Cheap 1% terminators and coax - VSWR testing! - Page 1
Forum section: electronics / general

author: KarlMonster
author_rank: Contributor
timestamp: June 23, 2013, 10:46:50 pm
content: Previously, my list of test equipment included the EXtech 330 DMM and a very finely calibrated HurtzDammit wrench.(I bought the Extech before finding Dave's blogs. He's not the only one who  likes them.)Having recently acquired my first analogue oscilloscope, and an interesting array of probes, it was time to get some RG-58 coax, fittings, and 1% terminators. After poking around the interweb for a bit, I discovered a company that sells some of its items through Amazon. Digikey carries some brands of 1% terminator for $14 or so, but SFcable.com has them for $1.50? What the heck? Obviously, these have to be the result of Chinese "dumping" practices. These were so cheap that I scratched a shipment of assorted RG-58 bits with free shipping from Amazon to get the lot of them all from SFcable.com - with significant ship costs! I figured that even if they are crap, it wasn't a lot of money - and I could have fun testing them to prove that they were crap.First of all, I can't fault their customer service. They called me on the telephone about an hour later to ask if it was alright if they sent me the type of 1% terminator that had the chain on it. I'm thinking that they were confused, because the pics I saw had the chain. I told them that it was perfectly fine; though I'll probably put stranded wire on the better ones. They arrived from CA after about 2 days. The quality of the coax fittings gets a thumb sideways. Maybe its just me, but my opinion is that the retaining lug diameter is about a bee's d*ck too small, and the detent for the closed position is too shallow by about at least the same amount. I should like more positive retention in my cabling than these provide. Or maybe that's normal these days? I'll get out my needle file later.I ordered 5 of the 1% terminators, and at first I wasn't sure how these should be tested, but I got the DMM out. The Extech shows 0.2-0.3 Ohms when I hold the bare probes together. (I'd have to find the manual before I could establish if I could calibrate it.) Three of the terminators measured 50.4 - 50.5 Ohms, with the other two at 50.7 and 50.9 Ohms. So its an average of 1.5%. I'm going to find some enamel paint to mark the outlier terminators, but until I discover otherwise, I don't see a reason not to use them as covers for ground or triggering ports. For that reason alone, I'll probably get more in the future.I wonder if you electron wranglers can help me design a *crude* VSWR test to see just how crappy these terminators, coax, and fittings really are?Test Equipment; Tek 2336, coax bits, brand new LM340 (5v) and 555 - because I burned up the old LM340, assorted TTL logic ICs, a pair of 741s, enough transistors to build a latch or two, and a wider array of capacitors than resistors. And I have a PDI 30v power supply coming in a few days; until then I can't properly power the 741s.How would you design a crude VSWR test? Or at least; show obvious reflection from poorly terminated coax?My immediate plan is to repeat the AND-NOT experiment with more attention to voltage measurements (attenuation near 100MHz), followed by 555/741 square(ish) wave signals at a wider range of frequencies.Edit: Whoops! "AND-NOT" refers to a continuously switching logic circuit that creates a AND-output signal wave form in the range of 80-150 ns.Edit 2: Forgot to mention. DMM reading of one end of a terminated cable was 51.2 Ohms, but I don't know if that is in any way significant.

author: madires
author_rank: Super Contributor
timestamp: June 23, 2013, 10:58:42 pm
content: You could use W2AEW's TDR pulse gen, cheap and simple :-)

author: KarlMonster
author_rank: Contributor
timestamp: June 23, 2013, 11:26:44 pm
content: Thanks for the suggestion.It's not *that* cheap. W2AEW specifically says in his comments that TTL logic circuits are insufficient to drive a 50 Ohm load, <wise-ass>I already know how long the cables are </wise-ass>, and I really should have mentioned that my 100 MHz Tek 2336 has a time range limit of 50 ns (0.05 us).That being said, I *do* want to make a pulse generator with nice sharp edges like that for some rise time studies on the assortment of probes that I have. (Which may also be beyond my bandwidth capabilities!) So I might opt for a more costly re-design of that pulse gen. And you're right, a quick pulse is going to be the best way to see reflection, and it should be perceptible - even on a tens of ns scale.Edit: Wait, I got that wrong, didn't I? His reflection at 15.x ns was due to the long length of coax that he used. Which has no bearing on other rise time or reflection studies that I may wish to do, unless I intentionally use a cable that long or longer to facilitate the delay. Not a terrible idea, but I'd like to look at attenuation first.

author: vk6zgo
author_rank: Super Contributor
timestamp: June 24, 2013, 04:35:08 am
content: Before you get to the reflection,the device is quite happily driving 50 Ohms,as that what the cable looks like.He specifically points out that the 220 Ohm resistors will protect the paralleled inverters.In any case,I really doubt that very small mismatches will be visible using this method.What you really need is a Spectrum Anyalyser with a Tracking Generator.plus a directional coupler.It's very easy,then!If you were a Ham,you would probably have a 2m radio & an SWR bridge,so you could wind the outputpower down & test the termination with that.Or,you could build an Impedance bridge & use it with the Ham Radio,or a Signal Generator,if you had one!A Noise Bridge is another thing you could use,if you had a HF receiver.

author: Rufus
author_rank: Super Contributor
timestamp: June 24, 2013, 05:35:15 am
content: Quote from: KarlMonster on June 23, 2013, 10:46:50 pmCrappy BNC terminators will be 1/2W 50 ohm resistors soldered into a BNC crimp plug with a metal or plastic cap stuck over the end. They used to be common and used for 10base2 (coax) network termination.If you haven't got anything operating or measuring at more than 100MHz crappy ones will be fine.I measured one and saw 1.03 SWR (35db return loss) at 100MHz. You need to go higher in frequency before the crappy construction causes real problems.

author: PA4TIM
author_rank: Super Contributor
timestamp: June 24, 2013, 09:58:30 pm
content: http://www.pa4tim.nl/?p=4052Just for fun I measured the worst dummyloads I could think of. I do not show the swr, but trace one is S11 in dB and that is return loss. a RL of >25dB is very close to a SWR=1 (the difference between 25dB and 50 dB is not much in SWR, most swr meters allready show a SWR of 1 around 20-22 dB return loss.But to be on topic. Look for TDR, build a good fast puls generator (the faster ristetime the better, and a step (fast Tr and long pulse duration is better as a semi-dirac pulse, frequency of 20 kHz upto a few hundered kHz is good. You do not need a high frequency, only a fast Tr)A (very) fast step from a Tek TDRTek has a book in the concepts range about TDR (time domain reflection), it is free downloadable on many sides.http://www.pa4tim.nl/?p=1213about TDR but it is in dutch, there is a translate button but the picture show some results.TDR can show you every miss match or help you to measure coax impedance. I have seen chinese testleads that had printed 50 Ohm on it but measured 75 O)hmAbout terminators.There are a few things important. Not always if it is very close to 50 Ohm. As long as you know the value, but for some things like TDR it is very Important. Also very important are stability, tempco, good contacts, low paracitic components and for serious work it must be completely documented. So the delay in ps, resistance, paracitic capacitance etc.Good BNC terminators are usefull upto 2 GHz , cheapys do not even reach 100 MHz. A good 50 Ohm terminator will be a few hundered dollar. A cheap serious callkit (open, load, short ) around 1500, a real good one (but not bnc) over 15K dollar.For 1 MOhm scopeinput use, an inline terminator is better as a T and terminator and the best result is a 10 or 20 dB attenuator combined stacked on an inline terminator. The 1 M degrades with frequency, My Rigol DS1102 degrades to something like 30 Ohm so adding a terminator will sometimes degrade things (30 Ohm parallel with 50 Ohm)http://www.pa4tim.nl/wp-content/uploads/2010/11/rigolDC.jpgThe rigol 1 M inputI have done measurements on this on several scopes and the best result is a build in 50 Ohm input. Second a 20dB attenuator.http://www.pa4tim.nl/wp-content/uploads/2010/11/2445ac10000ohmATT.jpga Tek 2445 1 M input with 20dB attenuatorLater I found out 50 inline termination plus a attenuator was even better.

author: KarlMonster
author_rank: Contributor
timestamp: June 27, 2013, 12:33:53 am
content: There will be a delay in this project, as the real world has interfered again. Once I get the Device working, these interruptions should cease.Altogether, I can marshal about 12' of coax (more than 2x what W2AEW used), so as long as I can generate a relatively sharp pulse, a detectable reflection shouldn't be a problem for my timescale.However, now you have me curious. Where in North America can I find GHz quality coax and terminators? Or is something other than RG-58 preferred?

author: PA4TIM
author_rank: Super Contributor
timestamp: June 27, 2013, 07:55:19 am
content: Bnc is still available upto 2GHz but you need to look in the network analyse conrner. Rosenberger in Germany can deliver bnc loads, Afaik Radial and Amphenol still make them too but most times today it is N, sms, 3.5 mm etc for high frequency work. In the 60's it was often GR.I use teflon 50 Ohm cable in mil spec. That is a teflon core, very fine woven ( it looks like silver) shieldng and a skin collored transparant outerlayer ( it is about as thick as RG58G/U but much stiffer)I have 3 pieces of 3 meter and for my vna two pieces of 30 cm. That is long enough for all my needs. But also belden makes high frequency 50 Ohm cable. That is just slightly thinner as RG58, it is double shielded. I used RG58 bnc connmectors from radial on that but I had to solder the shield to the body before crimping, then addee a crimphose around it and then the metal crimp thing. They are stiff to, i made 2 pieces of 1 meter and they work very well.On cheap coax you see the impedance change if you take it between you fingers and put pressure on it of fold it.A sharp short pulse ( Tr < 5 ns is usable but faster is better, around 100-500 ps) will do, but it is hard to see very small impedance changes. A dirac like pulse is used to find shorts, opens or really big mismatching.The advantage of a step is you see every small mismatch, even at very short distance.HP and Tek used steps in theire TDR.What is your purpose because it does not always have to be cutting edge

author: The Electrician
author_rank: Frequent Contributor
timestamp: June 27, 2013, 07:27:04 pm
content: Quote from: KarlMonster on June 27, 2013, 12:33:53 amIt's not necessary to pay a hundred dollars for a good BNC terminator--just don't buy it new. For example from eBay I recently bought 4 of these for $10 (I notice the price went up a little; now they're 3 for $10. Oh well):http://www.ebay.com/itm/50-Ohm-BNC-Terminator-CGLA26-lot-of-3-/290936786817?pt=LH_DefaultDomain_0&hash=item43bd2f0f81The funky little terminators like you bought are available for a few dollars on eBay also, but as Rufus said, they are just a 50 ohm resistor soldered into a BNC connector. They may be good up to a few hundred MHz. The one from the eBay link is good to a much higher frequency.You can evaluate these terminators by measuring their impedance over a frequency range with a network analyzer. For example, the good ones referenced above measured from 100 kHz to 3 GHz give this result:If it were perfect, the measurement would give a little yellow dot right in the middle of the Smith chart. I have zoomed in and you can see that the impedance of the terminator remains very near 50 ohms all the way up to 3 GHz.The impedance of these units are not spec'd to be 1%, however since I bought 4 of them I hoped at least one would be 1% or better. The measured DC resistances of the 4 units are 51.74, 49.56, 51.41 and 50.09 ohms. As you can see, I really lucked out on the one. But regardless of the DC resistance, these terminators are suitable for GHz use; the cheap ones aren't.Here a measurement of one of the cheap ones. You can see the impedance vary as the frequency rises. I was able to put markers at .5, 1.0, 1.5, 2.0 and 2.5 GHz so you can see the impedance vs. frequency. This terminator is reasonably close to 50 ohms up to a few hundred MHz:I took one of them apart to be sure of its construction, and it is indeed a 50 ohm resistor soldered into the connector.