| Products > Test Equipment |
| How much noise floor and other things matter in oscilloscope usability |
| << < (23/63) > >> |
| David Hess:
--- Quote from: Performa01 on December 26, 2021, 12:57:16 pm ---It is either HiRes or ERES - I'm not quite sure - but in any case it is a true acquisition mode, in the sense of a real time pre-processing. The sample memory gets halved in this mode, because it is expanded to 16 bits width as the captured raw data now consists of 10 bit samples. All the post processing, measurements and math are now using the 10 bit data. The firmware cannot tell the difference between this resolution enhancement (implemented in the FPGA) or a true 10 bit ADC. --- End quote --- Old Tektronix DSOs used 16-bit acquisition and processing memory so high resolution mode did not halve the record length. |
| Performa01:
--- Quote from: David Hess on December 26, 2021, 09:13:48 pm --- --- Quote from: Performa01 on December 26, 2021, 12:45:46 pm ---Yes, split path input buffer have been invented a long time ago – and it’s all the more baffling that most people don’t seem to be aware of it and make it sound as if an oscilloscope frontend still consists of a cascade of differential amplifiers. Maybe some even think it consists of just a high speed OpAmp… --- End quote --- Differential amplifiers are still routine and the highest performance digitizers have differential inputs. Usually the first stage after the low impedance attenuators converts from single ended to differential, and this stage is convenient for adding the combined position and offset signal is introduced. The various modern PGAs used in oscilloscopes are differential so they follow the same pattern, but since they replace the low impedance attenuators, position and offset are added after. DSOs with a separate offset control will add it before the PGA. Old designs which do this have to somehow add the offset before some of the attenuation stages which means moving some of the attenuators to the differential part of the signal chain which is relatively expensive. --- End quote --- It doesn’t make much sense to get philosophic about obsolete designs. We are talking about general purpose DSOs here, which ranges from entry level (low end) up to the midrange, but excludes high end gear, which is specialized and definitely not general purpose. At one point, at least after the invention of the digital readout, T&M industry noticed that a minimum of DC accuracy and stability was expected. Users were no longer willing to permanently turn the offset control of their scopes just to center the trace, as they used to do with their ancient CROs, but expected a decently stable offset position and some accuracy. So, the split path design has long become universal for all general purpose DSOs – despite its drawbacks, where the most obvious is the overload recovery issue. And this is unavoidable, even by a good design. Of course we find the cascaded differential stages in almost every HF IC, and in HF instruments like spectrum analyzers it might well be the only amplifier architecture required, but split path has become common in wideband general purpose oscilloscopes since they are supposed to work from DC up to the specified bandwidth. Btw, there are folks who have managed to build a balanced version of the split path input buffer, so you can have this with balanced inputs too. --- Quote from: David Hess on December 26, 2021, 09:16:42 pm --- --- Quote ---If you actually think the LF noise in a split path design would be reduced, you’re forgetting that the LF path has to be attenuated quite a bit (usually up to 10 times) in order to get the desired input protection and a decent offset compensation range. This has to be compensated for by a corresponding gain in the OpAmp. Together with the high source impedance of the divider (which has to have a total resistance of 1 meg) this can raise the noise floor by more than 20 dB below the crossover frequency. --- End quote --- That is a good point that I had forgotten, but the noise can still be lower even in old designs. Old designs which have two separate x10 high impedance attenuators limit the input range to the buffer to 1/10th the level of new DSOs, so attenuation on the DC path is also lower. The Tektronix 22xx series only attenuates by 1.33. Luckily for the discussion here, low frequency noise is irrelevant because wideband noise at 20 MHz and higher bandwidths dominates. --- End quote --- It’s not “old designs” that utilize two input attenuators. Of course you cannot build a good scope with vertical gain settings from 500 µV/div up to 10 V/div with just one single attenuator. For instance, every contemporary Siglent DSO has two input attenuator stages. Offset compensation voltage has to be added to the input in order to be effective (otherwise the input stage would require a totally unrealistic high common mode range), so this is part of the LF path of a split path input buffer design and topologically sits between the attenuators and the PGA. With low attenuation factors you either need high supply rails (old design) or you get only a very low offset compensation range. But does a Tek 22xx even have a split path design? The specifications of up to one division trace shift for variable gain and trace invert make me wonder. All the more so as the best sensitivity is not particularly high at 2 mV/div. Or maybe they use the cheapest FET-OpAmp with high Offset voltage and -drift without self-calibration in the LF path – but this would somehow scotch the whole idea of the split path approach? Above some 100 kHz the situation eases a lot and at 10 MHz and above we get noise figures in the realm of 2 – 3.5 nV/sqrt(Hz) with proper designs at least from Rohde & Schwarz, LeCroy and Siglent. --- Quote from: David Hess on December 26, 2021, 09:16:42 pm --- --- Quote ---So there is no way around the sad fact, that the usual general purpose DSO isn’t well suited for precision work at low frequencies because of the steeply rising noise floor down there. --- End quote --- I agree but if you include older instruments, then some general purposes DSOs are much better than others at low and/or high frequencies. I have not tested enough modern low end DSOs to know if they all have subpar noise performance. Even with older instruments though, I gave up on good low noise performance a long time ago with the exception of anything with the Tektronix 5A22/7A22/AM502. At low frequencies it is relatively easy to make a low noise amplifier, but since oscilloscopes lack the noise marker function for their FFT, I would like to have a low noise dynamic signal analyzer instead. --- End quote --- I do not know what you mean by “low end” DSOs. We are talking about serious instruments here, so low end would be the entry level class. But the problem is not limited to these – all contemporary scopes up to the upper midrange have the very same problem: rising noise at very low frequencies because of the special conditions in a split path input buffer design. If someone needs a superb instrument for low frequencies, then a Picoscope 4262 is one of the few options – apart from a DSA, that is. The 4262 only has 5 MHz bandwidth, but it is true 16 bits, has an SFDR of >96 dB and a near constant noise density from DC to its upper bandwidth limit. --- Quote from: David Hess on December 26, 2021, 09:16:42 pm --- --- Quote from: Performa01 on December 26, 2021, 12:57:16 pm ---It is either HiRes or ERES - I'm not quite sure - but in any case it is a true acquisition mode, in the sense of a real time pre-processing. The sample memory gets halved in this mode, because it is expanded to 16 bits width as the captured raw data now consists of 10 bit samples. All the post processing, measurements and math are now using the 10 bit data. The firmware cannot tell the difference between this resolution enhancement (implemented in the FPGA) or a true 10 bit ADC. --- End quote --- Old Tektronix DSOs used 16-bit acquisition and processing memory so high resolution mode did not halve the record length. --- End quote --- I was talking about a Siglent SDS2000X Plus, which provides 200 Mpts memory per channel pair, hence there is some headroom for this. How long was the memory in said old Tektronix DSOs? |
| David Hess:
--- Quote from: Performa01 on December 27, 2021, 01:14:52 am ---It doesn’t make much sense to get philosophic about obsolete designs. We are talking about general purpose DSOs here, which ranges from entry level (low end) up to the midrange, but excludes high end gear, which is specialized and definitely not general purpose. --- End quote --- The point is that the designs have not changed much. The PGA has replaced the low impedance switched attenuator but engineers then and now are solving the same problems. Early singed ended input digitizers were replaced with differential input digitizers, and differential signal paths were ultimately kept. Everything is more integrated now of course. --- Quote ---At one point, at least after the invention of the digital readout, T&M industry noticed that a minimum of DC accuracy and stability was expected. Users were no longer willing to permanently turn the offset control of their scopes just to center the trace, as they used to do with their ancient CROs, but expected a decently stable offset position and some accuracy. So, the split path design has long become universal for all general purpose DSOs – despite its drawbacks, where the most obvious is the overload recovery issue. And this is unavoidable, even by a good design. --- End quote --- The move toward the split-path design was not driven by performance; it was about cost. It happened as soon as low cost monolithic low input current operational amplifiers became available. The cost savings came from replacing the discrete dual matched JFET with a single unselected JFET even though the split-path design requires trimming of the compensation or gain or both. --- Quote ---Btw, there are folks who have managed to build a balanced version of the split-path input buffer, so you can have this with balanced inputs too. --- End quote --- Haha, I am one of those folks, but it was much lower noise, impedance, and bandwidth for low level DC differential amplification. I extended and improved an existing single ended design to fully differential and it worked perfectly on the first try, which pleasantly surprised me. --- Quote ---It’s not “old designs” that utilize two input attenuators. Of course you cannot build a good scope with vertical gain settings from 500 µV/div up to 10 V/div with just one single attenuator. For instance, every contemporary Siglent DSO has two input attenuator stages. Offset compensation voltage has to be added to the input in order to be effective (otherwise the input stage would require a totally unrealistic high common mode range), so this is part of the LF path of a split path input buffer design and topologically sits between the attenuators and the PGA. --- End quote --- Modern "budget" DSOs use only one input attenuator, which places much greater demands on the input buffer to handle larger signal levels. The mid-tier models I have considered still use two input attenuators. The presence of two input attenuators might be a good way to divide the lowest end budget DSOs from the next level up in performance. --- Quote ---With low attenuation factors you either need high supply rails (old design) or you get only a very low offset compensation range. But does a Tek 22xx even have a split path design? The specifications of up to one division trace shift for variable gain and trace invert make me wonder. All the more so as the best sensitivity is not particularly high at 2 mV/div. Or maybe they use the cheapest FET-OpAmp with high Offset voltage and -drift without self-calibration in the LF path – but this would somehow scotch the whole idea of the split path approach? --- End quote --- The Tektronix 22xx series does as shown below, and it might have been the first split-path design from them, but not all stages have balance adjustments in the 22xx series. It is split-path but DC coupled and the operational amplifier controls the source current of the JFET to produce the DC and low frequency output. Steve Roach discussed DC and AC coupled split-path designs in his article about oscilloscope signal conditioning. The offset null is used for balance which is a terrible idea for precision, but probably good enough for an oscilloscope. That might explain why one channel of one of mine has noticeable warmup drift. When I studied the design in detail years ago with an eye toward noise analysis, I got the feeling that the Tektronix engineers paid attention to proper distribution of noise and gain. Sensitivity was limited to 2 mV/div simply because greater sensitivity would require another preamplifier stage and noise was already greater than trace width, which seems funny now that modern oscilloscopes put up with even more noise. It is not shown below but the basic sensitivity is 5 mV/div. 2 mV/div relies on increasing gain by 2.5 times in the preamplifier instead of removing attenuation which was pretty common at the time but has disadvantages. The more modern AC coupled split-path amplifier allows AC and DC coupling to be implemented with the low frequency path instead of a high voltage RF relay which is a major advantage. --- Quote --- --- Quote from: David Hess on December 26, 2021, 09:16:42 pm --- --- Quote from: Performa01 on December 26, 2021, 12:57:16 pm ---It is either HiRes or ERES - I'm not quite sure - but in any case it is a true acquisition mode, in the sense of a real time pre-processing. The sample memory gets halved in this mode, because it is expanded to 16 bits width as the captured raw data now consists of 10 bit samples. All the post processing, measurements and math are now using the 10 bit data. The firmware cannot tell the difference between this resolution enhancement (implemented in the FPGA) or a true 10 bit ADC. --- End quote --- Old Tektronix DSOs used 16-bit acquisition and processing memory so high resolution mode did not halve the record length. --- End quote --- I was talking about a Siglent SDS2000X Plus, which provides 200 Mpts memory per channel pair, hence there is some headroom for this. How long was the memory in said old Tektronix DSOs? --- End quote --- The maximum record length on those old DSOs is tiny by modern standards at only 4k, but even though fast RAM was expensive in both cost and size, they still made it twice as wide as needed. Processing in the modern way would have doubled the record length without increasing the amount of installed memory. Tektronix would later advertise this as a "no compromise" feature. |
| Performa01:
--- Quote from: David Hess on December 27, 2021, 03:36:58 am ---The move toward the split-path design was not driven by performance; it was about cost. It happened as soon as low cost monolithic low input current operational amplifiers became available. The cost savings came from replacing the discrete dual matched JFET with a single unselected JFET even though the split-path design requires trimming of the compensation or gain or both. --- End quote --- Well, of course cost might have been a major consideration, even though I cannot see why back then a dual matched FET should have been more expensive than an IC that contains basically the same plus a bunch of additional transistors and other components. Today it’s a different story of course, because these are hard to get and expensive spare parts now, but back in the seventies a dual FET was about as affordable (or rather expensive) as a JFET OpAmp (like LF356) as far as I remember. The discrete differential stages usually did require trimming of the “offset balance”, as far as I remember the old circuit diagrams of up to 300 MHz frontends that did not use a split path topology. Even though your circuit diagram shows three trimmers, I don’t think we’ve seen this in recent designs. Self calibration takes care of the offset error and with modern low tolerance parts in the input and feedback networks the balance between both paths and the transition at the crossover frequency are good enough even without adjustments. --- Quote from: David Hess on December 27, 2021, 03:36:58 am --- --- Quote ---Btw, there are folks who have managed to build a balanced version of the split-path input buffer, so you can have this with balanced inputs too. --- End quote --- Haha, I am one of those folks, but it was much lower noise, impedance, and bandwidth for low level DC differential amplification. I extended and improved an existing single ended design to fully differential and it worked perfectly on the first try, which pleasantly surprised me. --- End quote --- Congrats – my hat goes off to you! This was (and still is) true design work, not very common anymore… --- Quote from: David Hess on December 27, 2021, 03:36:58 am ---The Tektronix 22xx series does as shown below, and it might have been the first split-path design from them, but not all stages have balance adjustments in the 22xx series. It is split-path but DC coupled and the operational amplifier controls the source current of the JFET to produce the DC and low frequency output. Steve Roach discussed DC and AC coupled split-path designs in his article about oscilloscope signal conditioning. The offset null is used for balance which is a terrible idea for precision, but probably good enough for an oscilloscope. That might explain why one channel of one of mine has noticeable warmup drift. When I studied the design in detail years ago with an eye toward noise analysis, I got the feeling that the Tektronix engineers paid attention to proper distribution of noise and gain. Sensitivity was limited to 2 mV/div simply because greater sensitivity would require another preamplifier stage and noise was already greater than trace width, which seems funny now that modern oscilloscopes put up with even more noise. It is not shown below but the basic sensitivity is 5 mV/div. 2 mV/div relies on increasing gain by 2.5 times in the preamplifier instead of removing attenuation which was pretty common at the time but has disadvantages. The more modern AC coupled split-path amplifier allows AC and DC coupling to be implemented with the low frequency path instead of a high voltage RF relay which is a major advantage. --- End quote --- Thanks for the excerpt from the circuit diagram. It is quite interesting. Yes, I’ve immediately noticed that it’s only DC coupled, which means a number of drawbacks, particularly the fact that the input goes open circuit in AC coupled mode, whereas good designs are supposed to have a constant input impedance regardless of the input coupling, or any other settings for that matter. The LF-path also doesn’t provide the offset control usually found in DSOs – just because it really is best placed here. But yes, with the low division ratio of the LF input network, the compensation range could not be huge anyway. Nevertheless I have to assume that the offset adjustment is done at a later stage, which means that it actually relies on the usable common mode range of the input buffer – which will of course work to a certain degree because of the relatively high rail voltages of +/- 8.6 V. A maximum sensitivity of 2 mV/div means 16 mVpp full scale. Even 5 mV/div is equivalent to 40 mVpp FS. Since this is hardly enough to drive the plates of a CRT, there has to be a lot of amplification after the programmable attenuator. In a DSO, the ADC would require at the very least several hundred millivolts (but usually up to two volts) full scale for proper operation. This is why integrated PGAs do not only provide attenuation, but amplification as well. Consequently, as the signal needs to be amplified anyway, there’s no need to stop at 2 mV/div. With 20 MHz bandwidth limit the total noise in a proper low noise design can be as low as 20 µVrms, so this should not be a problem for the trace width. |
| G0HZU:
--- Quote ---If someone needs a superb instrument for low frequencies, then a Picoscope 4262 is one of the few options – apart from a DSA, that is. The 4262 only has 5 MHz bandwidth, but it is true 16 bits, has an SFDR of >96 dB and a near constant noise density from DC to its upper bandwidth limit. --- End quote --- Yes, I've seen these and there are also some alternatives. Very tempting. At the moment I sometimes use a Tek RSA3408A 8.5GHz RTSA for looking at low frequency stuff. This has a low noise floor and it has the advantage (for me at least) of having a 50 ohm input impedance. The Picoscope should be a bit better although it is limited to a 5MHz BW. The Tek analyser can capture 40MHz but it is only a 14bit system. |
| Navigation |
| Message Index |
| Next page |
| Previous page |