75ohm to 50ohm at high frequenies - 6Ghz How? - Page 1
Forum section: electronics / testgear

author: rx8pilot
author_rank: Super Contributor
timestamp: March 17, 2017, 03:39:24 am
content: I am hoping to look at some signals from 3Ghz to 6Ghz on a 75ohm coax on my scope. I found all kinds of pads for this but the bandwidth is generally too low.Anyone have a manufacturer or model that may solve the problem of terminating a 75ohm transmission line to the 50ohm input of my scope? BNC to BNC connectors.

author: TheSteve
author_rank: Unknown
timestamp: March 17, 2017, 03:47:38 am
content: I've seen DC-2GHz and DC-3 GHz. Finding one that goes to 6 GHz may be possible but I really doubt it will have BNC connectors, I'd expect N at a minimum.How does it look directly connected with the mismatch?edit:You should be able to use an Agilent 11852B (with N to BNC adapters)See this page:http://www.keysight.com/main/editorial.jspx?cc=MZ&lc=eng&ckey=1000001978:epsg:faq&nid=-32490.1150487&id=1000001978:epsg:faqFor fun I just checked my MiniCircuits BMP-5075R+ matching pads - they are rated to 2 GHz and, well, it shows. The 75 ohm cable between two adapters is total crap though. Pic attached.I don't have any Agilent 11852B's to check - they were outside of my "fun" budget.

author: rx8pilot
author_rank: Super Contributor
timestamp: March 17, 2017, 04:09:05 am
content: The mismatch clearly shows a reflection and the amplitude is high - expected result.I guess an N to BNC should not be a real issue other than being huge. Maybe Pasternack could make a short N-BNC so I don'tbreak the input of the scope.BNC is a legacy connector that continues to be pushed beyond its original intention.Curious how much that Keysight model costs. I need two of then and have a suspicion that the price may be nuts. Lower cost than a pair of 6Ghz active probes.Sent from my horrible mobile....

author: TheSteve
author_rank: Unknown
timestamp: March 17, 2017, 05:10:47 am
content: As is often the case eBay is your friend - finding two for a good price might be tough if you're in a hurry. Also get name brand adapters if you do convert BNC to n.

author: Someone
author_rank: Super Contributor
timestamp: March 17, 2017, 05:35:46 am
content: Quote from: TheSteve on March 17, 2017, 03:47:38 amThere is a special order option for it: 11852B Option H12, Minimum loss attenuator tested to 12 GHzThough it might just be cheaper to layout a stripline adaptor and use some BNC connectors directly.

author: DaJMasta
author_rank: Super Contributor
timestamp: March 17, 2017, 05:38:42 am
content: Out of curiosity... does your scope even have close to that bandwidth? I've never heard of a scope with a range of several GHz or more bandwidth that still had regular old BNCs. I haven't even seen it used all that commonly with cables rated to over 1GHz.If not N connectors, SMAs are quite standard for this sort of bandwidth (not as high as N or some others, but there's convenience in the smaller connector and cables are cheaper).

author: TheSteve
author_rank: Unknown
timestamp: March 17, 2017, 06:07:57 am
content: He has a MSOX6004A - good to 6 GHz and still uses BNC's.

author: alm
author_rank: Super Contributor
timestamp: March 17, 2017, 10:09:42 am
content: As far as I know most 75 Ohm BNC connectors are only rated up to 2 GHz or so. So I imagine you'd definitely need to convert to 75 Ohm N connectors (and do not expect those adapters to be rated for 6 GHz either). And expect a less than perfect performance from those BNC connectors at 6 GHz. Pretty sure even for the 50 Ohm connectors you would need some pretty high grade connectors. Can you change the 75 Ohm connector?

author: rx8pilot
author_rank: Super Contributor
timestamp: March 17, 2017, 06:07:18 pm
content: Quote from: alm on March 17, 2017, 10:09:42 amThe scope BNC's are high-precision good well beyond 6Ghz - but only when mated with a similar precision part. I am wondering if I may need to get a custom high-bandwidth BNC to N cable to interface the scope and another 75 ohm N to BNC to connect to the transmission line.These are broadcast signals and BNC is THE connector. In the digital domain, they have gone from 270Mbps to 12Gbps over a single coax terminated with BNC. The rise times are crazy fast now. I am expecting a full spec from SMPTE soon on the signal architecture. The goal is to be able to test various equalizers and line drivers and the associated PCB connector layout to see if they meet the spec for rise/fall, amplitude, jitter, etc...

author: rx8pilot
author_rank: Super Contributor
timestamp: March 17, 2017, 06:58:57 pm
content: This will work....http://www.coppermountaintech.com/products/30/8AP50NM75NF/Just need a really nice way to adapt the N's to BNC's without too much damage.No surprise that it is $600, but in the big scheme of things - not a problem. I need two of course as well as two differential 6Ghz probes that are crazy expensive too.

author: nctnico
author_rank: Super Contributor
timestamp: March 17, 2017, 07:08:34 pm
content: AFAIK 50 Ohm N and 75 Ohm N connectors are slightly different so be carefull to get the right BNC to N adapters.

author: rx8pilot
author_rank: Super Contributor
timestamp: March 17, 2017, 08:02:58 pm
content: Quote from: nctnico on March 17, 2017, 07:08:34 pmYes - same with BNC too. Maybe Mini-Circuits has the right way to adapt. The Copper Mountain people suggested I ask them, specifically cautioned against Pasternack.

author: hagster
author_rank: Frequent Contributor
timestamp: March 17, 2017, 08:19:20 pm
content: The best and lowest loss way of doing it is with a tappered line impedence transformer(a length of PCB microstrip that is 75ohm at one end and narrows to 50ohm at the other)https://www.microwaves101.com/encyclopedias/tapered-transformers

author: alm
author_rank: Super Contributor
timestamp: March 17, 2017, 09:38:17 pm
content: Quote from: rx8pilot on March 17, 2017, 08:02:58 pmThis is false. Obviously matching a 50 Ohm connector with a 75 Ohm connector will result in an impedance mismatch either way (electrical issue), but there is no mechanical issue with BNC connectors with a different impedance (at least 50 and 75 Ohm) since the center pin has the same size, the only difference is the insulation. 75 Ohm N connectors, on the other hand, have a narrower center pin, which in the case of a male 75 Ohm N connector in a female 50 Ohm N connector can result in an intermittent connection, and the other way around in a damaged female connector. So with BNC the worst you will get is reduced performance (except maybe if your equipment gets damaged by reflected energy). With N connectors the worst you will get is a damaged connector. If this connector happens to be on some expensive bit of RF gear like a spectrum analyzer, this can be an expensive mistake.Back in the days before HDTV, it was actually fairly common to use 50 Ohm BNC connectors for 75 Ohm signals, since the impedance mismatch was quite small at the frequencies of an SD video signal. I believe one of the major manufacturers had (have?) a brand name like 'True 75 Ohm', because their 75 Ohm BNC connectors were, shocker, 75 Ohm.

author: rx8pilot
author_rank: Super Contributor
timestamp: March 17, 2017, 10:39:42 pm
content: Quote from: alm on March 17, 2017, 09:38:17 pmI was only worried about the electrical performance of the BNC - same with the N-connector. 50 Ohm connectors to 50 ohm connectors. 75 Ohm connectors to 75 ohm connectors and all will be well. The issue is that I don't want the adaptation from N to BNC to introduce too much insertion loss or additional return loss. Now that we have 12Gbps signals - impedance control is harder than ever. You either have to be perfect or have extremely short cables to be able to extract your signal on the other side.Quote from: hagster on March 17, 2017, 08:19:20 pmThis is pretty cool, although there is little information about a practical implementation from this article. Curious if this is how Copper Mountain makes their 8Ghz matching pad. I don't have a VNA or SNA so a DIY project is out of the question. If I was going to spend the $1200 on the pad plus some fancy adaptors - I would rather put that toward a VNA which I want to have anyhow. I could then go through the learning curve of making on my own (obviously not to save money, but because it would be fun). At 6Ghz +, I cannot imagine it would be much more than a simple (but precise) physical structure.

author: rx8pilot
author_rank: Super Contributor
timestamp: March 17, 2017, 10:56:22 pm
content: These are what I really need.....Keysight says they are affordable.

author: TheSteve
author_rank: Unknown
timestamp: March 17, 2017, 11:04:19 pm
content: They are low cost compared to the price of the scope.

author: DimitriP
author_rank: Super Contributor
timestamp: March 17, 2017, 11:09:06 pm
content: QuoteSo,just in case you get inspired, you can calculate values using this:http://chemandy.com/calculators/matching-pi-attenuator-calculator.htm...and then ..use this to come up with the specific resistor values using actual existing parts :http://www.qsl.net/in3otd/parallr.htmlThe $600 version claims 5.4db loss, using the calculator above the loss is in the 5.7db neighborhood.Not bad for a couple of BNCs and a few chip resistors..

author: TheSteve
author_rank: Unknown
timestamp: March 17, 2017, 11:17:55 pm
content: btw, Keysight has a premium used N2752a on ebay for 60% off - and you can still make an offer to get a better deal. Just do it!

author: pigrew
author_rank: Frequent Contributor
timestamp: March 18, 2017, 04:41:10 pm
content: I keep wondering how a "simple" 25 ohm resistor would perform in series with the scope. The distance between it and the internal 50 ohm resistor could be much less than a wavelength, so it might do somewhat ok (when mounted on a nicestriplineCPW)?Or how would a 10dB 75ohm attenuator work (assuming it is mounted directly on the oscilloscope port)?EDIT: Nevermind about the 75-ohm attenuators.... It seems that they are way too rare. I can only find up to 4 GHz. And for the 25 ohm-resistor on the cheap, looks like FR-4 may have a loss of on the order of 1 dB per in, so not too awful. CPW would be better controlled than microstrip, and I should have mentioned it instead.

author: rx8pilot
author_rank: Super Contributor
timestamp: March 18, 2017, 07:32:08 pm
content: Quote from: TheSteve on March 17, 2017, 11:17:55 pmSide note.... seeing if I can get this probe.Sent from my horrible mobile....

author: hagster
author_rank: Frequent Contributor
timestamp: March 19, 2017, 06:36:32 pm
content: Quote from: rx8pilot on March 17, 2017, 10:39:42 pmA tapered transformer is pretty easy to make. Just work out the width of the microstrip at each impedence. Then just gradually taper the track width across a length. The longer you make it the lower the frequency it will work to. You can do better with fancy curves for the track width, but the improvement isnt worth the hastle for most application. I have build one of these and it worked perfectly. It is however rather large at low frequencies.

author: PA0PBZ
author_rank: Super Contributor
timestamp: March 19, 2017, 06:54:09 pm
content: I'm probably missing a point or 2 here, but what about the N2882A? 75 to 50, BNC, good to 8GHz...http://www.keysight.com/en/pd-1651984-pn-N2882A/75-to-50-bnc-adapter?cc=US&lc=eng

author: TheSteve
author_rank: Unknown
timestamp: March 19, 2017, 07:13:06 pm
content: Hah, that looks pretty ideal.

author: nctnico
author_rank: Super Contributor
timestamp: March 19, 2017, 07:16:16 pm
content: Quote from: PA0PBZ on March 19, 2017, 06:54:09 pmGood find! And they seem to cost little over $100 each so that seems like a no-brainer to me to solve the problem.

author: Howardlong
author_rank: Super Contributor
timestamp: March 19, 2017, 08:05:56 pm
content: Although that BNC minimum loss pad is the neatest solution.Another jury rig option is a 75 ohm RF termination on microstrip and use a FET probe rated to 6GHz or higher probing across the resistor. In fact, even a lo-Z 500 ohm x10 probe would work with minimal effect, use a 91 ohm termination resistor instead.The biggest problem is finding BNC connectors that'll work reasonably well to 6GHz, making that mnimum loss pad much more palatable.

author: rx8pilot
author_rank: Super Contributor
timestamp: March 19, 2017, 08:44:45 pm
content: The cable and BNC manufacturers are making tighter and tighter tolerances to keep up with the bandwidth requirements of TV/Film where the connector has outlived its original intention.I am trying to get the Keysight N2752A 6Ghz probe which will allow analysis of the whole signal path instead of simply the 75 ohm output. Ideally, I would have two of them, but one will give me the measurements and confidence needed to put the new designs out in the wild. Baby steps and simple things first, then I can get another one in time. If I am successful in getting the probe - it eliminates the need for the impedance matching.This is my first dive into high-speed digital - made possible by the fancy Keysight 'Test to Impress' win. Pretty excited to be in this place for sure. First up is a few general purpose I/O devices where I can get my feet wet with a modest challenge. The next year is designing FPGA hardware to manipulate the data I bring in. Fun.

author: G0HZU
author_rank: Super Contributor
timestamp: March 19, 2017, 08:47:32 pm
content: Quote from: nctnico on March 19, 2017, 07:16:16 pmBut the RF performance of that Agilent adaptor is very poor up at 6GHz? eg very poor VSWR. Worse than 2:1.If I had to do this I'd experiment with a direct transition from (decent) 75R semi rigid cable 'slotting' into a tiny PCB and experiment with different (pi) 75R to 50R attenuator values using 0603 resistors. I'd probably start with 11dB as this gives three very friendly resistor values for use up at 6GHz. I'd exit the PCB with a decent SMA end launcher connector. I'd also experiment with a simple 25R series resistor although I'd expect to be able to beat this with the pi pad method.It would probably take me several attempts but I'd hope/expect to get better VSWR performance up at 3-6GHz when compared to the Agilent adaptor that uses a BNC connector at the 75R input.I'd also consider the tapered line approach using microstrip but this will only give a good match above a certain frequency. eg 1500MHz. Fine if you only want to look across 3-6GHz but not really a wideband solution if you want to maintain performance down to DC.

author: Howardlong
author_rank: Super Contributor
timestamp: March 19, 2017, 08:50:57 pm
content: 54006a lo-Z 6GHz probes appear on ebay quite frequently at reasonable prices, make sure you get the original BNC/SMA adapter, in line DC blocking cap and walking sticks with spare resistors.

author: rx8pilot
author_rank: Super Contributor
timestamp: March 19, 2017, 09:34:18 pm
content: The listing says they will charge 7.5% sales tax which seems weird because it is coming from Malaysia and going to California. Not sure if that is right or not and they have not yet responded to the question. The buy-it-now price includes shipping while the offers do not - not sure if they will simply upcharge the shipping to make up for my lower offer. The shipping is not stated in the counter-offer. Anyway...should have an answer soon.I find it almost funny that I am trying to buy a probe for $2k+ and feeling like it is a bargain because it is normally $5.8k. My previous scope - a used Tek TDS754C was only about $750.

author: rx8pilot
author_rank: Super Contributor
timestamp: March 19, 2017, 09:43:32 pm
content: Quote from: G0HZU on March 19, 2017, 08:47:32 pmBuilding my own solution would be a fun personal project for sure, but I am limited in options since I don't have an SA/TG or a VNA to properly characterize the finished device. The primary goal is to ensure signal integrity and match it to the published specification. I certainly cannot guess my way through the design without any testing. It is, however, a good excuse to get more test equipment in due time.For now, I can only look at verified commercial solutions.

author: G0HZU
author_rank: Super Contributor
timestamp: March 19, 2017, 10:08:29 pm
content: QuoteBut are you going to get that with the Agilent BNC adaptor? Looking at the datasheet the VSWR at the 75R port is worse than 2:1 across a fair bit of 1GHz thru 6GHz. 2:1 could mean 150R or 37.5R resistive or anywhere else on the 2:1 circleLooking at the (mismatch induced?) ripple in the S21 plot and the shape of the S11 plot (75R port) I'd suspect that the input impedance is going around a loop shape on the smith chart that breaks the 2:1 VSWR circle fairly regularly across the band.A 2:1 VSWR could mean an input impedance that looks like 150R or 37.5R resistive or some other impedance on the 2:1 circle on the smith chart. I guess it depends if that matters or not for your application?

author: rx8pilot
author_rank: Super Contributor
timestamp: March 19, 2017, 10:34:25 pm
content: Quote from: G0HZU on March 19, 2017, 10:08:29 pmNope - I would clearly have to do way better than that to get any useful information. The limitations you already pointed out ensure it does not help. Some of the other commercial options end up being about $1k after adaptations which is more than I expected.BREAKING NEWS: Keysight took the offer and the N2752A 6Ghz probe is on the way. VERY excited about that!

author: G0HZU
author_rank: Super Contributor
timestamp: March 19, 2017, 10:49:55 pm
content: FWIW here's a quick and dirty simulation of what I think the insides of that Agilent adaptor will contain. The simulation shows thru loss and input match at the 75R port.This model took a few seconds to produce based on my experience of stuff like this. But you can see that the model and the official plots (on the right) agree even at this simple level.There are only a few components in the model. To refine it would mean creating a very complex model but the quick and dirty model shows that this Agilent adaptor is a crude and unsuitable device if you expect it to give good performance up at 6GHz (no wonder it is so cheap).

author: rx8pilot
author_rank: Super Contributor
timestamp: March 19, 2017, 11:05:47 pm
content: What a mess, nuts. Thanks for the quick look.With T&M - it is very rare to truly find a bargain. Low price, as you said, generally has a reason. The list price of the probe I just purchased is completely wild at $5,800. Since I can count on its performance (up to 7Ghz) and got it for a little over $2k - it kinda feels like a bargain. I only have to sell a handful of the resulting circuits to cover that cost and Keysight knows that. I can only sell them if it is very reliable and meets the published specs too.Looking forward to putting all the learning efforts to understand high-speed signal integrity to use - even if only on a small scale at the moment. I am still quite deficient in T&M for what I am after - but time and determination will fix that.

author: TheSteve
author_rank: Unknown
timestamp: March 19, 2017, 11:33:48 pm
content: Please do post some sample scope shots once you receive it and connect it to the DUT.

author: G0HZU
author_rank: Super Contributor
timestamp: March 19, 2017, 11:44:07 pm
content: The other thing the model predicts is the group delay for that BNC 75R to 50R adaptor and it shows a ripple as high as 37ps peak to peak. See the plot below:Obviously, this is just a model but it would be interesting to see what the official spec is for the group delay response. For a lot of users that Agilent adaptor will be fine even up at many GHz but only you can decide if the performance is good enough for your needs. It isn't a terrible device to be avoided at all costs, it just isn't a precision component up in the GHz region. If you can afford to splash out big money on probes you could still consider the adaptor for casual measurements or for use at lower frequencies?

author: rx8pilot
author_rank: Super Contributor
timestamp: March 19, 2017, 11:54:53 pm
content: Quote from: G0HZU on March 19, 2017, 11:44:07 pmI financed a probe that was 60+% off list for 6 monthsNot much of a cash splash, lol. I just recently paid off the various power related probes that I needed - they were also expensive, but allow me to design better circuits and therefore pay for themselves after I release a new product. Each T&M tool I purchase has to have a specific path to paying for itself within a modest timeframe.I am considering some lower cost scopes and probes for the more common daily tasks that could risk the expensive stuff. It is a slow crawl at the moment. I better get something new in my catalog to make sure I can pay for this thing.

author: G0HZU
author_rank: Super Contributor
timestamp: March 20, 2017, 01:45:51 am
content: All my RF probes here are either homemade or salvaged from a skip and repaired.For a bit of fun I had a go at making and testing a very crude pad to match 75R to 50R using cheapo SMD resistors.I know how my SMD resistors here behave up at several GHz so I knew this pad would not be that great. If I had access to more choice in terms of SMD resistor package (like I have at work) I could do much better.But see below for a plot of the response of a pad that cost about a penny in parts. OK, it's on a low loss test fixture made with a decent PCB but I have de embedded the result to just show the response of the resistor section. This is a real pad measured on a VNA calibrated with an Ecal module.You can see that the RF performance is a bit better behaved. I'd probably embed 75R semi rigid into a real design and use a decent 75R N connector at the input and then use a decent 50R connector at the output. With better SMD resistors and some fiddling I think I could get the VSWR at 6GHz much better than thisThe performance begins to unravel just above 5GHz in the plots below but it manages to keep the VSWR below 1.4:1 even with this early/crude attempt.

author: nctnico
author_rank: Super Contributor
timestamp: March 20, 2017, 02:10:12 am
content: What kind of connectors are on your impedance matching circuit? I have a feeling the BNC's aren't helping on Keysight's product.

author: rx8pilot
author_rank: Super Contributor
timestamp: March 20, 2017, 02:13:03 am
content: That's not bad for a such a quick effort. This is my primary reason for wanting a VNA - literally to just do experiments for the sole purpose of learning and having fun. Thanks for sharing EXCELLENT!Sent from my horrible mobile....

author: G0HZU
author_rank: Super Contributor
timestamp: March 20, 2017, 02:25:30 am
content: Quote from: nctnico on March 20, 2017, 02:10:12 amIt's (kind of) 'connector-less' because I've de embedded the (50R SMA) connectors and the PCB microstrip used in the test fixture. So my reference plane is right at the input to the resistors. If I was to make a real version of this I would go straight in via 75R semi rigid and avoid the BNC connection. I think there are a few things that are less than ideal about the Agilent pad and I included this in my attempt to model it. I don't think it is just the BNC connector type although that obviously doesn't help.

author: G0HZU
author_rank: Super Contributor
timestamp: March 20, 2017, 02:35:33 am
content: If you are interested, here is an old plot of a precision 6dB splitter I made a couple of years ago. It was designed to be 'very' accurate up to 1GHz to allow me to test my old HP vector voltmeter but it ended up working a lot better than this. I tried quite a few resistor types and once I found the best, I then did a select on test to find the most accurate resistors before building it. I also used Sonnet EM on a PC to model the PCB layout to optimise the VSWR across the full BW.But the plots below include the connectors and it was all cal'd measured using a 4 port VNA and a 4 port Ecal module. Obviously, I only needed 3 of the 4 ports and the plots below are of the s parameter data exported from the VNA. The phase matching and VSWR and amplitude match are all very good. As good as some of the 'real' offerings from the big boysIf I was to try and make a precision 6GHz 75R to 50R match pad I would try similar tricks to get it as good as possible. With care you can make some decent accessories like this quite cheaply

author: 1audio
author_rank: Frequent Contributor
timestamp: March 20, 2017, 04:00:14 pm
content: Getting past 3 GHZ at 75 ohms is really hard. Keysight doesn't have a VNA for 75 Ohms above 3 GHz. I just looked. I have an 8753E 75 Ohm which was the best Agilent had at the time. The internal hardware could go to 6 GHz but no support etc. was available and its limited to 3 GHz. I don't know if you can get a test kit today good for much above 3 GHz @ 75 Ohms. Its all uncharted territory. If you can't get a termination for 75 Ohms good for much past 3 GHz its difficult to know if anything else is OK. I have a 75 Ohm air line but its terminated in 75 Ohm GR 874 connectors so 6 GHz is not likely.And be sure to keep the 75 Ohm N connectors etc well away from the 50 Ohm N connectors. A casual mistake can be devastating if its on the front of the VNA.The active probe may be your best option.

author: Someone
author_rank: Super Contributor
timestamp: March 20, 2017, 10:26:51 pm
content: Quote from: 1audio on March 20, 2017, 04:00:14 pmIts not uncharted territory:Quote from: Someone on March 17, 2017, 05:35:46 amSo with a calibration (de-embeddinghttp://literature.cdn.keysight.com/litweb/pdf/5989-5765EN.pdf) of the setup you can use 75Ohm systems (or other impedances with your own adaptors) out beyond 3GHz. How do you think people measure waveguides at their arbitrary impedances?

author: G0HZU
author_rank: Super Contributor
timestamp: March 20, 2017, 11:20:37 pm
content: QuoteI've taken this quote from the link in Someone's post above. These techniques are vital for the stuff I do at work because I have to do wideband design up to many GHz. I have to be able to examine/model various components or structures with a VNA and I can only do this using advanced de embedding techniques.When I upgraded to my 4 port VNA just over a year ago for home use I spent nearly 2 months playing with the Ecal module and perfecting the de-embedding for several test fixtures I have here. That's virtually all I did with it in those first few weeks because this stuff is so important if you want to get meaningful/accurate results from your test setup. 'Having the gear' is only the first step.Get this stuff right and combine it with a decent RF simulator and it enhances your capabilities way beyond the typical/casual VNA user

author: 1audio
author_rank: Frequent Contributor
timestamp: March 21, 2017, 04:03:49 pm
content: Thanks for the link. Lots to learn. The extended characterization moves the cost from $67 to $753. Clearly a lot of time invested in the measurements and number crunching. I expect there is some selection up front as well.

author: rx8pilot
author_rank: Super Contributor
timestamp: March 21, 2017, 08:18:20 pm
content: Some great info for sure.Quote from: G0HZU on March 20, 2017, 11:20:37 pmYes, indeed. I have spent the last year just learning how to approach the high-speed circuits I have been hoping to design. I suspect that it will continue to be an uphill learning curve as I dig into the practical realities. There are so many test and measurement traps that can send the engineer on a useless tail chasing adventure. It does seem that there is always one missing probe, adapter, or cable that costs a ton of money though.That is why this forum is so useful and fun - there is always someone that has ben stuck in the same trap and willing to talk about it. So much to learn, so little time......