Intentionally injecting noise into supply rails - Page 1
Forum section: electronics / projects

author: IDEngineer
author_rank: Super Contributor
timestamp: June 06, 2018, 11:31:23 pm
content: Before I start digging into this, I thought I'd avoid reinventing the wheel if possible. A bit of Google searching hasn't revealed much because the search terms always match up to articles about REDUCING power supply noise.My interest is in testing how well various circuits work when the incoming power isn't lab-quality. All bench power supplies strive to provide the purest power possible, but that's not always the real-world environment. We do a lot of design work for gas and diesel engine powered environments where the combination of lead-acid batteries, alternators, coarse-step voltage "regulators" (to be VERY loose with that term!), ignition and other RFI, asynchronously switching loads of various sizes, etc. all conspire to make the incoming power incredibly noisy.We'd like to simulate that environment. And we can't be the first people to do so. But as I said, bench supplies compete on cleanliness, not on "adjustable noise levels". We thought about connecting an electronic load in parallel with the DUT and modulating the power that way, but the whole time you'd be fighting, almost abusing, the power supply as it actively seeks to offset the effects of the load.As I said, searches on anything involving "power supply" and "noise" inevitably return sales sheets bragging about LOW noise. So hopefully someone here knows of such a product, or at least a starting point. In my mind, the ideal would be a traditional supply with a BNC input to which you could connect a function/AWG and have that signal imposed upon the output(s).Thanks!

author: CopperCone
author_rank: Super Contributor
timestamp: June 06, 2018, 11:35:33 pm
content: You raise the RF impedance of your power supply and feed a signal into your circuit with a coupler.To get the right waveforms you would probably sample the rails that you have at work, do a FFT, determine what the dynamic range and frequency range requirements are, along with the power levels at your load impedance.You would then most likely have the easiest time using a arbitrary waveform generator fed into a wideband power amplifier which is connected to your load. If the requirements are vast you might need to use several generators and several power amplifiers connected through the appropriate RF stuff like Diplexersa slow option might be a Kepco brand 'four quadrant' power supply that you can find on ebay, they are basically giant high voltage op-amps, but the frequency response is not great.The circuitry used for an efficiently regulated DC power supply won't really be designed for RF modulation, there are internal time constants that will integrate your HF response.

author: chris_leyson
author_rank: Super Contributor
timestamp: June 07, 2018, 12:23:43 am
content: "Conducted immunity testing" would be a good place to start searching. A current injection probe, also known as BCI, bulk current injection, would be a good start and you could build one with a good quality RF grade ferrite ring core. A CDN or coupling/decoupling network is also something that isn't too difficult to build. Last but not least, an EM clamp will provide good inductive and capacitive coupling over a wide range of frequencies and if used "fixed" wiring then you could use ordinary ferrite ring cores instead of half cores and you probably don't need to build it 1m long. For "half cores" you could probably use U-I cores if you want to build your own couplers.A current injection probe is a good start as you can use these to not only inject noise but also to measure it as well.A bit of background readinghttp://www.elmac.co.uk/PNP_61000-4-6.pdf

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: June 07, 2018, 12:25:46 am
content: Hmm, start simple--a single resistor decouples the your power supply and allows noise to be directly injected using a signal generator.Start with that.

author: chris_leyson
author_rank: Super Contributor
timestamp: June 07, 2018, 12:37:37 am
content: QuoteDidn't think of that. Plus a wideband current probe to measure what you're really up against.

author: IDEngineer
author_rank: Super Contributor
timestamp: June 07, 2018, 12:38:53 am
content: All good early comments, thank you.I had thought about a filter between the power supply and the noise source, but if the filter has any Q at all it would have to be changed depending upon the frequency/bandwidth of the noise. And I'm still a bit hesitant to "fight" a power supply, since any such variation will be opposed by its design - with the efficiency of its ability to oppose the noise dependent upon the frequency response of the power supply's own regulation circuitry. Finally, I'm unsure how a nice function/AWG will appreciate having a DC bias of many volts imposed upon its output stage... that's not exactly how they're intended to be used. So then a series coupling cap, and you start thinking about a buffer amp with some substantial current capacity....Things get weird fast, which is why I was starting to think about designing a power supply whose control loop intentionally accepts such an injected signal. Then you'd go from fighting the power supply and all the odd secondary effects from that, to the supply actually helping create and sustain the desired test environment.I'll read the link Chris provided and see where my brain goes. More comments invited... thanks!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 07, 2018, 12:39:25 am
content: Yup. ^^What you're looking for is a bias tee. You may also need an attenuator or transformer, to get a lower impedance at the supply -- lots of devices won't be stable above the minimum supply impedance, so you need to provide that at least. The effective circuit is a bulk capacitor with ESR, except instead of passive ESR, it's your RF source. Simply using a resistor divider is probably quite adequate -- you won't need too many volts of injection to measure PSRR or cause upsets.I've seen people using the ChipWhisperer and such, making some really weird choices about switch size (this is a carefully-timed supply brown-out generator, using a switch to shunt the supply, or a capacitor to inject a pulse), capacitors, local bypasses and so on. When you think of the supply as a signal network like any other, with impedances and filtering characteristics, it all becomes so much clearer what is needed.Tim

author: james_s
author_rank: Super Contributor
timestamp: June 07, 2018, 12:40:04 am
content: Depending on how precise you need to be, you could use cheap switchmode regulators, those tend to be noisy. Put a few brushed DC motors on the line if you want even more noise.

author: Mr. Scram
author_rank: Super Contributor
timestamp: June 07, 2018, 12:51:27 am
content: What the Dickens? I've been thinking about the same thing the past few days and also ended up wondering whether an AWG could deal with being hooked up to a DC source. I'd like to test microcontroller supply filtering in real life to confirm effectiveness.Following this thread with interest, and suspicion. Just so you know, I'm wearing my tin foil hat from now on. I got my eye on you lot.

author: IDEngineer
author_rank: Super Contributor
timestamp: June 07, 2018, 01:00:52 am
content: Quote from: james_s on June 07, 2018, 12:40:04 amTrue (unless you've measured it you can't imagine how noisy those little encapsulated DC-DC modules are - I suspect spark gap transmitters are quieter) but I'd like to be able to specify and control the noise, not just throw whatever on there. So you're correct, that's a fast way to get noisy but without much control.

author: IDEngineer
author_rank: Super Contributor
timestamp: June 07, 2018, 01:01:32 am
content: Quote from: Mr. Scram on June 07, 2018, 12:51:27 amDarn, he's on to us. Now where are we going to get our good ideas?!?

author: IDEngineer
author_rank: Super Contributor
timestamp: June 07, 2018, 01:04:38 am
content: Quote from: T3sl4co1l on June 07, 2018, 12:39:25 amThat's for rather high frequencies. I'm thinking more in the neighborhood of 1MHz and under. Though a bias tee would start getting useful in the 10's of MHz.The noise we deal with is generally low frequency transients with fast edges, so response up to a MHz or so would cover it. I might go a bit higher to prevent rolloff but we don't need microwave response.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 07, 2018, 01:07:43 am
content: Quote from: IDEngineer on June 07, 2018, 01:04:38 amI wasn't aware there was any accepted cutoff for a "bias tee".To me, it's simply a diplexing filter. "DC" in one port, "AC" in the other port, both at the EUT port. It's a tee. That you use to bias things. You know.This works whether it's a 10Hz cutoff in an inductive coupled tube amp (100H+), or GHz with 1/4 wave transmission line segments.Tim

author: AG7CK
author_rank: Regular Contributor
timestamp: June 07, 2018, 01:56:18 am
content: Most bigger HP Harrison PSUs (lots on fleabay - cheap/normal shipping cost in US) have a connection row on the back panel. Some variant of this:One of the terminals is for remote programming (impedance one or a few kiloohms IIRC). If the frequency is not too high, you can put your signal generator / noise source there.

author: C
author_rank: Super Contributor
timestamp: June 07, 2018, 02:57:30 pm
content: Think of using a DC coupled audio amp as your power source.Your DC supply is now powering the audio amp.The audio output is your DC power + noise. TheThe audio input is your noise input.An audio mixer is an adder, so you can have many noise inputs.Now the audio amp is acting like a filter that is protecting the power supply.The output of the audio amp is a tap between two shunt controlled circuits. A positive power supply is just one shunt controlled circuit with a possible weak second shunt for pull down.Going full differential would add negative output control where you could add negative lead noise(ground noise) that is different from positive noise.The question then becomes is the bandwidth of the DC coupled audio amp high enough for all the different types of noise you want to add.Keep in mind that a differential output can be better at removing power noise. The power needed to drive outputs is more local.C

author: ejeffrey
author_rank: Super Contributor
timestamp: June 07, 2018, 03:29:50 pm
content: Quote from: Mr. Scram on June 07, 2018, 12:51:27 amI wouldn't directly couple an AWG to a DC supply, you want at least a DC blocking capacitor. Most AWGs with a 50 ohm output can drive a zero ohm load with no problem. The issue is that since the DC supply is low impedance, much of the injected current will go towards the supply rather than the load. That is why you put an inductor on the DC input -- to raise the impedance and force the AC current to flow to the AC+DC port of the bias tee.

author: awallin
author_rank: Frequent Contributor
timestamp: June 07, 2018, 07:11:30 pm
content: FWIW here's a circuit for driving/testing high psrr/low-noise regulatorsLinear AN159, fig 13http://www.analog.com/media/en/technical-documentation/application-notes/an159fa.pdf

author: Mr. Scram
author_rank: Super Contributor
timestamp: June 07, 2018, 07:26:19 pm
content: Quote from: ejeffrey on June 07, 2018, 03:29:50 pmYes, I figured a DC blocking capacitor would make sense even if just to be sure. I wasn't familiar with a bias tee, so that's something to take a closer look at. I haven't really dealt with RF much.

author: ejeffrey
author_rank: Super Contributor
timestamp: June 07, 2018, 07:45:33 pm
content: As Tim explained, bias tees are commonly used at RF to combine DC power/bias and RF signals but they don't have to be RF. Anything that combines a DC and AC signal to DC+AC is a bias tee.

author: IDEngineer
author_rank: Super Contributor
timestamp: June 07, 2018, 09:27:02 pm
content: Quote from: awallin on June 07, 2018, 07:11:30 pmThanks! Figure 13 is along the lines of what I had in mind. One problem I see is that the output impedance is rather high for a power supply. Five 0.2 ohm resistors in parallel is still 40mOhms. Furthermore, at the specified 5A and presuming perfect load sharing, each of those resistors will dissipate (I^2 / R =) ((1A^2) / 0.2R =) 5W (!!!). Might work for the specific application in the app note but may need some tweaking for a general purpose tool. Perhaps a FET design, where the impedance would be "dynamic" as the supply seeks to maintain its output, though I seem to remember some little gotcha about problems with FET "inherent load sharing" when running them in linear (as opposed to saturation) mode....

author: prasimix
author_rank: Unknown
timestamp: June 08, 2018, 06:56:17 am
content: Quote from: IDEngineer on June 07, 2018, 09:27:02 pmPerhaps you can connect FETs as Liv did in hispower supplyfor "pass" element.