Chinese Terminator - Page 1
Forum section: electronics / testgear

author: jjoonathan
author_rank: Frequent Contributor
timestamp: December 20, 2017, 08:50:37 pm
content: Teardown of an aliexpress 50 ohm SMA terminatorEDIT: this probably belonged in tools and gadgets, my bad.

author: Kilo Tango
author_rank: Regular Contributor
timestamp: December 20, 2017, 08:58:26 pm
content: did they say what frequency it was good for ?Ken

author: DC1MC
author_rank: Super Contributor
timestamp: December 20, 2017, 09:06:12 pm
content: Quote from: jjoonathan on December 20, 2017, 08:50:37 pmIf they build them like that SkyNet will fail miserably

author: mc172
author_rank: Frequent Contributor
timestamp: December 20, 2017, 09:19:21 pm
content: That is a common construction technique. It's very efficient at being predominantly resistive and not capacitive or inductive. What were you expecting?

author: edpalmer42
author_rank: Super Contributor
timestamp: December 20, 2017, 10:18:33 pm
content: The biggest problem I see is that the resistor is fifty-oneohms. It's possible they selected for a -1.9% tolerance, but I find that really hard to believe.

author: jjoonathan
author_rank: Frequent Contributor
timestamp: December 20, 2017, 10:20:38 pm
content: Quote2.5GThey didn't specify return loss at 2.5G, thoughQuoteI was always taught that axial resistors were quite inductive compared to SMD resistors. I have a VNA and homebrew SMD terminator handy, so I swept them both. See attached. I'm happy with what I was taught.QuoteI was hoping to at least find several SMD resistors and possibly, if I was lucky, a crude thick-film termination. Before you laugh, I've torn down one of those peeling-green-paint SMA attenuators and found thick-film, which adjusted my expectations upwards for dubious aliexpress listings. Alas, my expectations have been adjusted back down.

author: mc172
author_rank: Frequent Contributor
timestamp: December 20, 2017, 11:01:07 pm
content: Fair enough, but terminations of this type have a very widespread, legitimate use. They're not intended for 26 GHz and you know thay by the price you paid.Most of the cable network across Europe and America uses them because they are extremely cheap and are on the end of probably 12 cascaded taps, directional couplers and often 8 or 16-way splitters, where return loss of the termination itself doesn't really matter any more, as long as it's not 6 dB. The cable network only operates up to 1 GHz if you're lucky. It's also all 75 Ohms, not that that makes a difference but worth pointing out because the idea behind your 50 Ohm one was probably nicked from the 75 Ohm gang.They aren't meant to be good, they're meant to be just better than leaving 15 ports unterminated on a 16 way splitter because only one house has subscribed to cable, until more do.

author: mc172
author_rank: Frequent Contributor
timestamp: December 20, 2017, 11:08:21 pm
content: Quote from: mc172 on December 20, 2017, 09:19:21 pmJust quoting myself to clarify... It's very cost efficient. Terminations like these (especially the F connector flavour) can be made incredibly cheaply. They often use the leg of the resistor as the pin. You wouldn't make a calibration kit like that but they aren't meant for that.You are right about axial resistors being horrible but unfortunately the whole cable network is driven by cost. Performance is always a compromise and they always, always buy what is just about good enough.It's good for companies that sell the equipment because the cable operators are continuously having to upgrade the network as a result of everything being just good enough.I think your excample is a particularly crusty example, though, what with the dodgy bent leg contact? I've got some F terminations at work that will achieve about 15 dB return loss up to 3 GHz that are made of nothing more than a resistor and a solder joint.

author: Bud
author_rank: Super Contributor
timestamp: December 20, 2017, 11:12:42 pm
content: I thought i will find in this thread a photo of Schwarzenegger browsing Shenzhen electronic shops.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 20, 2017, 11:16:41 pm
content: Only thing I see wrong is the lack of soldered ground connection: there could be a hole in the backside that the lead is fed through, and soldered, leaving a little bump or nipple on the end. A plastic end cap might cover this up.Whether the sprung connection is reliable, depends on how tightly it fits. It might be quite reasonable.This would be more acceptable for BNC (which tops out around 2GHz), than SMA, but it's probably not that much worse, and you get what you pay for, eh?Tim

author: Pinkus
author_rank: Frequent Contributor
timestamp: December 20, 2017, 11:32:08 pm
content: Quote from: blueskull on December 20, 2017, 11:19:45 pmI am curious. Do you want to tell us more about it?

author: jjoonathan
author_rank: Frequent Contributor
timestamp: December 21, 2017, 01:06:58 am
content: I understand the need for value engineering but is an axial resistor and long brass can really more cost efficient than a SMD resistor and a short brass can? I would expect otherwise and aliexpress prices seem to agree.The SMD terminators are cheaper *and* have much better performance. Knowing which to buy is a win-win for those in the value end of the test equipment market.

author: jjoonathan
author_rank: Frequent Contributor
timestamp: December 21, 2017, 01:47:37 am
content: Sure, but a single SMD resistor terminator can be assembled with a single touch of a soldering iron and it still performs better at 26.5GHz than the axial performs at 2.5GHz. I just tried it; see attached.EDIT: The aliexpress prices are of course a better reflection of manufacturing cost, but they too reflect a cheaper SMD version. It's a little cheaper and a lot better.

author: Andrey_irk
author_rank: Regular Contributor
timestamp: December 21, 2017, 01:55:58 am
content: Quote from: blueskull on December 21, 2017, 01:11:22 am10 resistros in parallel is not a good idea. It will definitely decrease the inductance, but it will also add too much capacitance or the size of the thing will be unacceptable.

author: Bud
author_rank: Super Contributor
timestamp: December 21, 2017, 02:24:35 am
content: 2x 0603 100 Ohm resistors was what Agilent recommended.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 21, 2017, 02:45:22 am
content: Of course, unless you're using a dozen in parallel, the power rating is lower.Tim

author: Bud
author_rank: Super Contributor
timestamp: December 21, 2017, 04:34:00 am
content: 0402 would be difficult to solder onto a SMA from practical perspective. 0603 are manageable and fit better. Still, directly soldering resistors onto a SMA connector is not a terribly good idea because the center pin may move under pressure and eventually crack the resistors. Depends on the quality of the connector used. With a reasonable quality connector for home use should be fine.

author: smarteebit
author_rank: Contributor
timestamp: December 21, 2017, 09:43:54 am
content: I'm curious. Can real resistor(s) (axial or SMD, in parallel or not) work well at frequency up to 26GHz? As we know, a real resistor has ESL and stray capacitance, which bring its expected impdance away as the frequency increases.

author: Mr. Scram
author_rank: Super Contributor
timestamp: December 21, 2017, 10:06:26 am
content: Quote from: Bud on December 21, 2017, 04:34:00 amIndeed. Soldering SMD components directly to metal reminds me of the recent powe supply failure episode. You don't need a lot of movement for problems to arise. A chancy Chinese plug with stuff rigidly soldered to it doesn't sound like a good plan.

author: bd139
author_rank: Super Contributor
timestamp: December 21, 2017, 10:33:59 am
content: This sort of crap is why I've started buying good quality cables and terminations.I was playing around with TDR using a pulse generator I made (74lvc - nothing fancy) and popped my shitty Chinese terminator on the end with 20m of high quality RG58 which actually has decent braid coverage unlike the cheap stuff. Ooh got some reflections already? What's going on here? Checked everything thoroughly. I suspected my pulse generator at first so I loaded it with a pair of 1% 100 ohm resistors and measured the output impedance. Nope spot on at 50.2 ohms. What's going on here?Eventually I got to that terminator which was marked up as 50 ohms. Nope not 50 ohms, 150 ohms.... I assume that it had three 150 ohm resistors inside it and two had broken off.Now have Cinch, Amphenol and TE Connectivity stuff only. I have one RS Pro inline terminator which is a bit hooky which is due for replacement as well.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 21, 2017, 11:18:30 am
content: Quote from: smarteebit on December 21, 2017, 09:43:54 amIndeed, but even more generally, the pads and body make up a transmission line structure.This is why resistors close to Zo, specifically Zo in the PCB environment, have the most flatness (highest bandwidth), say 30 to 300 ohms. In higher values, the equivalent capacitance dominates -- but it is only capacitive equivalent at frequencies where the resistor is still electrically small; and conversely for lower values, equivalent inductance takes over.At frequencies where the resistor is not electrically small, the reactance flips sign; a transmission line model catches this, whereas an RLC model needs to be ever more intricate to approximate it at higher and higher frequencies.Tim

author: mnementh
author_rank: Super Contributor
timestamp: December 22, 2017, 04:30:10 am
content: Quote from: jjoonathan on December 21, 2017, 01:47:37 amI think you just found the Higgs Boson there...mnemOut. Standing. In my field.

author: tautech
author_rank: Super Contributor
timestamp: December 22, 2017, 04:57:14 am
content: Quote from: mnementh on December 22, 2017, 04:30:10 amNah, these are just learners compared to what GK has done:https://www.eevblog.com/forum/projects/t20347/msg898898/#msg898898

author: mnementh
author_rank: Super Contributor
timestamp: December 22, 2017, 05:14:45 am
content: So what you're saying is... my [Random Mode] isn't nearly random enough?[/Random mode]mnem'Tis crackers to slip a rozzer; the dropsy in snide.

author: Bud
author_rank: Super Contributor
timestamp: December 22, 2017, 06:29:56 am
content: Quote from: bd139 on December 21, 2017, 10:33:59 amYou better do, because Chinese poorly mechanicaly made connectors can ruin your equipment connectors.

author: Mr. Scram
author_rank: Super Contributor
timestamp: December 22, 2017, 08:06:08 am
content: Quote from: Bud on December 22, 2017, 06:29:56 amI imagine they can, but do they do this enough to be a real problem?

author: jjoonathan
author_rank: Frequent Contributor
timestamp: December 22, 2017, 03:54:29 pm
content: Yep, it's a real problem. Tight tolerances, tiny delicate structures, and relatively large forces (8 in-lb) are a recipe for connector destruction. Cheap ebay equipment frequently comes with horribly chowdered up input connectors.A slightly-too-long male pin will split a slotted female pin. A slightly-too-wide or burred male pin will catch a "finger" of the female pin and push it back. A female pin with bad edges will scratch up a male pin. Twisting the connectors during mating will occasionally catch the female fingers on the male pin, bending one and scratching the other. It will always cause abrasion, which is quite visible at 50GHz. I see three of these (too long, bad edges, abrasion) frequently. Pin width seems to be better controlled; my guess is swiss lathe programmers/operators tend to know what they're doing. Everyone blames connector failure on twisting-during-mating, but I privately suspect that this accounts for only a small fraction of ruined connectors.The male SMA connector for the subject of this thread was almost 10 mil out of spec lengthwise in the bad, potentially harmful direction, FWIW. Good connectors are typically 1-2mil in the good direction. We can speculate about what it would have done to the precision slotless connector on my VNA had I attached it, but we will never find out :-)EDIT: see the precision 3.5mm female conector here in all its glory:https://www.microwaves101.com/encyclopedias/how-to-not-trash-a-calibration-kit

author: mnementh
author_rank: Super Contributor
timestamp: December 22, 2017, 05:02:26 pm
content: Quote from: jjoonathan on December 22, 2017, 03:54:29 pmYeah, but SMA/RP-SMA are WAY overused in many applications where frequent connection/disconnection is part of normal service. Depending on the manufacturer, most are rated at only 200-500 mating cycles; I understand there are some all SS variants rated for more, but they are frightfully expensive.mnem*Relurk*

author: Mr. Scram
author_rank: Super Contributor
timestamp: December 23, 2017, 05:47:09 pm
content: My mistake, I thought we were talking about BNC connectors too.