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Ypur 50 ohm thing will be problem for most sources. You can probably also start with a 10.000 or whatever V source and then use precision dividers or you might try looking for precision gains and start from a 0.500V reverence. 10/50 is 200 mA.
I notice you pulled a fast one: 0.5, 0.2; but you get the idea.
I had a Eurotherm 239 calibrator at work to use and it was really really nice. I don;t no the actual specs, but here **broken link removed** is one on ebay. It misses your 10 V mark by 1 mV. i.e. 9.999V It had a mV, V and 12V as well as a Thermocouple mode and you could easily slect two voltages and zero using the thumbwheels and switches.
I would think you could get there using a 0.1% (or better) precision voltage reference, an opamp, a push-pull buffer stage and a set of feedback voltage dividers comprising 0.1% tolerance resistors.
0.25% precision is 400:1 accuracy so a calibrated 4 digit DMM is all you need with an adjustable source. If you really need 10V into 50 Ohms then that would be a 2 watt source. Use a PC PSU Source of 12V, 5V to be a source. with FET feedback loop and op Amp with trimmmers and selector switch.
I was hoping to make a replacement tool for the PG506 calibration for the TEK 24xx series scopes at those voltages. Perhaps oven temp. controlled like a OCXO.
Possibly as a DIY kit as that PG506 is quite expensive.
I don't think it's necessary to calibrate the oscilloscope with a 50Ω input setting. Doing it with the standard 1MegΩ input impedance should be fine.
In that case you don't need to generate a significant amount of current at the calibrator output, significantly simplifying the circuit.
If you start out with an accurate voltage reference IC (at least 0.1%) and then use 0.1% resistors with a low offset op amp, you should be able to achieve (or at least get very close to) your goal of 0.25% accuracy.
Do you need help in selecting the appropriate devices?
Where would you obtain the parts?
I could use any advice in selecting the devices.
If we're looking at a 1M load then scaling the voltage with 0.1% resistors would be doable. Perhaps starting at the 10.000V target and then scaling down.
Parts...perhaps Digikey?
I will be getting one of these soon: https://www.tequipment.net/RigolDM3068.html
My reply is similar to Tony's reply, except here we will use a DAC.
For example, a 12bit DAC and op amp output buffer. Find the input code that gives you the desired outputs (measured with a good volt meter), store them, then look them up when you need to get the desired outputs again. Use a good temperature stable voltage reference with the DAC. With a 4.096 reference and 12bit DAC you can get 1mv resolution. With a 2x amplifier you'll get to over 8v with 2mv resolution, etc., etc.
There's almost no limit to what you can do with this setup because you can always go to a higher resolution DAC also, one way or another, and if you find you need another voltage in the future that's easy to get too.
Note that in the other group post, the PG506 is assumed to be connected to a 1M 0.1% resistor. (I think that's right). The suggestion was to allow 1 KHz because it creates an auto-baseline and it used for calibration.
Voltages were specifically use din the cal procedure. It's 1-2-5 steps with some missing.
Rise time is anther issue altogether. There's a pulsar discussion somewhere on the eccentric guys site in Australia.
It is...I have 3 of these 24xx series scopes...and I was looking at investing in cal. eqpt...then I ran into what the PG506 costs....
It'll end up in Hackaday -open source if i get it going. I'll use a uC front end & a 16 x 2 LCD to make it flexible like MrAl suggested
See Mosaic and I have interests in at least 4 groups that we share interests in. That's 4 that I know of.
Some of the 24xx series are really nice: e.g. The 2467B, Dunno if you have it,
I'm envious.
I have a working TDS340 that I need to replace the Dallas chip and replace the floppy with an emulator.
and a Working Kikisui 100 MHz Analog scope And a couple 21x series scopes that need work and 4 internal plastic parts that could be 3D printed.
Don't you need to generate waveforms for calibration, along with DC, what about 3dB bandwidth check? The scope spec for accuracy is +/- 2% @ 1MΩ, +/- 3% @ 50Ω, so I don't think you need the accuracy you listed above. I did not check the spec for the DMM portion of the scope.
TG501 Time mark generator
PG506 Calibration generator
TM500 Mainframe
Tunnel diode pulser
and a variety of generic tools:
400MHz bench scope
DMM
Various high quality (and short) bnc cables and connectors
5x, -14dB attenuator
Ceramic alignment tools, straight and phillips and 1 tiny one.
Linear power supply
Sine-wave gen <= 2MHz
Like many topics on ETO, I've been reading this one of yours with interest. MrAl's suggestion of using a digital to analog converter seems like a great idea and opens up a whole raft of possibilities, even adding a microcontroller in the future. After mulling over your requirement for a few days, I ended up doing the attached schematic outlining an additional approach.
As KeepItSimpleStupid and Tony Stewart imply, the requirement for 10V with a 50Ω load is a little challenging. I agree, so the design only provides a maximum output of 5V. crutschow makes some good points about calibrating scopes, but on rare occasions you may need to calibrate a 50 Ω input on a scope so this design provides a 50 Ω output.
Mikebits mentions about checking the frequency response of a scope. Many scopes have built-in HF calibrators typically providing a 100mv, 1 Khz square wave with fast edges and flat tops and bottoms. If using a scope probe you need a good square wave to set up the probe compensation. My suggested design should produce a reasonably good square wave, but it would be much better to have a separate circuit on the scope calibrator designed especially for this purpose. The same general approach could be used, but simplified with only one low output voltage and using fast components.
I don't suppose a negative output voltage would be acceptable, because if so, it would allow a much wider choice of the voltage reference and operational amplifier. Both greatly affect the accuracy.
Here is a high-level description of the suggested scope calibrator:
The essence of the design is that a precise voltage controls a precise constant current generator that feeds into a precise 50Ω resistor to generate a precise output voltage. All those 'pecises' may sound good but every one represents an error that detracts from the overall accuracy. The suggested approach is fairly common in traditional oscilloscope calibrators, certainly the ones that I have worked on. I have also used the general circuit for many other applications, from power supplies to scope time base generators and it seems to work quite well. I've even designed and built a scope voltage calibrator using this technique.
Two output connections are provided: '50Ω OUTPUT' and 'VOLTAGE OUTPUT'. The first has a constant impedance of 50Ω and the second has an impedance of pretty much 0Ω at DC for load currents up to 20mA, depending on the choice of operational amplifier, N2. Outputs from 10mv to 5V in standard 1:2:5 ratios are selectable by a 9 position switch. Other output voltages could be configured by simply changing the value of one resistor. Another switch selects an output of either DC or a square wave at 1 kHz.
The suggested scope calibrator requires a stabilised 12V supply and consumes 120 mA, worst case, when the 5V output is selected.
The circuit should meet your accuracy specification which I take to be +-0.25% without trimming. I haven't done an error budget, but worst case calculations would probably show around +- 0.5%. That would be if all the errors go the same way. Similarly I haven't calculated a thermal error budget but I don't expect thermal effects to be significant with adequate cooling and layout.
To answer the initial question in your opening post, I suggest that the approach shown in the schematic would be a good starting point for your project, and could be developed into a reliable scope calibrator. Please remember though that this is not a fully developed design and is certainly not optimised. I didn't spend any time researching the components shown on the schematic so no doubt these could be improved on. None of the components should be prohibitively expensive. The precision 0.1% high stability resistors cost about 85p but I haven't checked the cost of a high quality switch.
I hope this helps and if you spot any mistakes, there always are, or need any clarification please let me know. I have put some more information in the APPENDIX. As you may have guessed, I would love to have a go at this project myself. Do let us know how you get on if you decide to go ahead with it.
The 50Ω output should produce a well-shaped square wave with fast edges and little ringing, and the well-defined 50Ω impedance helps minimises reflections, with 50Ω cables that is. Another important characteristic of this approach is that all the precision circuitry works at DC and is isolated from the outside world by the transistor in the current generator. This greatly simplifies things. Finally it provides a very robust output, essentially a resistor and the drain/collector of a transistor. The 'VOLTAGE OUTPUT' is a different story and would need comprehensive protection in a practical scope calibrator.
For even better absolute accuracy the value of R15 could be changed slightly; a trim potentiometer would not be suitable as the calibrator temperature stability may be ruined.
If a negative output voltage was acceptable a series type voltage reference could be used. These generally have a better performance than the shunt type shown in the present design. And to a lesser degree, the operational amplifier in the constant current generator would be operating with a better input voltage. Having said that, it may be possible to use a shunt voltage reference with the positive version of the calibrator, but this is another area I haven't investigated.
In case you're wondering about the parallel resistors, they are just there to dissipate power: high power, high frequency, precision, 50 Ω resistors are very expensive. The capacitors across the supplies etc are only notional decoupling; this is an important area to get right in a practical design. Tantalum for the electrolytics and ceramic for the non-polarised capacitors would be a good choice.
The LM555 is only there to provide a quick and easy way to get a square wave for development. Apologies for the spaghetti schematic layout around the LM555 but I just grabbed a part from the library and didn't bother to redraw it. In the final version I suggest a square wave generator with an xtal oscillator and accurate 1:1 mark-to-space ratio, all pretty straight forward and cheap to do. This would turn the calibrator into an accurate frequency generator as well. You could even provide a range of frequencies selectable by an additional switch. The highest frequency should probably be 100kHz to keep things simple though. You could go a lot higher if a separate frequency calibrator was built into the scope calibrator but it would be best not to run the main circuit at high frequencies.
There is absolutely no reason why, for initial development purposes, standard non-precision parts couldn't be used. The resistors could just be ordinary types and you could even start with a PNP bipolar transistor for the constant current generator. For prototyping you wouldn't need to worry too much about input currents and low offset voltage so a lesser operational amplifier could be used provided that it will operate with its inputs just 1V down from the 12V supply rail. The best approach may be to build a negative output version of the calibrator first to establish the fundamentals of the circuit operation. The decoupling components and layout etc must be the real deal though.
MOSFET, Q1, will need careful selection to ensure a decent square wave, and low leakage, especially as it will be dissipating quite a bit of power at the higher output voltage settings. Maybe another type of transistor would be more suitable. Of course, the big advantage with FET devices in this case is that the current flowing up the source is the same current that flows out of the drain. The big drawback with FET devices though is their leakage currents and capacitances and I have a feeling that in the end a couple of bipolar transistors will do a better job.
Another area to watch out for is stability: although the constant current generator is only working at DC it has a very high gain and will be prone to oscillations without a good layout and adequate decoupling. It may even be necessary to add some frequency compensation in the feedback loop. The operational amplifier is operating with its input at only 1V more negative than the 12V supply rail. This should not be a problem and the OPA192 shown should do the job well, but as the upper difference amplifier in the chip is operating it is not ideal (see my post on the OPA192).
Apologies if I'm preaching to the converted, but here are a few general comments. The devil is in the detail is the adage to keep in mind when developing accurate analogue designs like this. HiFi amplifiers are the same. A very good power supply is absolutely essential, certainly not a switch mode type, for initial development anyway and solid grounding is vital. The physical layout and screening would also be critical to achieve the performance. And heat is always your enemy. One last piece of cracker-barrel advice, you need to use 'four terminal connection' when the precision parts are wired into the circuit or when the printed circuit board is laid-out. Without this, the accuracy will be lost. Believe it or not, you also need to use solder that gives a good solid joint with low resistance and minimum galvanic voltages: some of the zero-lead types are very poor in this respect and they get worse as they age!
The switch for selecting the output voltages is in a critical place in the circuit as far as accuracy and frequency stability is concerned, so it should be a good quality low resistance type, ideally with gold contacts. Rather than mounting the switch on the front panel and having long leads to the precision resistors, I'd strongly advise mounting the switch to optimise the electrical performance and keeping the leads as short as possible. You can always use a mechanical method to operate the switch from the front panel.
Here is a list giving the resistances and currents for the range of output voltages
THIS VERSION IS NOW OBSOLETE> PLEASE SEE REVISED CIRCUIT IN POST 34 of 05 November 2015.
ERRATA (items in red are important and affect the fundamental function of the circuit) (1) R15 should be 15K not 25K (Typo) With R15 at 25K the circuit will still function ok but the output voltages will only be 70% of the values shown on the schematic. (2) Add 100nf ceramic capacitor between the positive and negative supply pins of N1. (good practice to help keep N1 stable)
I do plan to do the project and I have acquired the 16bit DAC and 12 bit ADC µC as well as a few instrumentation grade OPAs. The ADC has a 10µS rise time so it's only good for low freq square waves, but it can produce pyramid and stepped waves for accurate V amplitude checking.
I also have gotten a sub 50 picoseond rise time comparator chip to create reference pulses for scope bandwidth evaluations (0.35 x Tr) and doing a HF time mark gen for horiz. timing cals.
Rather than the switches I'll go with the crystal controlled µC running the 16bit ADC into the prec. opa driven constant current ref. I got some RF (25Ghz spec , 2.5Ghz operating) bipolar transistors to use as the output end. I have some precision 50Ω and 49.9 Ω resistors and I have a 6.5 digit DM3068 arriving next week to use as my V, I and Ω ref checker. It'll pick up any solder joint or connector milliohm issues.
By using ADC oversampling the system to 20 bits (@ 0.08%% prec V ref), it should be able to self test itself and even self calibrate amplitudes by creating offsets to run the DAC for different temp environments as assessed by an onboard thermistor.
Because of the picoseond comparator capability, I'll have to study up PCB layout etc, stripline design and generally make things tiny. Prob. try to do opposing SMA and BNC outputs to direct connect to the scope or spectrum analyzer with no cabling.
I didn't think you needed all the basic advice but I'm new here and I read in the instructions for ETO to explain everything to help less experienced readers understand the posts.
Sound like a great project. That's some scope calibrator you are designing! As I said please let us know how it progresses.
Re strip-line design, I expect you have seen this: **broken link removed**
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