On Fri, 14 Oct 2016 12:00:08 -0600 "Cube Central" <cubecent...@gmail.com> wrote:
> How would one go about testing power supplies and seeing how noisy they > are? I have the standard suite of tools, an oscilloscope and a little > (dangerous) know-how. I am just not sure what to look for or how to safely > hook it up to test. There are different frequency ranges one looks at when measuring power supplies: 1) Long term stability over seconds to hours 2) Low frequency noise between 0.1Hz and 10Hz 3) Mid frequency noise between 10Hz and 20MHz 4) High frequency noise beyond 20MHz The borders of these different ranges of noise are kind of arbitrary and different people use different values. They are motivated by the need of the consumer (of power) and by the limitations of different measurement equipment. The long term stability is what we usually call "wander" and is mostly dominated by the thermal stability and aging of the power supply's voltage reference and its control loop. To measure it correctly one usually uses a high resolution DMM in the 7.5 or 8.5 digits range. One can also build a homebrew system using a high resolution ADC (like 32bit delta-sigma converters: AD7177-2, LTC2508-32, AD1262) and a very stable voltage reference (LTZ1000, LM399). I'd like to refer to the volt-nuts mailinglist on how to properly do this, as my knowledge in this area is rather limited. The low frequency range is what we usually call the 1/f region, although the long term stability also belongs to it. But unlike the long term region you don't have to sacrifice a virgin to get decent measurment data. Jim William's appnote has lots of details how to measure noise in this region. There are slightly more modern circuits by Todd Owen/Amit Patel and Gerhard Hoffmann. I recently stumbled over a similar amplifier by Enrico Rubiola and Franck Lardet-Vieudrin. Both  and  explain why for low impedance sources (like power supplies) a BJT input stage would be a better choice than jFETs and also cover the influence of temperature on the measurement.  gives some additional info on how to design the differential input stage. I wonder how an active offset voltage cancelation scheme for the differential pair input stage using one of the chopper stabilized opamps (eg LTC2057) would change the temperature dependence and long term stability (aka 1/f^a noise) of the circuit, but I have not seen any measurements of a system like this yet. The mid frequency range is mostly influenced by the telecom noise requirements, which for historical reasons cover the 10Hz to 20MHz range. It is probably the easiest region to measure with homebrewn instruments. A decently fast ADC with a low noise voltage reference (like the LTC6655) are all you need. Depending on how accurately you want to measure the noise, it makes sense to further split this range into a lower range up to ~500kHz and an upper range above 500kHz. The reason is that there are today several high resolution ADCs available that support sampling rates of up to 1Msps (and some beyond),eg: AD7982, 18bit 1Msps AD7984, 18bit 1.33Msps AD7960, 18bit 5Msps LTC2386-18, 18bit 10Msps LTC2378-20, 20bit 1Msps LTC2368-24, 24bit 1Msps These would allow to accurately measure the noise range that is IMHO most interesting for most applications. Interesting because a lot of applications are insensitive to noise below 1Hz or even below 10Hz and noise above several 100kHz becomes easy to filter out using inductors, ferrit beads and ceramic capacitors. When choosing an ADC for this range make sure you check the actual SNR/SFDR performance as it a higher output resolution not necessarily corresponds to the actual performance delivered. This becomes especially pronounced when going higher with the sampling rate to cover the higher noise frequency ranges. Beyond 5-10Msps 16bit is the best you can get and conversly the SNR is limited to something around 90dB-95dB. The high frequency noise, I mentioned above 20MHz, but probably the limit is more in the 1-10MHz range, is where radiation becomes interesting. Ie with increasing frequency it becomes harder and harder to "isolate" electronics against noise and it becomes necessary to shield it with metal plates. This range is important for the switched power supplies that started this thread. These supplies have usually switching frequencies between a couple of 10kHz to low 100kHz for high power and AC/DC supplies and goes up to a few MHz for the low power (where low power is relative and can be several W) low voltage DC/DC supplies. And as these supplies are switching hard they produce lots of harmonics. A badly designed supply can easily produce very noticable spikes at 100MHz. Measuring this type of noise is probably easiest with a good digital oscilloscope with a sufficiently high analog bandwidth and sampling rate. Alternatively specturm analysers are a good choice too. Going homebrew in this range is kind of difficult, as high resolution ADCs max out around 100-125Msps, with the odd exception of LTC2107 that gives 16bit up to 210Msps (and still an 80dB SNR). Ie the maximum achievable bandwidth is 40-90MHz for single ADC configurations. But interfacing these high-speed ADCs requires an FPGA and thus considerable effort. An alternative approach is to build a spectrum analyser like setup with a tunable oscillator and a down-mixer. The problem here would then be to get a flat frequency response and the required calibration. Another approach is to use an SDR system with a low lower frequency limit, but this also requires to kind of calibrate it as these are not ment for power measurements and thus their frequency response is not really flat. But they give at least a good indication whether you have any spikes that stick far out. One thing I haven't mentioned yet is that you will need to have some form of load that can be switched. Power supply noise is highly dependent on the current flow, especially for switched power supplies. But because the load will also create noise, you probably want to just have a bank of relay switched power resistors. In case you want to measure the response on load switches, you should replace the relays by power transistors (otherwise the bouncing of the relay will confound the measurement). Attila Kinali  "775 Nanovolt Noise Measurement for A Low Noise Voltage Reference", by Jim williams, Linear AN124, 2009 http://www.linear.com/docs/28585  "Measuring 2nV/sqrt(Hz) Noise and 120dB Supply Rejection on Linear Regulators", by Todd Owen and Amit Patel, Linear AN159, 2016 http://www.linear.com/docs/47682  "A 220 pV/sqrt(Hz) low noise preamplifier", by Gerhard Hoffman, 2014 http://www.hoffmann-hochfrequenz.de/downloads/lono.pdf  "Low Flicker-Noise DC amplifier for 50Ω Sources", by Enrico Rubiola and Franck Lardet-Vieudrin, 2004 http://rubiola.org/pdf-articles/journal/2004rsi(rubiola)low-flicker-dc-amplifier.pdf http://arxiv.org/abs/physics/0503012  "Some Considerations for the Construction of Low-Noise Amplifiers in Very Low Frequency Region", by Sikula, Hashiguchi, Ohki, Tacano. 2004 http://dx.doi.org/10.1007/1-4020-2170-4_27  "Some Tips on Making a FETching Discrete Amplifier", by George Alexandrov and Nathan Carter, Analog Dialog 47-10, 2013 http://www.analog.com/library/analogdialogue/archives/47-10/discrete_amplifier.html -- Malek's Law: Any simple idea will be worded in the most complicated way. _______________________________________________ time-nuts mailing list -- email@example.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.