What sort of features does one forego buy buying a used scope Vs. a new scope - Page 1
Forum section: electronics / testgear

author: ImABeginner
author_rank: Contributor
timestamp: March 10, 2020, 08:03:40 pm
content: I'm looking forward to buying my first 5Gs/s scope very soon for RF applications. I've noticed that I can buy a good used scope on E-bay for a good discount from what a new scope would be. I could even, for the same price, get a scope which would have better performance specs. These are often older, discontinued models. So I am curious. What features of modern scopes am I missing out on if I purchase an older model with the same specs?

author: nctnico
author_rank: Super Contributor
timestamp: March 10, 2020, 08:18:27 pm
content: Care to be specific about models you are looking at?

author: ImABeginner
author_rank: Contributor
timestamp: March 10, 2020, 08:26:10 pm
content: Sure, On the new side, I'm looking at theRigol DS7014, and on the used side, I like the look of theLeCroy DDA-5005 on this page.

author: profanum429
author_rank: Regular Contributor
timestamp: March 10, 2020, 08:47:24 pm
content: I believe that DDA-5005 is only 50ohm input impedance, so careful on that.I actually have a Waverunner 6030A showing up today that I'm looking forward to playing with some. You might want to look at the 6100A or something if you're looking for a more general scope but I can't give much advice towards use on them since I'm just getting my 6030A.

author: tggzzz
author_rank: Super Contributor
timestamp: March 10, 2020, 09:22:12 pm
content: Quote from: ImABeginner on March 10, 2020, 08:03:40 pmYour first task is to define what measurements you need to make, and whether a scope is the right tool.Usually with RF away from baseband, a scope is the wrong tool.Pay particular attention to the linearity, noise, and (for a digitising scope) ENOB specs. Understand how they will limit what a scope can show you.,

author: ImABeginner
author_rank: Contributor
timestamp: March 10, 2020, 09:41:17 pm
content: that's good advice, I was under the impression that under sampling theorem, If I were designing 2.4GHz hardware, that I'd need a sampling rate of 4.8Gs/s. I'll be sure to look more into the non-linear properties of these scopes.

author: tggzzz
author_rank: Super Contributor
timestamp: March 10, 2020, 10:05:07 pm
content: Quote from: ImABeginner on March 10, 2020, 09:41:17 pmNo, you don't; that's a common misunderstanding of the sampling theorem.I have a 4GHz scope with a 40kS/s front end. 30 years ago a top end HP 1GHz scope managed 40MS/s.

author: nctnico
author_rank: Super Contributor
timestamp: March 10, 2020, 10:27:44 pm
content: Quote from: ImABeginner on March 10, 2020, 08:26:10 pmThe first is a general purpose scope (but rather new and Rigol isn't known for fixing their software quickly) and the second is a very application specific DSO with 50 Ohm only inputs. Using the latter as a general purpose oscilloscope will be difficult but the DDA5005 will be fantastic for doing RF measurements. However the DDA5005 is likely to achieve full bandwidth on 2 channels only because only then the samplerate will be anough to allow a 5GHz bandwidth. IIRC a forum member has figured out the connector type and for sure there will be someone to help out liberating all the options.Regarding samplerate: if you want to do single shot acquisitions then you need to have a samplerate of at least 2.5 times the bandwidth. If the signal is repetitive you can use equivalent time sampling to reach much higher effective samplerates BUT the signal must be 100% repetitive because the resulting trace will consist of multiple acquisitions.

author: 0culus
author_rank: Super Contributor
timestamp: March 10, 2020, 11:04:07 pm
content: If you're doing RF, you should consider spending your money on a spectrum analyzer instead. Far more useful for the types of measurements needed. Plus, if you find yourself needing time domain, most spectrum analyzers have an output that samples the last IF (often 21.4 MHz, which will work with almost any oscilloscope that has a pulse).

author: james_s
author_rank: Super Contributor
timestamp: March 11, 2020, 02:03:48 am
content: The main thing you'll forgo buying used is a much lighter wallet. Then there's the warranty if you care about that, I never did. Also new scopes will typically be smaller, lighter and have deeper memory. Many also have protocol decoding but a scope is not really the best tool for that job IMO.

author: helius
author_rank: Super Contributor
timestamp: March 11, 2020, 02:18:31 am
content: I'd echo James's points, and add that when you find a used scope you want, it may require maintenance and/or cleaning that wouldn't be needed for a new scope. This shouldn't be much of a problem for hobbyists or hands-on technicians but it wouldn't be feasible if you were equipping a lab for others.Newer scopes may have LXI (ethernet) whereas older ones typically have GPIB (IEEE-488) or RS-232 interfaces. So the computer software used for saving configs, waveform printouts, etc, may be different.

author: ImABeginner
author_rank: Contributor
timestamp: March 11, 2020, 03:34:39 am
content: This is such a good lot of info. The only other scope I've owned is a dinky 50Mhz hantek scope. It was great for a beginner scope, but it's clear I've outgrown it, and this information is very helpful. My thanks.

author: james_s
author_rank: Super Contributor
timestamp: March 11, 2020, 05:52:51 am
content: Yeah if I were spending someone elses money setting up a company lab I'd just buy new equipment, and if one is a hobbyist with basic needs and not a lot of repair experience then something like a Rigol or Siglent is so cheap that it's hard not to recommend it but prices rise sharply beyond 100MHz. Also for me the fixing and polishing up part is half the fun, but if you just want a scope to use that might not be the best way to go.The further beyond 100MHz you need, the more it's likely to make sense to buy used.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: March 11, 2020, 06:26:04 am
content: Lots of people saying you should get a specan instead of scope for RF: really depends on the type of RF work. If you are having to do stuff like modulation analysis, a scope with vector analysis software could be more usefull. Modern specans can do this kind of stuff too, but usually with much more limited bandwidth (EG, I don't know of a single specan that can do it over a BW of more than 1 GHz, and I think even that requires a scope as ADC).

author: james_s
author_rank: Super Contributor
timestamp: March 11, 2020, 06:29:41 am
content: It really depends on what you want to do. If you're JUST interested in RF then the SA is useful. If you lean heavily toward RF then you might consider buying an older, cheaper less capable used scope *and* a spectrum analyzer which almost necessitates used gear unless you have a big budget. A spectrum analyzer is a wonderful luxury for RF stuff but a scope is a much more versatile instrument.

author: tautech
author_rank: Super Contributor
timestamp: March 11, 2020, 06:32:25 am
content: Quote from: TheUnnamedNewbie on March 11, 2020, 06:26:04 amSSA3000X Plus, SVA1000X and the soon to be released SSA3000X-R RTSA can all do Modulation analysis. Models range from 1.5 GHz to 7.5 GHz.Each are well featured SA's in their own right and the SVA range a dual purpose SA/VNA instrument.

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: March 11, 2020, 07:49:58 am
content: Quote from: ImABeginner on March 10, 2020, 08:03:40 pmThat's like saying "I need to buy a vehicle for commuting to work". What "RF applications" are these? What signals will you be looking at? What information do you want to get out of it?The answers to these questions determine what you should be buying, which may or may not be a scope (or a spectrum analyzer).Besides, sample rate isn't the only relevant criteria for a digital scope. What about analog BW (5GSa/s gives you enough real-time BW for 1.3GHz, i.e. you'd be looking at an 1GHz scope)? Memory depth? What analysis tools (if any) do you need?QuoteNot much, really, if you are looking at newer scopes (i.e. made after 2000). Mostly waveform rates but unless you're buying a simple scope the update rate isn't very relevant as you'd have a sophisticated trigger suite anyways. Also, sample memory and screen sizes are generally smaller than on newer scopes, it will have an old operating system like Windows XP, and network ports may be slower.On the other hand, you can get a used high bandwidth high performance scope for the price of a new mid-range or even entry-level scope.Quote from: ImABeginner on March 10, 2020, 08:26:10 pmWell, you didn't say what you want to do but if you need FFT then the Rigol will be very limited (1Mpts FFTs) while the DDA-5005 (which is essentially a WavePro 8500) can (if I remember right) achieve up to 25Mpts FFT (and you can run multiple). The DDA-5005 will also have a lot more analysis tools (it can even do signal analysis, although not officially). As others said the full 5Ghz BW can only achieved in single and dual channel mode. And it's a pretty huge and very loud scope sucking a lot of power.There might be other alternatives but since we don't know what the instrument will be used for it's difficult to make any recommendations.Quote from: ImABeginner on March 10, 2020, 09:41:17 pmFor scopes, you should really look at a sampling frequency which is 3x the max BW. For real-time sampling you need a sample rate higher than the Nyquist frequency (2 x fin), not equal. So for a 1GHz signal you want something with more than 2GSa/s. And since you want every period of the signal adequately sampled a fractional sample rate (i.e. 2.5GSa/s) won't work very well (the signal will be wobbly because some of the periods will be under-sampled), so the fraction of sample rate and signal BW should really be an integer.Of course, depending on what you want to do (you didn't say) you might well be able to achieve your goals with equivalent time sampling. In this case you might want to consider something like an Agilent 89600 Series scope (DSA) or a LeCroy WaveExpert.But again, without you giving us some more details it's all just guesswork.