Cheap BNC 50Ω Terminators - Page 1
Forum section: electronics / testgear

author: mawyatt
author_rank: Super Contributor
timestamp: January 09, 2025, 06:25:44 pm
content: We normally didn't require any 50Ω terminators as all our older analog analog and newer digital scopes had 50Ω inputs. However, the very new SDS824X HD doesn't have 50Ω inputs and a couple times we rummaged thru a bunch of connectors to find 50Ω terminations for this DSO. We never bothered to check just how good or bad these were, just a quick DC Ohms verified ~51Ωs.In another thread discussion about how good these 12 bit new DSOs behave wrt to amplitude flatness, and how flat are some AWG generators. We had measured our 2 AWGs with a SA a few times and knew they were OK and had measured our older DSOs also. For the SDS824 we used a 50Ω terminator to measure it's low frequency characteristics, and in another thread shownherewe noticed a similar 50Ω terminator in use and decided to check this out.BTW noted the "50Ω Terminator" on the DSO BNC Input which is connected to a "T". If you remove the back cap on the Terminator, it simply pulls off as held by grooves in the BNC which is just a standard BNC connector for a crimped braid on the barrel. The ones we have were just single leaded resistors (cheap 1/6 watt) between the BNC center pin and then bent over the BNC barrel for the braid. The cap is what makes the ground side connection for the terminating resistor and in one the cap had cut the resistor lead which created an intermittent connection (why we decided to investigate by removing the cap).A rough guess is this setup adds 6~8nH inductance in series with the R and 1~2pF shunt capacitance. We probably should look into creating a model based upon VNA measurements sometime.Edit: Curiosity got the best of us {see below} and we fired up the VNA and did a quick measurement. This is the better of the two cheap 50Ω terminators we have, and a quick simple model created.Decided to create this thread with what we foundFirst off we have 2 of these horrible terminators, one measures 43Ω at DC, the other 50.6Ω. Don't know how we missed the 43Ω one, which shows how often we utilize them!! After removing the cap we soldered the resistor lead to the BNC barrel as shown, the lead was just bent over and relied on the cap to make contact with the BNC ground, and you can see how the 1/6 watt leaded resistor is used which creates considerable inductance due to lead length.Next we attached to our VNA and did a port extension to cover the length of the N to BNC adapter, VNA was One Port Calibrated with Type N Standards, and range set to 100KHz to 1GHz with various Markers collected along the way.Note that the original estimate of lead inductance effects based upon resistor physical length and parasitic shunt capacitance wasn't too far off as a simple model was created and achieves a good fit to the VNA results over this range, more tweaking could bring the model closer but really no need for such.Anyway, hope this finds some interest and alerts folks to just how bad these cheap 50Ω BNC Terminators can beEdit: Added 43Ω BNC Terminator VNA Plot (PNG2).Best

author: norbert.kiszka
author_rank: Frequent Contributor
timestamp: January 09, 2025, 06:46:04 pm
content: In another thread, people was laughing at me, because instead of buying cheap terminators as they suggested, I added resistors inside the scope, 1 W in total. I did it only for one channel, because that was good enough for me in most cases. Also it doesn't change distance - with terminator it uses physical space in front of the scope, which is not very helpfull.I already know how it ends buying cheap things - it's a waste of money and cheating yourself. Buy a better one or make one by yourself.

author: iMo
author_rank: Super Contributor
timestamp: January 09, 2025, 07:12:51 pm
content: For demanding apps you would need a quality termination, indeed.Operating up to 300MHz does not seem to be so demanding, at least based on the sim below (or perhaps we need a much better model for the terminator).Of course, the cheapo ethernet network terminators produced in 80ties/90ties in billion quantities are not the best stuff today, but still usable (provided it has got at least the 51ohm resistor in there) especially with our budget scopes..

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: January 09, 2025, 07:23:59 pm
content: I've never used the simple terminators, but get the ones with a M-F on the ends. Otherwise you might be tempted to add a T-fitting! I use surplus Tek ones, but even those can get weird after some years, or of somebody overloaded them.

author: norbert.kiszka
author_rank: Frequent Contributor
timestamp: January 09, 2025, 07:27:16 pm
content: Quote from: iMo on January 09, 2025, 07:12:51 pmWith Your data, border will be more like 50-100 MHz, due to phase shift. Also, good luck with burning small 0.25 W resistor.

author: tautech
author_rank: Super Contributor
timestamp: January 09, 2025, 09:00:35 pm
content: Quote from: Conrad Hoffman on January 09, 2025, 07:23:59 pmYes, once you become aware that at RF frequencies, all you are adding is a stub.Decades back I got a pair of Tek 50 Ohm feedthroughs, 1x and 10x although I've since acquired a couple more from other sources.Not that long ago and wanting to compare their frequency characteristics I put them all on our 4 port VNA and did this S11 SWR sweep.Clearly shows those old Tek ones are still pretty good.....

author: TimFox
author_rank: Super Contributor
timestamp: January 09, 2025, 09:09:35 pm
content: As shown in the two Tek graphs: agood10:1 (20 dB) attenuator ahead of not-so-good stuff (iffy 50 ohms and some capacitance) will present a good 50 ohm load to the source (good VSWR).

author: iMo
author_rank: Super Contributor
timestamp: January 09, 2025, 09:40:59 pm
content: I've been using Suhner 3dB and 20dB attenuators (sma, up to 18GHz) with nanovna, but that is a different story..

author: joeqsmith
author_rank: Super Contributor
timestamp: January 09, 2025, 09:43:46 pm
content: For the scope, you may want a bit of parasitic to improve the performance.Simulationshttps://www.eevblog.com/forum/rf-microwave/bnc-50ohm-feed-thru/msg5229120/#msg5229120Video demonstrating the assembled terminatorhttps://www.eevblog.com/forum/testgear/banggood-50-ohm-bnc-feed-through-terminator-a-quick-review/msg5460968/#msg5460968

author: Fungus
author_rank: Super Contributor
timestamp: January 09, 2025, 10:00:17 pm
content: SDS824X HD is a hobby-grade 'scope. For serious work you need one with internal 50 Ohm terminator.

author: iMo
author_rank: Super Contributor
timestamp: January 09, 2025, 10:01:51 pm
content: We have to ask Shahriar from TSP what kind of termination he would recommend for our Rigols and Siglents

author: tautech
author_rank: Super Contributor
timestamp: January 09, 2025, 10:19:22 pm
content: Quote from: Fungus on January 09, 2025, 10:00:17 pmReally ?If and if you knew how to interpret VNA modelling it is plain to see all 4 feedthroughs tested would be just fine with a 200 MHz class DSO.

author: thm_w
author_rank: Super Contributor
timestamp: January 09, 2025, 10:50:28 pm
content: Quote from: norbert.kiszka on January 09, 2025, 06:46:04 pmSo what is the performance difference between external and internal? Now one of your channels is useless for >10V signals.Its an absurd solution.If you want to make your own optimal terminator to match the scope, you can do that, then at least you can remove it when it is not needed.

author: coppercone2
author_rank: Super Contributor
timestamp: January 10, 2025, 12:01:26 am
content: those cheap terminators are for old style LAN cards that have the BNC connector on the back and similar hardware. Its for digital signal networks. At least I think thats how someone decided to manufacture that crapI had a whole bunch of them and they are +-10%and the mechanical contact is often awful too, they all malfunction with a wiggle testYou want the stuff for analog signals, not absolute cheapest digital networkshttps://www.ebay.com/itm/275087559647and they are 50Ohm versions, even though that might be 75I just throw those out, its a different part for a different purpose with the same shape lolI think there is ttroves of them from early 90's offices where it was a upgrade from token ringI got mega uninterested about BNC connectors in the trash when I figured out they are basically junk from old network cards. Because you can find a bunch of BNC hardware and think its something from a RF lab but really its outdated office computer stuff.

author: coppercone2
author_rank: Super Contributor
timestamp: January 10, 2025, 12:15:37 am
content: But to make a good one out of a through hole resistor maybe what you can do is instead of soldering the lead, is to put a copper cylinder with a hole in it on the resistor lead and solder it all together hoping that the solder wicks in. then it should terminate to the inside of the barrel and practically be SMT lead length You could trim the barrel down too a minimum length, so you can use a 'copper disk' instead of a cylinder.cutting a slice off the closest fit size brass bolt might be the easy way to get something close.It won't be as nice as a SMT part RF wise, but you can still have a minimum of leads (say 1mm) to give it some play, because the center pin will micro wiggle when its mated with whatever, and you don't want force going into the smt resistor like that IMO.