Re: [time-nuts] OT: AC voltage standard
); SAEximRunCond expanded to false Errors-To: [EMAIL PROTECTED] RETRY Henk ten Pierick wrote: On Nov 6, 2007, at 22:43, Bruce Griffiths wrote: NIST's AC standard is currently useful for generating frequencies up to 100kHz with 10MHz the projected useful limit for a 10Gb/s bit stream. With say a 1Mb/s bit stream output frequencies up to 1KHz or so should be feasible with high amplitude stability. The stated 10Gigabits/s for a bit stream, is that a typo? Henk No typo, a stack of Josephson junctions excited by a 15GHz RF source is used together with a 10GBb/s data source see papers listed at: http://www.acdc.nist.gov/acdc.nist.gov/ac%20JVS%20page.html In particular: http://www.acdc.nist.gov/acdc.nist.gov/ac%20JVS%20page_files/acjvs2006_extAbstract-tel.pdf Bruce ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] OT: AC voltage standard
); SAEximRunCond expanded to false Errors-To: [EMAIL PROTECTED] RETRY On Nov 6, 2007, at 22:43, Bruce Griffiths wrote: NIST's AC standard is currently useful for generating frequencies up to 100kHz with 10MHz the projected useful limit for a 10Gb/s bit stream. With say a 1Mb/s bit stream output frequencies up to 1KHz or so should be feasible with high amplitude stability. The stated 10Gigabits/s for a bit stream, is that a typo? Henk ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] OT: AC voltage standard
); SAEximRunCond expanded to false Errors-To: [EMAIL PROTECTED] RETRY Bruce, Great! Henk On Nov 8, 2007, at 22:11, Bruce Griffiths wrote: Henk ten Pierick wrote: On Nov 6, 2007, at 22:43, Bruce Griffiths wrote: NIST's AC standard is currently useful for generating frequencies up to 100kHz with 10MHz the projected useful limit for a 10Gb/s bit stream. With say a 1Mb/s bit stream output frequencies up to 1KHz or so should be feasible with high amplitude stability. The stated 10Gigabits/s for a bit stream, is that a typo? Henk No typo, a stack of Josephson junctions excited by a 15GHz RF source is used together with a 10GBb/s data source see papers listed at: http://www.acdc.nist.gov/acdc.nist.gov/ac%20JVS%20page.html In particular: http://www.acdc.nist.gov/acdc.nist.gov/ac%20JVS%20page_files/ acjvs2006_extAbstract-tel.pdf Bruce ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/ time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] OT: AC voltage standard
); SAEximRunCond expanded to false Errors-To: [EMAIL PROTECTED] RETRY I suggest you use a CMOS multiplexer driven by a clock and a flip-flop (to guarantee 50% duty cycle) to switch the Weston cell's output voltage on and off at 50% ratio. Knowing the ON resistance of the CMOS multiplexer will allow you to calculate the effect of loading if you know the impedance of the meter you are driving. Other than that, the output voltage will be an AC voltage with a peak-to-peak value equal to the cell voltage. Possible problem: the output will be chopped DC, not true AC. Alternately, you could make the circuit a little more complicated and build a full bridge with the multiplexer, so that the output voltage is either the cell voltage or the cell voltage reversed, for a DC coupled true AC (+/-) output. If you don't see what I mean, I can draw a schematic for you. Didier KO4BB -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Joe McElvenney Sent: Tuesday, November 06, 2007 5:57 AM To: time-nuts@febo.com Subject: [time-nuts] OT: AC voltage standard Hi, Excuse the topic but is does push the same buttons as it were. After calibrating my old HP54502A 6-bit digitizing scope I'm left with an error I can't quite believe and so am trying to determine which of my instruments is telling me lies. Anyone know of a simple way of producing an AC voltage standard suitable for general workshop use without reference to another one? About one percent would be good enough, wave shape and frequency accuracy not important (wash my mouth out). I have a Weston Cell for DC voltage calibration, a Rb one for frequency but nothing for AC volts. Perhaps there is a chip out there that clocks between accurate limits that I could use as a source? Thanks - Joe ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] OT: AC voltage standard
); SAEximRunCond expanded to false Errors-To: [EMAIL PROTECTED] RETRY Joe McElvenney wrote: Hi, Excuse the topic but is does push the same buttons as it were. After calibrating my old HP54502A 6-bit digitizing scope I'm left with an error I can't quite believe and so am trying to determine which of my instruments is telling me lies. Anyone know of a simple way of producing an AC voltage standard suitable for general workshop use without reference to another one? About one percent would be good enough, wave shape and frequency accuracy not important (wash my mouth out). I have a Weston Cell for DC voltage calibration, a Rb one for frequency but nothing for AC volts. Perhaps there is a chip out there that clocks between accurate limits that I could use as a source? Thanks - Joe Joe NIST (Then NBS) started with thermal voltage converters as AC to DC transfer standards and later built AC standards using DACs to produce computable AC waveforms. You could always use a DAC to synthesize an AC waveform, for low frequencies ( 50kHz) at least. Or just roll you own with a CMOS shift register and a bunch of resistors. For higher frequencies, if one keeps the distortion down a pair of matched diodes can be used in an AGC or other leveling loop to regulate the output of an oscillator. Fluke used to produce an AC voltage standard. Bruce ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] OT: AC voltage standard
); SAEximRunCond expanded to false Errors-To: [EMAIL PROTECTED] RETRY On Tue, 06 Nov 2007 11:57:27 +, Joe McElvenney [EMAIL PROTECTED] wrote: Anyone know of a simple way of producing an AC voltage standard suitable for general workshop use without reference to another one? About one percent would be good enough, wave shape and frequency accuracy not important (wash my mouth out). I have a Weston Cell for DC voltage calibration, a Rb one for frequency but nothing for AC volts. Perhaps there is a chip out there that clocks between accurate limits that I could use as a source? If a simple square wave will do then one of the simplest and yet most accurate sources is to switch a known DC reference using a mercury wetted reed relay. A dry reed will work but will bounce a little. The small reed relays as were ubiquitous in data acquisition systems up into the 80s can switch at 150 hz or better. Driving the coil of the relay with stepped-down line voltage is a good solution. You probably know this already but I'll mention it anyway. You can't draw any appreciable current from that weston cell and it remain within specs. Even a 1 meg scope probe is too much. I'd use a DC power supply or battery and a quality DVM (which is probably more accurate than the standard cell) instead. I started in metrology in the time when the standard cell was the best there was. Boy, am I glad that era is gone. I still have one just to stimulate old memories but my lab standard is a precision 10 volt reference IC. I don't recall the part number but both National and Burr-Brown make 'em. Untrimmed accuracy is something like 0.01%. That's better than my boat anchor Fluke meter calibrator! John -- John De Armond See my website for my current email address http://www.neon-john.com http://www.johndearmond.com -- best little blog on the net! Tellico Plains, Occupied TN Save the whales, collect the whole set! ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] OT: AC voltage standard
); SAEximRunCond expanded to false Errors-To: [EMAIL PROTECTED] RETRY Hi John: How does it compare to the Linear LTZ1000? Have Fun, Brooke Clarke http://www.PRC68.com http://www.precisionclock.com http://www.prc68.com/I/WebCam2.shtml 24/7 Sky-Weather-Astronomy Cam Neon John wrote: ); SAEximRunCond expanded to false Errors-To: [EMAIL PROTECTED] RETRY On Tue, 06 Nov 2007 11:57:27 +, Joe McElvenney [EMAIL PROTECTED] wrote: Anyone know of a simple way of producing an AC voltage standard suitable for general workshop use without reference to another one? About one percent would be good enough, wave shape and frequency accuracy not important (wash my mouth out). I have a Weston Cell for DC voltage calibration, a Rb one for frequency but nothing for AC volts. Perhaps there is a chip out there that clocks between accurate limits that I could use as a source? If a simple square wave will do then one of the simplest and yet most accurate sources is to switch a known DC reference using a mercury wetted reed relay. A dry reed will work but will bounce a little. The small reed relays as were ubiquitous in data acquisition systems up into the 80s can switch at 150 hz or better. Driving the coil of the relay with stepped-down line voltage is a good solution. You probably know this already but I'll mention it anyway. You can't draw any appreciable current from that weston cell and it remain within specs. Even a 1 meg scope probe is too much. I'd use a DC power supply or battery and a quality DVM (which is probably more accurate than the standard cell) instead. I started in metrology in the time when the standard cell was the best there was. Boy, am I glad that era is gone. I still have one just to stimulate old memories but my lab standard is a precision 10 volt reference IC. I don't recall the part number but both National and Burr-Brown make 'em. Untrimmed accuracy is something like 0.01%. That's better than my boat anchor Fluke meter calibrator! John -- John De Armond See my website for my current email address http://www.neon-john.com http://www.johndearmond.com -- best little blog on the net! Tellico Plains, Occupied TN Save the whales, collect the whole set! ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] OT: AC voltage standard
); SAEximRunCond expanded to false Errors-To: [EMAIL PROTECTED] RETRY To All, Let me repeat what John De Armond stated. It is most important. A standard cell, whether saturated or unsaturated, cannot be loaded under any circumstances. Any current draw will upset the cell chemistry and cause the cell to change value and the odds are it will never return to its previous value. Even with a 10 megohm load. If you can come by any reasonable thermocouple made for measuring AC, even a so-called RF amp meter, then you could, using a good deal of caution, apply an AC signal and then apply a DC voltage to come to the same level. Remember to feed the DC voltage both ways to account for the reversal error in the thermocouple. If you want to do the Flip-Flop reversal process then use a very well regulated DC source. This method was used by Tektronix in their older tube type oscilloscopes. Their method of calibrating that function of the scope was to pull a tube out and set the DC voltage to some predetermined level. He exact procedure escapes me at the moment. Within the ability of your EYE to see the same point, you could just use a scope and compare the line shift using a DC source adjusted to the same level of the AC signal on the scope screen. With careful comparison you could do a little better then 3 %. That may be all you need to check yourself for sanity. BillWB6BNQ An old, used and tired EX-metrologist Joe McElvenney wrote: Hi, Excuse the topic but is does push the same buttons as it were. After calibrating my old HP54502A 6-bit digitizing scope I'm left with an error I can't quite believe and so am trying to determine which of my instruments is telling me lies. Anyone know of a simple way of producing an AC voltage standard suitable for general workshop use without reference to another one? About one percent would be good enough, wave shape and frequency accuracy not important (wash my mouth out). I have a Weston Cell for DC voltage calibration, a Rb one for frequency but nothing for AC volts. Perhaps there is a chip out there that clocks between accurate limits that I could use as a source? Thanks - Joe ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] OT: AC voltage standard
The problem with a mercury relay is that the switching delay is significant and not well controlled, so the duty cycle of the resulting waveform is not well controlled, and so would be the RMS value. I believe CMOS analog switches would provide better control, and with series resistance that is easily below 10 ohm, that would give you negligible error when driving loads in the megohm, such as a voltmeter. Four switches in a full bridge configuration will give you a true AC square wave output, and if you know the DC voltage feeding the bridge (using your voltmeter calibrated with the Weston cell), you will have an accurate AC source that will not require further calibration, at least good enough for most home lab uses. Now, for a sinewave, it's another matter, but Bruce's suggestion of a DAC powered from a precise DC source would work extremely well (limited by the DAC) and provide a low distortion sinewave, which is just as important as controlling the peak voltage. A simple microcontroller is all that's required to drive the DAC. Make sure you understand the delays involved with making software loops. Alternately, a counter driving a suitably programmed EPROM driving the DAC will take software out of the equation, but it sounds like the 70's all over again... Didier KO4BB Neon John [EMAIL PROTECTED] wrote: On Tue, 06 Nov 2007 11:57:27 +, Joe McElvenney [EMAIL PROTECTED] wrote: Anyone know of a simple way of producing an AC voltage standard suitable for general workshop use without reference to another one? About one percent would be good enough, wave shape and frequency accuracy not important (wash my mouth out). I have a Weston Cell for DC voltage calibration, a Rb one for frequency but nothing for AC volts. Perhaps there is a chip out there that clocks between accurate limits that I could use as a source? If a simple square wave will do then one of the simplest and yet most accurate sources is to switch a known DC reference using a mercury wetted reed relay. A dry reed will work but will bounce a little. The small reed relays as were ubiquitous in data acquisition systems up into the 80s can switch at 150 hz or better. Driving the coil of the relay with stepped-down line voltage is a good solution. You probably know this already but I'll mention it anyway. You can't draw any appreciable current from that weston cell and it remain within specs. Even a 1 meg scope probe is too much. I'd use a DC power supply or battery and a quality DVM (which is probably more accurate than the standard cell) instead. I started in metrology in the time when the standard cell was the best there was. Boy, am I glad that era is gone. I still have one just to stimulate old memories but my lab standard is a precision 10 volt reference IC. I don't recall the part number but both National and Burr-Brown make 'em. Untrimmed accuracy is something like 0.01%. That's better than my boat anchor Fluke meter calibrator! John -- John De Armond See my website for my current email address http://www.neon-john.com http://www.johndearmond.com -- best little blog on the net! Tellico Plains, Occupied TN Save the whales, collect the whole set! ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] OT: AC voltage standard
Didier Juges wrote: The problem with a mercury relay is that the switching delay is significant and not well controlled, so the duty cycle of the resulting waveform is not well controlled, and so would be the RMS value. I believe CMOS analog switches would provide better control, and with series resistance that is easily below 10 ohm, that would give you negligible error when driving loads in the megohm, such as a voltmeter. Four switches in a full bridge configuration will give you a true AC square wave output, and if you know the DC voltage feeding the bridge (using your voltmeter calibrated with the Weston cell), you will have an accurate AC source that will not require further calibration, at least good enough for most home lab uses. Now, for a sinewave, it's another matter, but Bruce's suggestion of a DAC powered from a precise DC source would work extremely well (limited by the DAC) and provide a low distortion sinewave, which is just as important as controlling the peak voltage. A simple microcontroller is all that's required to drive the DAC. Make sure you understand the delays involved with making software loops. Alternately, a counter driving a suitably programmed EPROM driving the DAC will take software out of the equation, but it sounds like the 70's all over again... Didier KO4BB A DDS chip with its own DAC could be used to generate a programmable frequency sinewave with amplitude stable to better than 1%. Bruce ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] OT: AC voltage standard
Didier Juges wrote: The problem with a mercury relay is that the switching delay is significant and not well controlled, so the duty cycle of the resulting waveform is not well controlled, and so would be the RMS value. I believe CMOS analog switches would provide better control, and with series resistance that is easily below 10 ohm, that would give you negligible error when driving loads in the megohm, such as a voltmeter. Four switches in a full bridge configuration will give you a true AC square wave output, and if you know the DC voltage feeding the bridge (using your voltmeter calibrated with the Weston cell), you will have an accurate AC source that will not require further calibration, at least good enough for most home lab uses. Now, for a sinewave, it's another matter, but Bruce's suggestion of a DAC powered from a precise DC source would work extremely well (limited by the DAC) and provide a low distortion sinewave, which is just as important as controlling the peak voltage. A simple microcontroller is all that's required to drive the DAC. Make sure you understand the delays involved with making software loops. Alternately, a counter driving a suitably programmed EPROM driving the DAC will take software out of the equation, but it sounds like the 70's all over again... Didier KO4BB One can always adapt the techniques used in NIST's quantum AC standard. This device is basically a single sigma delta DAC that turns a very accurate and stable voltage source on and off at 10GHz or so. The modulator output is then low pass filtered. To generate a stable AC voltage, at least for low frequencies, use CMOS switches controlled by a sigma delta bitstream to switch the input to a low pass filter between ground and the stable voltage. AC couple the output to eliminate the dc offset. Using a single bit DAC simplifies the calibration process over that when using a multibit DAC. NIST's AC standard is currently useful for generating frequencies up to 100kHz with 10MHz the projected useful limit for a 10Gb/s bit stream. With say a 1Mb/s bit stream output frequencies up to 1KHz or so should be feasible with high amplitude stability. Bruce ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] OT: AC voltage standard
); SAEximRunCond expanded to false Errors-To: [EMAIL PROTECTED] RETRY Anyone know of a simple way of producing an AC voltage standard suitable for general workshop use without reference to another one? About one percent would be good enough, wave shape and frequency accuracy not important (wash my mouth out). I have a Weston Cell for DC voltage calibration, a Rb one for frequency but nothing for AC volts. Perhaps there is a chip out there that clocks between accurate limits that I could use as a source? Modern CMOS drivers are very close to rail-rail for low loads. So for quick/dirty, I'd just look at a handy clock signal and assume it goes from ground to the power supply. Be sure you don't look at one with a terminator, aka heavy load. Series termination would be OK. TI says for their HC04 at 4.5 V Vcc and 20 uA load, 25 C: Min 4.4, typ 4.499 I like the DAC suggestion. -- These are my opinions, not necessarily my employer's. I hate spam. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] OT: AC voltage standard
); SAEximRunCond expanded to false Errors-To: [EMAIL PROTECTED] RETRY On Tue, 6 Nov 2007 13:52:13 -0500, Didier Juges [EMAIL PROTECTED] wrote: The problem with a mercury relay is that the switching delay is significant and not well controlled, so the duty cycle of the resulting waveform is not well controlled, and so would be the RMS value. Not at all. The early edition of the Berkeley Nucleonics precision pulser, capable of delivering a monotonic amplitude pulse to a 4096 multichannel analyzer, used a pair of off-the-shelf Claire MWRs, one to switch the reference voltage to a capacitor and another to switch the charge to the pulse forming network. In later models they changed to a complicated solid state circuitry that never was quite as stable. None of that is particularly relevant here because he needs a simple circuit to check the accuracy of a 6 bit ADC in a scope. The RMS value doesn't matter, as the output is a simple square wave that swings between 0 volts and precisely the value of the DC source. Neither does the frequency. The advantage of the reed relay approach, in addition to precision, is that the circuit can be thrown together on a bench using jumper clips in 5 minutes, assuming a MWR is on hand. A voltage source (battery even), a good DVM, the relay and a 6 volt filament transformer to drive the coil is all that is needed. More than good enough for a 6 bit application. -- John De Armond See my website for my current email address http://www.neon-john.com http://www.johndearmond.com -- best little blog on the net! Tellico Plains, Occupied TN Democracy is three wolves and one sheep voting on what to have for supper. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] OT: AC voltage standard
); SAEximRunCond expanded to false Errors-To: [EMAIL PROTECTED] RETRY I thought the question was: Anyone know of a simple way of producing an AC voltage standard suitable for general workshop use without reference to another one? About one percent would be good enough, wave shape and frequency accuracy not important (wash my mouth out). I have a Weston Cell for DC voltage calibration, a Rb one for frequency but nothing for AC volts. Perhaps there is a chip out there that clocks between accurate limits that I could use as a source? There was no reference to a 6 bit scope. Obviously, what you need to calibrate a scope's vertical system is quite different from what you need to calibrate, say, a bench AC voltmeter. If timing is unimportant and RMS value is unimportant and wetted relays are available, then that solution would work. In my shop, CMOS switches are more readily available than wetted relays, and their on-resistance is well low enough for the job, with the advantage of being much faster, but at that point, it's a matter of what you have available and personal preference. There is still an issue that the relay will switch the voltage ON, and there is no equivalent circuit to switch it off, leaving that to an RC time constant of some sort. I would think it would be of interest to have a symetrical waveform. If you use another relay to switch the voltage off, there will be timing issues, unless you use a PFN of some sort, in which case calibration will be another story. I thought the more general question about a general purpose AC source that could be built and calibrated using a precise DC voltage source and used to calibrate AC voltmeters for instance is interesting though, because I have that issue. That will be for another thread :-) Didier KO4BB Neon John [EMAIL PROTECTED] wrote: On Tue, 6 Nov 2007 13:52:13 -0500, Didier Juges [EMAIL PROTECTED] wrote: The problem with a mercury relay is that the switching delay is significant and not well controlled, so the duty cycle of the resulting waveform is not well controlled, and so would be the RMS value. Not at all. The early edition of the Berkeley Nucleonics precision pulser, capable of delivering a monotonic amplitude pulse to a 4096 multichannel analyzer, used a pair of off-the-shelf Claire MWRs, one to switch the reference voltage to a capacitor and another to switch the charge to the pulse forming network. In later models they changed to a complicated solid state circuitry that never was quite as stable. None of that is particularly relevant here because he needs a simple circuit to check the accuracy of a 6 bit ADC in a scope. The RMS value doesn't matter, as the output is a simple square wave that swings between 0 volts and precisely the value of the DC source. Neither does the frequency. The advantage of the reed relay approach, in addition to precision, is that the circuit can be thrown together on a bench using jumper clips in 5 minutes, assuming a MWR is on hand. A voltage source (battery even), a good DVM, the relay and a 6 volt filament transformer to drive the coil is all that is needed. More than good enough for a 6 bit application. -- John De Armond See my website for my current email address http://www.neon-john.com http://www.johndearmond.com -- best little blog on the net! Tellico Plains, Occupied TN Democracy is three wolves and one sheep voting on what to have for supper. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] OT: AC voltage standard
Nice catch, John. A solution provider would do well to understand the problem before presenting the solution. The first paragraph of this morning's 5:52 AM (UTC-6) posting outlined the problem. A 1% solution will satisfy a 6 bit calibration. The reading is likely to be Average, not RMS, so the standard can be a 50 +/- 0.5% chopped standard DC voltage. The RMS value of a square wave is the same as the Average value, but for a sine wave the Average is 0.9 times RMS. For an RMS display of an Average-reading device, multiply the answer by the inverse of 0.9. Do the same with evenly chopped DC to get the predicted reading of the AC digital display. This is why true RMS meters are required if the shape is not a sine wave. True RMS meters either heat something or do the integration. There are ways to use the standard cell, but they are overkill for 1% accuracy. The old thermocouple millivoltmeters used a standard cell and galvanometer to set the current in a set of precision resistors. The galvanometer was then used to balance the resistor set to the unknown millivoltage. If the scope has chopped sweep and you have suitable 0.2% resistors to divide the source of chopped DC down to the standard cell volts, then you don't need a galvanometer. Stabilize the cell, put both scope probes on the resistors and crank up the vertical gain. Remove most of the DC offset with vertical position, and trim one of the channel gains to get a single trace line. Now briefly touch the standard cell with one of the probes and note the difference. Most standard cells outside of a controlled standards lab are not accurate to 6 figures, and will not be permanently affected by a scope probe for the accuracy desired here. 100 megohm impedance is required for 6 figures, so a megohm probably drops that to 4 figures. Standard cells are calibrated by comparing them, not by reading across each of them. You know how you short the terminals of sensitive analog meters for shipment, so the needle won't bang around? I got a standard cell from the bay with a short between the terminals. It was not repairable. Bill Hawkins -Original Message- From: Neon John Subject: Re: [time-nuts] OT: AC voltage standard None of that is particularly relevant here because he needs a simple circuit to check the accuracy of a 6 bit ADC in a scope. The RMS value doesn't matter, as the output is a simple square wave that swings between 0 volts and precisely the value of the DC source. Neither does the frequency. Not quite. The Average/RMS AC coupled value of a square wave varies with the duty cycle. That's why PWM works. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] OT: AC voltage standard
); SAEximRunCond expanded to false Errors-To: [EMAIL PROTECTED] RETRY On Tue, 6 Nov 2007 17:41:21 -0500, Didier Juges [EMAIL PROTECTED] wrote: There was no reference to a 6 bit scope. First paragraph of the original post: Joe McElvenney wrote: Hi, Excuse the topic but is does push the same buttons as it were. After calibrating my old HP54502A 6-bit digitizing scope I'm left with an error I can't quite believe and so am trying to determine which of my instruments is telling me lies. -- John De Armond See my website for my current email address http://www.neon-john.com http://www.johndearmond.com -- best little blog on the net! Tellico Plains, Occupied TN Give Blood. 8 Billion Mosquitoes can't be wrong. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
