Interest in a 2 channel 16 bit 250MSPS o'scope? - Page 1
Forum section: electronics / testgear

author: boeserbaer
author_rank: Contributor
timestamp: August 18, 2014, 05:55:07 pm
content: Hi all,I am considering producing a 2 channel 16 bit 250MSPS ethernet connected data acquisition system which would be packaged as an digital oscilloscope, making precision lab measurement a snap, but mostly targeted for lab data acquisition. Does anyone thank that such a system would be useful to their applications. Imagine something the size of a RIGOL DS-1052, or a little larger depending on LCD size.Possible feature set:1. 2 channels, each acquiring 16 bits at 250 MSPS (BW 100MHz). Up to 512M samples single shot capture (either 25M per ch or 512M 1 ch)2. Free programming API in 3 parts:a) Simple API over gbit ethernet to control and acquire data. Example remote display and acquisition program.b) Lower Level API running on the LINUX (ZYNQ) to allow tighter control3. FPGA source avail under NDA4. Normal scope operation with triggered (on either waveform or ext) operation5. Spectrum analyzer functionality6. Initial product would offer 50ohm inputs only. Maybe even including optional front end bypassing and direct VinP and VinN along with buffered ADC common mode out (BW ADC limited to around 700MHz).7. Battery power as an option?What features would make his an indespensible instrument?Pricing in the $5k rangebest regards, mike

author: Dongulus
author_rank: Regular Contributor
timestamp: August 18, 2014, 06:30:26 pm
content: I don't get it. You sound like you are just referring to a DSO with a LAN connection to the PC.Such products already exist.By the way, 16 bit samples is totally unnecessary for an oscilloscope. Most oscilloscopes are 8-bit with few others that I have seen that are up to 10 bits. Also, 250 MSPS is not sufficient for capturing signals up to 100 MHz. The rule of thumb for oscilloscope sampling is 10x, so 250 MSPS would only be reasonable to capture 25 MHz signals.

author: David Hess
author_rank: Super Contributor
timestamp: August 18, 2014, 06:37:18 pm
content: Quote from: boeserbaer on August 18, 2014, 05:55:07 pmWhat kind of triggering? Does this preclude equivalent time operation?Quote50 ohm inputs with only 100 MHz of bandwidth strikes me as a contradiction in application and without a switched attenuator input, it is a specialize digitizer rather than an oscilloscope.QuoteEthernet provides galvanic isolation which is a weak point in USB connected instruments. I would think that POE support is more useful than battery power if Ethernet has to be used anyway.

author: boeserbaer
author_rank: Contributor
timestamp: August 18, 2014, 06:41:50 pm
content: Hi Dongulus,Thank you for your reply. I totally agree that 16 bits is not needed for an oscilloscope. What I am wondering is whether there are lab users who are making precision measurements that need such an instrument. it is more of an scope enabled precision data acquisition system than a LAN enabled DSO. Also the 100MHz referred to the BW limit on the front end. There are applications, where sampling is used above Nyquist.Best Regards, Mike

author: boeserbaer
author_rank: Contributor
timestamp: August 18, 2014, 06:48:04 pm
content: Hi David,Planned triggering would be basic threshold crossing with hysteresis on either waveform, or external logic signal.regarding equiv time sampling, planned operation on analog triggered waveforms would indeed include equiv time sampling.as for the 50 ohm inputs, the reason I posted the questions here was to get feedback on usability. The intent was to preserve the 16 bits S/N. While it is more of aspecia;ized digitizer than an o'scope, maybe it does need a scope probe amplifier to be usable generally.Thanks for the POE suggestion.Best Regards, Mike

author: Maxlor
author_rank: Frequent Contributor
timestamp: August 18, 2014, 07:23:38 pm
content: 16 bit resolution sounds interesting. I can think of several situations where it would have been nice to more accurately measure short events, e.g. the voltage drop during 3ms battery-powered zigbee TX events. Continuous sampling and transmission over ethernet (even if it's limited to 5MS/s... or maybe you can do gigabit ethernet?) would also sound interesting.At $5k, I'd probably get a picoscope instead though, sorry.

author: alex.forencich
author_rank: Frequent Contributor
timestamp: August 18, 2014, 07:24:12 pm
content: VXI-11 support as that is the standard Ethernet instrument control protocol. You can certainly have a higher performance custom protocol as well.Also, it would be very handy in some instances to be able to add various decoding or highly non-standard triggering configurations. It might be very cool if it would be possible to install custom HDL-based triggering or decoding logic. Not sure what implementation would make the most sense here - open source the whole design, partial reconfiguration, separate FPGA for custom components, etc. Separate FPGA would probably be the simplest to support if you don't want to open up the whole FPGA configuration. A small Spartan 6 that's supported under the free version of the Xilinx toolchain would be really sweet.

author: jeremy
author_rank: Super Contributor
timestamp: August 18, 2014, 10:04:42 pm
content: This sounds kind of like the Red Pitaya. I have one of those, for the exact reasons you describe. 14bit at 125Msps I think. And you can put whatever you want on the zynq FPGA. It's only $500 though.

author: boeserbaer
author_rank: Contributor
timestamp: August 18, 2014, 10:31:27 pm
content: Hi Jeremy,Thanks for pointing out the red PIYATA. I had looked at it quite a while back, and passed because my applications required a higher sampling rate. That said, the red PIYATA may meet most peoples requirements. Not everyone needs 16 bits at 250MSPS. Also, if I were to leverage my design off of the red PIYATA, I probably could lower my price, as the development (software side mostly) would be greatly reduced. I was planning to package it as a bench top instrument, including display, power, cooling... The kind you can carry around the lab, quickly hook up to your signal, and verify everything is working. Additionally, embeddable into experiments easily. I personally don't like being having to be connected to a computer for a quick signal check, but I can see where many/most won't be willing to pay the price difference for a full product.And maybe the inexpensive 8 bit scopes meet all of the portable setup requirements, and you would just want a stripped down data acquisition module for most experiments.Again your feedback is most helpful.Best Regards, Mike

author: nctnico
author_rank: Super Contributor
timestamp: August 18, 2014, 11:00:51 pm
content: Quote from: jeremy on August 18, 2014, 10:04:42 pmAnd that is not the only instrument that exists in that class. For one of my customers I made FPGA firmware for an oscilloscope which synchronises and communicates over a fiber optic network based on Cern's White Rabbit project (http://en.wikipedia.org/wiki/The_White_Rabbit_Project). But this kind of equipment has a very specialised and limited clientele.@boeserbaer: With a resolution of 16 bits you really need to be careful about the analog design regarding frequency response, sub picosecond clock jitter (very important!), harmonic distortion, etc. Designing a proper analog front-end is a big project in itself. And then there is the software part. I have been down this road before with an USB oscilloscope before Rigol flooded the low end market and found out that creating a meaningful API takes a lot of time.

author: David Hess
author_rank: Super Contributor
timestamp: August 18, 2014, 11:16:59 pm
content: Quote from: boeserbaer on August 18, 2014, 06:48:04 pmThis makes sense if the triggering is done using the sampled data stream which is how modern DSOs work although most apparently forgo equivalent time sampling. I wonder now if they leave out equivalent time sampling because the external trigger path can not support it anyway. I have often wondered if such a design can result in aliasing affecting the trigger itself but have not had an opportunity to test it yet.Quote50 ohm inputs are not significantly less noisy than high impedance 1 Mohm inputs on oscilloscopes because the shunt capacitance associated with the high impedance input low pass filters high frequency noise. The high relative noise level compared to other instruments like spectrum analyzers is because an oscilloscope does usually operate with narrow bandpass filtering.This also shows why 8 bits of resolution is usually sufficient when working in the time domain with wide bandwidth. Without some type of noise reduction like bandwidth limiting or synchronous demodulation in the form of averaging, wide bandwidth results in too much noise for a higher resolution ADC to be useful.If *that* was not enough of a problem, achieving 16 bit settling times at high bandwidths is somewhere between difficult to impossible.So what does the above mean? You can take advantage of 16 bit resolution with lower bandwidths if slow settling times are acceptable as well. I find it difficult to believe there is a market for such an instrument because a cheaper design could use a fast 8 bit digitizer for the DSO functions in parallel with a 24 bit delta-sigma ADC for precision measurements at low bandwidths.

author: boeserbaer
author_rank: Contributor
timestamp: August 19, 2014, 01:09:11 am
content: Hi nctnicoI occasionally check on CERNS open hardware, I spent 20 years designing nuclear instrumentation, including very early sampling ADC based spectroscopy for 100s of channels that had to resolve time of arrive to sub ns levels, while simultaneously recording gamma ray energy at over 1million cts/s.Lately I have been designing boards with up to 4GSPS at 12 bit ADCs. there you can really see the impact of time jitter, as was recently evidenced using a newly released 2Ghz oscillator that had only 18dB suppression of the 1GHz fundamental.For nuclear spectroscopy, there is certainly a place for the 16bit 250MSPS ADC's, but it appears that the consensus on this list is that most engineers wouldn't value it as a general purpose instrument.Your's and everyone else's considerate feedback is really helpful prior to sinking a lot of time and money down this path.best regards, mike

author: boeserbaer
author_rank: Contributor
timestamp: August 19, 2014, 01:19:23 am
content: Hi David,I may have mis-spoken regarding the method on equivalent time sampling. What I have done in the past is rebin the amplitude data based on the interpolated trigger position. This works well if the slope is sufficient. I don't usually make o'scopes, just use them, and sometimes emulate their functionality on data acq boards.Regarding the 16 bit conversion, I have made boards that had over 500MHz input bandwidth, and only a few counts RMS noise. Certainly such that a 14bit converter would have unable to see the noise.All that said, while I would buy the instrument, I do not see any "Wow, with that I could move mountains" from the list.Thanks for the feedback.Best Regards, Mike

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 19, 2014, 01:44:22 am
content: If you're going to run 16 bit ADCs at that speed, you'll need self calibration methods to make it useful. Be prepared to include a >14 bit equivalent accuracy voltage reference, signal path switches (rated for the bandwidth), and probably a variable gain or step generator or something in there, to calibrate different levels and AC/DC gain. Problem being, those ADCs are all tuned for SDR or ultrasound apps, where DNL must be relatively small, but INL doesn't even matter. Usually, the 14 and 16 bit models in a series are no better on INL than the 12 bit version.Definitely do not bring out ADC pins, unless you're prepared to find a socket for the chip and leave spares in the case!Tim

author: jeremy
author_rank: Super Contributor
timestamp: August 19, 2014, 01:45:27 am
content: Very little equipment in our lab moves around (optics lab). Probably the most portable things are DSOX2002 scopes. This is mostly because the experiments are extremely sensitive, and the instruments are very heavy. Almost everything runs off NI DAQ cards which are connected to a PC, or instruments which have a PC in them anyway (like an optical spectrum analyser). It would be interesting to have such a fast 16bit ADC, but what I really want is a fast, high res ADC that will dump to an SSD. Sometimes the phenomena I am investigating are rare and not well understood, so I simply need to record data for a long time to try and catch it.

author: marshallh
author_rank: Unknown
timestamp: August 19, 2014, 01:51:27 am
content: Trying to design a 100mhz frontend for that 16bit adc is mind blowing. I salute you for even entertaining the idea.

author: David Hess
author_rank: Super Contributor
timestamp: August 19, 2014, 02:51:06 am
content: Quote from: boeserbaer on August 19, 2014, 01:19:23 amThat is how equivalent time sampling works. With an analog trigger, each waveform acquisition has an associated measurement of the time between the trigger and sample clock. With a digital trigger, the interpolated trigger point allows the same measurement of the delay between the trigger and sample clock in a way which is very similar to how centroid timing and transition midpoint timing time delay counters work.QuotePeak to peak noise is usually more meaningful with an oscilloscope. If the RMS noise is 3 counts at 16 bits and the peak-to-peak noise is 5 times greater, then that reduces you to 12 bits of resolution which coincidentally is what LeCroy's high resolution oscilloscopes use.QuotePicoscope has some oscilloscopes with high resolution ADCs. It may be worth checking out their products to get an idea of what is available.

author: jeremy
author_rank: Super Contributor
timestamp: August 19, 2014, 04:10:44 am
content: The downside to picoscopes is that they cost a bomb. When I looked at them in the past, they were more than 2x more expensive than an equivalent Rigol (which has a screen and buttons and everything). That was for 8bit scopes though.

author: edavid
author_rank: Super Contributor
timestamp: August 19, 2014, 04:14:15 am
content: The AD9467 is only $141 each, where is the rest of the money going?

author: nctnico
author_rank: Super Contributor
timestamp: August 19, 2014, 09:12:02 am
content: Quote from: boeserbaer on August 19, 2014, 01:09:11 amWhat would be interesting is a low cost ($1k) scope which goes to several GHz. There is a gap in the market.

author: eurofox
author_rank: Unknown
timestamp: August 19, 2014, 10:19:45 am
content: Quote from: nctnico on August 19, 2014, 09:12:02 amIf you can manage with a sampling scope that is available in this price range.Now I expect again people say that they need a real time scope ..... let them spend 1/2 million on such a scopeFor "normal" use a 500Mhz or 1Ghz scope is perfect. I use a MSOX3104AIn HF and ultra fast pulse analyses signal are often recurrent and for this a sampling scope is a perfect tool.

author: David Hess
author_rank: Super Contributor
timestamp: August 19, 2014, 01:29:06 pm
content: Quote from: eurofox on August 19, 2014, 10:19:45 amI have often thought so as well and went as far as preliminary designs for a DSO which shared as much as possible between a real time digitizer with 100 to 200 MHz of bandwidth at relatively low sample rates of 100 to 200 MS/s and a sampling front end with 4+ GHz of bandwidth and sample rates of 10s to 100s of kHz.I am dissuaded however by the existing low cost sampling oscilloscopes which are neither inexpensive nor in demand. The only unique things I considered are random sampling support like a Tektronix 3T2 or 7T11 so a pretrigger signal is not necessary and TDR support like the Tektronix SD series of sampling heads.

author: nctnico
author_rank: Super Contributor
timestamp: August 19, 2014, 04:13:18 pm
content: Quote from: eurofox on August 19, 2014, 10:19:45 amMaybe going realtime isn't that far fetched. It seems boeserbaer has been working with fast ADCs a lot. If he manages to interleave a few he could get close to 10Gs/s or more. The theory behind a sampling scope is quite simple but turning it into a useful instrument is hard.

author: David Hess
author_rank: Super Contributor
timestamp: August 19, 2014, 04:43:43 pm
content: Quote from: nctnico on August 19, 2014, 04:13:18 pmThe problem is not the digitizer. The problem is the analog signal path before the digitizer and the sampling of the signal before conversion.Tin (time constant) << t (sampling pulse width) samplers have a very predictable sin(x)/x frequency response unburdened by external bandwidth limitations because the signal in a transmission line environment is sampled directly. This makes 4+ GHz performance easy compared to a design with linear elements preceding sampling.It is much easier to design and fabricate a DC to 4 GHz sampling head than a DC to 4 GHz linear amplifier or switched attenuator and that is not even fast for a sampler. Just swapping the diodes in a Tektronix S-2 sampling head which uses a discrete design with modern surface mount RF schottky diodes improves its performance from 4 GHz to 8 GHz or so.Of course there is a price to be paid for this. Tin << t samplers have very limited dynamic and offset range and have to operate at relatively low sample rates. On the other hand, they inherently have fast (instant) overload recovery which a linear amplifier cannot match making them uniquely useful in applications where this matters.

author: nctnico
author_rank: Super Contributor
timestamp: August 20, 2014, 11:50:40 am
content: Now add a trigger to that sampling scope.... That is the whole problem with a sampling oscilloscope: creating a trigger system. For example the TDS810 from Tektronix can deal with signals up to 6GHz but the trigger system only works up to 2GHz. There really is a big advantage of using interleaved ADCs and an FPGA instead of a diode sampler if you look at the entire oscilloscope and not just the sampling part.

author: David Hess
author_rank: Super Contributor
timestamp: August 20, 2014, 04:32:18 pm
content: Quote from: nctnico on August 20, 2014, 11:50:40 amIf you mean the Tektronix TDS820, it is pretty low performance compared to its ancestors.High frequency triggering on a sampling oscilloscope is not difficult if only because synchronous triggering with some form of phase locking may be used. It may even be added externally. The old Tektronix sampling sweeps (and some of their linear sweeps) support injection locking (countdown triggering) for this. The 7T11 I mentioned triggers to 1 GHz using level triggering and 12.4 GHz using a built in countdown trigger. The more modern but old CSA803 and 11800 series use a countdown trigger head to support triggering to 20 GHz.I suspect high frequency triggering like this was dropped in general purpose sampling oscilloscopes but retained in the more specialized communications signal analyzers.What kind of input bandwidth is available in the ADCs you would want to use? If you interleave them and perhaps even if you do not, then you are faced with some type of DC to GHz input driver which itself will limit bandwidth and overload recovery performance. I did a cursory search but did not find any integrated or hybrid ADCs that would be suitable while I have several examples of sampling head and trigger designs which operate into the 4 to 16 GHz range. If someone makes an ADC with 4+ GHz bandwidth at 100+ MHz (or slower) sample rates I would like to see it.An integrated high bandwidth track and hold or sample and hold with a separate relatively slow ADC is another option but I have never seen one short of a discrete sampler.