understand exactly what a T terminator does, because your description sounded too good to be true. Turns out you got it right, what I was missing is that these auto-terminating Tees contain terminating resistors that areautomatically physically switched out when a plug is connected. Very clever, although the curmudgeon in me would prefer the simplicity and reliability of a plain T with a terminator chained to it. I wouldn't want to suspect a dodgy auto-termination switch when debugging something.

author: Electro Fan
author_rank: Super Contributor
timestamp: May 21, 2015, 03:28:16 am
content: In the attached photos you can see the flexibility of the "Auto Term 50 Ohm" T Terminator. In Photo 1 with just one scope connected to the T terminator the scope sees 50 Ohms (making it an easy alternative to a straight/feed through terminator). In Photo 2 with both scopes connected to the T terminator each scope sees 50 Ohms.Note the 2247A on the bottom needs a 50 Ohm terminator because the scope only offers 1M Ohm inputs (like the OP's Rigol 1054 or a Rigol 2072), whereas the 2467B on the top is switch selectable (like a Rigol 2072A); in the photos the 2467B is set to 50 Ohms (so it doesn't need an external terminator).

author: Paul Moir
author_rank: Frequent Contributor
timestamp: May 21, 2015, 07:02:57 am
content: EDIT: Hey, there's a second page to this thread...Yes, don't forget the coax cables. Terminators are not to be used on probes.

author: tggzzz
author_rank: Super Contributor
timestamp: May 21, 2015, 07:43:51 am
content: Quote from: Paul Moir on May 21, 2015, 07:02:57 amTerminatorsmustbe used with low-impedance Z0 probes - which are the best type of probe for digital signals when using scopes >=100MHz bandwidth. (Unless you can afford active probes!)(BTW, as a separate issue, see a simple homebrew modification that improves 10:1 high impedance probes with digital signals:https://entertaininghacks.wordpress.com/2015/04/23/scope-probe-accessory-improves-signal-fidelity/)

author: Paul Moir
author_rank: Frequent Contributor
timestamp: May 21, 2015, 07:51:50 am
content: I'm afraid the OP will use the 50 ohm terminator on a common (ie, the one that came with his scope) 1x or 10x probe. He has mentioned buying Tees and through terminators but not mentioned owning or buying coax. Also have a look at his other thread.However I think he's developing his understanding very quickly!

author: tggzzz
author_rank: Super Contributor
timestamp: May 21, 2015, 08:28:10 am
content: Quote from: Paul Moir on May 21, 2015, 07:51:50 amJust so, except that I haven't looked at his other thread and can only hope he is learning.

author: ez24
author_rank: Super Contributor
timestamp: May 21, 2015, 07:49:05 pm
content: QuoteSorry the pictures were made using a coax directly from the Wavetek to the Rigol. I have bought some cheap and expensive 50 ohm T adapters to test out the signal. I did try the probes and the signals looked the same, it was easier to use the coax to take pictures. I should have been more careful with the voltages because of this. I started with the probe and did not think when I changed to the coax.thanks everyone