Most "affordable" way to get a really fast oscilloscope? - Page 1
Forum section: electronics / testgear

author: 0culus
author_rank: Super Contributor
timestamp: March 18, 2019, 06:33:02 pm
content: As I plan out acquisitions to help me with the >1 GHz RF projects I want to do, as much as I love my old analog 2465B, it's just not fast enough to be useful beyond getting a rough frequency estimate at 1 GHz. Beyond that it's entirely useless, obviously.Unfortunately, new real time digital scopes faster than 1 GHz are already well outside my budget. Even older ones are still 4-5k on ebay, such as the Tek TDS7404. I suppose I should just save some money and brace myself. Any suggestions?

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 18, 2019, 06:37:26 pm
content: Double brace. Probes, cables, connectors and all the other goo you need gets expensive at those frequencies.

author: macboy
author_rank: Super Contributor
timestamp: March 18, 2019, 06:56:22 pm
content: Buy a used Lecroy. Easily well under $1k per GHz.There are lots of other threads here discussing various models.

author: David Hess
author_rank: Super Contributor
timestamp: March 18, 2019, 07:04:13 pm
content: Spectrum and network analyzers are generally more useful that oscilloscopes for RF development. If you must have an oscilloscope, then a used sampling oscilloscope might be acceptable depending on exactly what you want to do. These two types of instruments overlap with something like the discontinued with no replacement HP Microwave Transition Analyzer which is a sampling oscilloscope optimized for RF applications.

author: edpalmer42
author_rank: Super Contributor
timestamp: March 18, 2019, 07:06:16 pm
content: Are you sure that you need a scope for those frequencies? Typical instruments in that frequency range are spectrum analyzers, frequency counters, and RF level meters.Ed

author: 0culus
author_rank: Super Contributor
timestamp: March 18, 2019, 07:10:33 pm
content: I'd like to be able to look at time domain too, basically. But it may be wayyy too expensive.I already have a good collection of HP spectrum analyzers and counters up to 20 GHz, so I'm set for those. VNA is another thing I'd love to have but $$$  perhaps better to save the money for a really fast 'scope and put it towards a VNA?

author: gf
author_rank: Super Contributor
timestamp: March 18, 2019, 07:13:35 pm
content: Is 20GHz enough? I happened to see this one on eBay:https://www.ebay.de/itm/Tektronix-11801A-Oszilloskop-Digitizing-Sampling-Oscilloscope-20GHz-SD26-SD22/223448118324I have no idea, though, what the final price will be when the auction is finished.Or is it supposed to be real-time?

author: 0culus
author_rank: Super Contributor
timestamp: March 18, 2019, 07:17:04 pm
content: Sampling would probably be ok, budget might even demand it.

author: hendorog
author_rank: Super Contributor
timestamp: March 18, 2019, 07:34:19 pm
content: Quote from: 0culus on March 18, 2019, 07:10:33 pmIf any of your existing devices has an IF output then you could use that with your existing scope to view the time domain.i.e. you use the SA as a downconverter. Otherwise you could just DIY a downconverter with a mixer and an LO.

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 18, 2019, 07:35:17 pm
content: Quote from: macboy on March 18, 2019, 06:56:22 pmto add to this... if you dont mind to bargain, you can get Lecroy 7GHz differential probe at $500 from some china man. an impossibility in Tektronix even more so in Agilent world (where you will have a cut connector still at premium). but if that is still too expensive, if patient you can get lecroy 1GHz active probe at $100+ (there are few in ebay now @ $200). if that is still too expensive well, stick with the DS1054Z. edit: nctnico has produced his GHz differential probe at EUR100+ might be the world saviour worth a look, fwiw ymmv.

author: DaJMasta
author_rank: Super Contributor
timestamp: March 18, 2019, 07:37:59 pm
content: +1 to LeCroy and do you actually need it? Teks, Agilent, and Rohde & Schwarz tend to carry pretty high premiums even on the used market.The big advantage of a scope is that you can look at a very wide bandwidth in realtime, but you do sacrifice detail for it (8 bits of resolution while similar spectrum analyzers can manage much better). If you're working with RF stuff and want to acquire data from it, you generally only need to be acquiring from a band, so the internal demodulation bandwidth of your SA or an external regular bandwidth scope on the IF output of the SA will give you all you really need.Sampling is an option, but then you get the negatives of the scope's limited resolution with the extra restrictions of requiring repetitive signals, so some of the fault finding that you'd get from a very high bandwidth scope that you wouldn't get with an SA you wouldn't be able to see with a sampling scope (and you'd occasionally be able to see with an SA).In any case, LeCroy has options to 2-3GHz that have standard high performance BNC inputs, beyond that their scopes (and many others') need special adapters to connect normal cables to, and high bandwidth probes are normally pretty expensive. Higher bandwidth also means easier to damage frontends, so it's common for very fast scopes not to even offer over 1-2V per division input range and for fast probes to be only up to +-4V or so plus some configurable DC offset.

author: 0culus
author_rank: Super Contributor
timestamp: March 18, 2019, 07:41:41 pm
content: Good points, all. Thanks!Quote from: hendorog on March 18, 2019, 07:34:19 pmThey do...good idea!Quote from: Mechatrommer on March 18, 2019, 07:35:17 pmI wouldn't mind spending a little money. However, I also want to eventually have the capabilities of a DSO/MSO instrument. Maybe better to take the suggestion of using IF out on the SA and instead of getting a super expensive scope, get a slower but more fully featured real time DSO.Quote from: DaJMasta on March 18, 2019, 07:37:59 pmGood points. I was just reading an article about sampling vs. real time:https://www.electronicdesign.com/test-amp-measurement/what-s-difference-between-real-time-and-sampling-oscilloscopes

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 18, 2019, 07:51:36 pm
content: Quote from: 0culus on March 18, 2019, 07:41:41 pmget a Tek TLA715 or something... or simply stick to saleae logic... ymmv.Quote from: 0culus on March 18, 2019, 07:41:41 pmafaik, IF works with 50 ohm impedance (i'm not aware of IF that effectively has >1Kohm impedance but i maybe wrong i'm not RF guru) so you will be limited to 50 ohm probing and setups. you can as well go with just 50 ohm coax cable on high end Lecroy. ymmv.i've edited my previous post on nctnico's probe @ EUR100+... might worth it to have a look.

author: tggzzz
author_rank: Super Contributor
timestamp: March 18, 2019, 08:02:45 pm
content: Before taking the plunge and getting a scope to look at microwave signals, do some sanity checks.What's the power/voltage of the signals you are looking at? Scopes are very insensitive compared to spectrum analysers.What's the power/voltage of the noise in the bandwidth? Wide bandwidth scopes have to be noisier than narrower bandwidth spectrum analysers.What harmonic distortion is tolerable? Scopes have a lot of distortion compared with spectrum analysers.What's the bandwidth of thesignalyou are interested in? If you are looking at (say) a speech signal on a microwave carrier, you don't need a scope with GHz bandwidth!The only case I can think of where a scope would be necessary are with UWB transmissions or where multi Gbit/s signals are being transmitted over microwave links. Other suggestions welcomed.

author: 0culus
author_rank: Super Contributor
timestamp: March 18, 2019, 08:04:17 pm
content: Quote from: Mechatrommer on March 18, 2019, 07:51:36 pmAlready have a TLA series LA, so I don't strictly need an MSO.It's most likely 50 ohm. When working with higher power RF you're going to want to be using a sampling tee (and possibly attenuators + dc block) anyway to keep the levels at your test equipment under control.I'll take a look at the probes!

author: 0culus
author_rank: Super Contributor
timestamp: March 18, 2019, 08:15:31 pm
content: Quote from: blueskull on March 18, 2019, 08:09:33 pmUS, and my ultimate goal is to build a microwave transverter for ham purposes. However, in the meantime, I might have a few ISM band ideas. Not sure yet, still researching. Thanks for the offer! I'll keep it in mind.

author: 0culus
author_rank: Super Contributor
timestamp: March 18, 2019, 08:18:04 pm
content: If it comes down to taking you up, I'll share my location via PM rather than in open forum.

author: bd139
author_rank: Super Contributor
timestamp: March 18, 2019, 08:34:14 pm
content: Interesting thread. Do you really need to look at something that fast? I could probably get away with a 20MHz scope for ham purposes on any band if I keep my mits in the modulation domain.If not you can probably get away with a counter (or god forbid lecher wires) and something to measure power and keep it at that. I have no hope of being able to get anything useful above about 250MHz of bandwidth to be fair and going back to the 1970s a lot of amateurs didn't even have a scope and they managed to work microwave bands fine.To date, looking at 10GHz band, most of it's narrow band signals i.e. SSB / CW / anyway. See:https://eshail.batc.org.uk/nb/

author: nctnico
author_rank: Super Contributor
timestamp: March 18, 2019, 11:23:49 pm
content: Quote from: 0culus on March 18, 2019, 06:33:02 pmThere is a difference between the asking price and the selling price. The older Lecroys or the Agilent 54835A (which can be uphacked to the 54845A which is 1.5GHz on 2 channels) can be bought around the $1k ball park. Given the age it won't matter much if it is in working or 'for parts' condition. You have to do your homework to investigate possible issues either way.

author: 0culus
author_rank: Super Contributor
timestamp: March 19, 2019, 12:36:36 am
content: I think in light of the advice in this thread, I'd rather save my money for a VNA system. If I really need time domain view, I can use the 21.4 MHz IF out on my SAs with a slower scope.

author: David Hess
author_rank: Super Contributor
timestamp: March 19, 2019, 12:39:55 am
content: Quote from: 0culus on March 18, 2019, 07:10:33 pmI have a 14 GHz sampling oscilloscope and have used it to look at RF signals but they are never very interesting. Modulation bandwidths are so small compared to the carrier frequency and harmonic distortion is low enough that basically only the carrier is displayed as a sine wave.

author: hendorog
author_rank: Super Contributor
timestamp: March 19, 2019, 12:48:35 am
content: Quote from: 0culus on March 19, 2019, 12:36:36 amI think you are making a sensible decision.I found a VNA to be an incredibly useful tool for gaining an intuitive understanding of 'whats going on' - without having to relearn all of the maths I've forgotten.

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 19, 2019, 01:08:52 am
content: Quote from: 0culus on March 19, 2019, 12:36:36 ami think its a wise decision. S11 (DUT input impedance) and return loss/reflection is difficult to see without a VNA, try to make an antenna without it is like doing it with eyes closed (blind) think of VSWR and/or Zinp tools etc. but a VNA at the same BW as a used DSO is much much more expensive, even used. but i agree, save as much as you can for a VNA first, dont waste the money on DSO if you dont have a clear purpose for it, it will just delay your possesion of a VNA. but then , there are recent discussions on using a DSO and pulse generator to achieve VNA functionality, not in real time i guess and have to do much manual processing and math stuffs, you may want to have a look. after i got my KC901V (cheapest and portable) VNA to know why my video transceiver toasted (now i know why due to the china mushroom antenna crap), the hunt is on the hi BW DSO some years later, and the lecroy one i got recently have a lot of signal integrity / eye diagram snapshot captures. i guess it was used for that specific purpose during its life with previous owner, consultancy job? or some compliance test service etc. if you have that in mind, then you might consider a HBW DSO, TDR sampling scope may also do the same SI. if just radio amateur stuffs to see how strong transmitter or receiver antenna, i guess a SA will be enough. ymmv.