Designing a DC block for a spectrum analyzer for power supply noise measurements - Page 1
Forum section: electronics / projects

author: matthuszagh
author_rank: Frequent Contributor
timestamp: June 27, 2021, 06:29:57 pm
content: I'm designing a DC block for spectrum analyzer so that I can use the SA to perform very-low noise power supply noise and ripple measurements. I'd like to be able to measure AC noise down to about 1uV RMS from 10Hz (or lower) up to as high as I can go (preferably 1GHz+). The spectrum analyzer presents a 50ohm input resistance and would have its mixer damaged by >100mA input current (5V). The DC block must also work for negative supply voltages. I would use this circuit by connecting it in parallel to a resistive load, so that I measure the supply noise under load.My plan is to use one or more very high capacitance (and high voltage) film capacitors in parallel (something likehttps://www.digikey.com/en/products/detail/avx-corporation/FFLR6C0188KJE/13580188). The number depends on the low frequency cutoff I decide I need. I was also going to add a ~100nF high-voltage ceramic capacitor in parallel (e.g.,https://www.digikey.com/en/products/detail/kemet/C1812X104KDRACAUTO/8109977) to present a low impedance at high frequencies. The film cap listed above has an ESL of about 60nH and I'm expecting the ESL of the ceramic cap to be <1nH. This device would be placed in a metal enclosure with BNC connectors at the input and output. The signal and ground wires will be kept as short as possible and twisted together to minimize series inductance (the high-frequency behavior of this circuit is obviously very sensitive to series inductance).A few additional thoughts. These capacitors support DC voltages up to ~1kV, which is way more than I need. Although the film capacitor is way more expensive than an equivalent-value aluminum electrolytic, I'm ok with the cost. The film capacitor can withstand much higher DC voltages and does not have a polarity (which means I won't damage my SA by connecting the device the wrong way). I've also heard that film capacitors are less failure-prone than aluminum electrolytics.I've simulated the transfer function using the capacitance values and their ESLs into a 50ohm load and it works from 10Hz to 1GHz (I have not added additional series inductance into the simulation).Does this design/plan seem reasonable? Have I failed to consider anything important? Is it reasonable to expect that I can keep the series inductance sufficiently low with proper mounting and wire connections? Will this device be sufficiently robust? Any other thoughts, or better ways to construct a device with the same specs?

author: David Hess
author_rank: Super Contributor
timestamp: June 28, 2021, 03:49:13 am
content: To get good high frequency response out of a physically large capacitor, it needs to be implemented as part of the transmission line structure. In practice this means using a high impedance input so that the coupling capacitor can be much smaller, or implementing a differential comparator function so that DC coupling can be used.Such a large value of capacitance will not protect the instrument input from damaging overload during the charge step. Even old oscilloscopes with better overload protection than modern ones implemented a "precharge" function for AC coupling applications, and that was with less than 0.1 microfarads.

author: jonpaul
author_rank: Super Contributor
timestamp: June 28, 2021, 05:53:03 am
content: Check Mini-Circuits for 50 and 75 Ohm N, BNC and other connector types DC blocks.https://www.minicircuits.com/WebStore/dc_blocks.htmlvery high quality and wideband.For DIY, we use a small Pomona case with BNC M-F connectors.10-100 nF/50V is fine.Finally notice that a DC block WILL generate a transient when a DC containing source is connected that CAN blow out the SA mixer.A transient protector is another issue.Bon Chance,Jon

author: matthuszagh
author_rank: Frequent Contributor
timestamp: June 28, 2021, 03:09:19 pm
content: Quote from: David Hess on June 28, 2021, 03:49:13 amCan you clarify what you mean by "the coupling capacitor can be much smaller"? If you're referring to a smaller impedance value at some frequency, I believe I've accounted for this. At low frequencies, say 100 Hz, the circuit will look like a voltage source feeding a voltage divider consisting of the large capacitor as the top impedance and the 50ohm resistor as the bottom impedance. The impedance of say, a 1.8 mF capacitor at 100 Hz is about 0.88j ohms. That has a voltage gain of very close to 1. Even at 10 Hz, I see a gain of about -0.13 dB. In terms of transmission line effects, at 1 GHz I'm at the boundary of the device extent starting to look non-negligible compared to the wavelength (roughly 30 cm). lambda/10 is 3 cm, which is smaller than I can achieve. But, if need be, I can use 50 ohm coax for as much of the wire connections as possible, with only short segments provided by hook-up wire. I can probably keep these short segments around 3 cm or less. The small capacitor will likely be placed on a PCB on which I can use a 50 ohm microstrip structure.Quote from: David Hess on June 28, 2021, 03:49:13 amThis is a very good point and something I completely failed to consider. Would a TVS diode be a good choice here? I can use a bidirectional one to allow it to work with positive and negative supplies. I'll also keep pay special attention to the parasitic capacitance and leakage current, since these could deteriorate the small-signal behavior of the device. Obviously, this would make the device unidirectional, which is unfortunate, but still acceptable.I could try to implement a precharge function by using a switch connecting the DC block output to either ground or the RF input, but that makes me nervous. If I forget once, I'll damage my SA. I'd like to keep the device unpowered, so I'm also reluctant to use a more sophisticated precharge capability.

author: matthuszagh
author_rank: Frequent Contributor
timestamp: June 28, 2021, 03:13:13 pm
content: Quote from: jonpaul on June 28, 2021, 05:53:03 amI like Mini-Circuits, but unfortunately every DC block I could find (including from Mini-Circuits) had too high of a low cutoff frequency. I've already gothttps://www.centricrf.com/dc-blocks/2.92mm-sma-dc-blocks/c0927-sma-dc-block-9khz-27ghz-inner-block/, which has a 9 kHz low cutoff, which is about as good as I could find. Still, I'd like to detect noise from linear regulators well below 9 kHz.That transient is what concerns me now. I replied to David about using a TVS diode for this. Would be great to get your input on that as well.

author: David Hess
author_rank: Super Contributor
timestamp: June 28, 2021, 04:53:15 pm
content: Quote from: matthuszagh on June 28, 2021, 03:09:19 pmThe frequency breakpoint depends on the capacitance and input resistance. Oscilloscopes with 1 megohm inputs can use capacitors which are 20,000 times smaller for the same low frequency cutoff as a 50 ohm input.Low frequency AC coupled noise amplifiers, like those suitable for measuring reference and regulator noise, use high impedance inputs for exactly this reason.QuoteThey make low capacitance shunt limiters for RF applications. Common TVS diodes may have too much capacitance. There are various diode clamp designs which can be used. A current driven diode bridge also makes a fantastic limiter. Of course in all of these cases, if the capacitance is large enough, then the limiter may be damaged anyway.They make fuses for RF applications. Maybe this would be a good place for one. Some old high input impedance amplifiers use a combination of shunt protection and a light bulb as a PTC and fuse for series protection. This was also a trick that old multimeters used to protect the output of their ohms converter.QuoteOn oscilloscopes the precharge function was activated by a switch position between GND and AC coupling so engaging AC coupling activated the precharge automatically. Of course if the oscilloscope was set to AC coupling before the probe was attached or power was applied, then this did not help.

author: TimFox
author_rank: Super Contributor
timestamp: June 28, 2021, 05:06:46 pm
content: The self-inductance of the film capacitors will give you problems, probably above 1 MHz. Bypassing the film capacitors with smaller ceramic capacitors will give you "interesting" resonant modes between the inductance of the large capacitors and the capacitance of the small capacitors.Given the noise referred to the 50 ohm input of the SA, you might be better using a low-noise preamplifier (with a higher input impedance) that can drive the 50 ohm load, being careful about the capacitor charging transients. You might need more than one preamp to cover the total range from < 60 Hz to 1 GHz. For example, an -hp- 465A amplifier has a 1 megohm input impedance, bandwidth 5 Hz to 100 MHz, and can drive 50 ohms. HP had other amplifiers in this series with different bandwidths.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: June 28, 2021, 05:53:37 pm
content: Which SA? I'm no expert but most I've worked with in RF only go down to 9KHz. The only SA I've seen that goes down to "DC" are audio analyzers.What SA are you using?

author: TimFox
author_rank: Super Contributor
timestamp: June 28, 2021, 06:10:56 pm
content: I don't know what SA the OP is using. However, there are low-frequency swept-frequency analyzers such as the venerable -hp- 3580A that go down to 5 Hz. My Marconi 2380/2382 analyzer (400 MHz max frequency) goes down to nominal 100 Hz, and can be used (barely) at 60 Hz. Unfortunately, most RF SAs nowadays start at 3 kHz or 9 kHz.

author: matthuszagh
author_rank: Frequent Contributor
timestamp: June 28, 2021, 07:11:35 pm
content: Quote from: TimFox on June 28, 2021, 05:06:46 pmI considered the ESL and resonant frequency issues, but I didn't see them as problems. The ESL of the large (1.8 mF) cap is specified in the datasheet as 60 nH. Using a single 100 nF ceramic cap in parallel, as I have currently, would produce a resonant frequency around 2 MHz. However, when I calculate the gain into a 50 ohm load, it seems fine. I've posted the schematic used for this calculation below (the ceramic cap does not specify an ESL, but does say the device has low ESL, so I used 1 nH, which I believe to be conservative).The code to calculate the gain isCode:import numpy as npdef impedance(c, l, f):omega = 2 * np.pi * freturn 1j * omega * l + 1j / (omega * c)cs=100e-9ls=1e-9cl=1.8e-3ll=60e-9fname="data/gain.dat"freq = np.geomspace(1e0, 1e10, 501)zs = impedance(cs, ls, freq)zl = impedance(cl, ll, freq)ztotal = 1 / (1 / zs + 1 / zl)gain = 20 * np.log10(np.abs(1 / (1 + ztotal / 50)))with open(fname, "w") as f:f.write("{:^12} {:^12} {:^12}\n".format("f", "z", "g"))for (frequency, z, g) in zip(freq, np.abs(ztotal), gain):f.write("{:^12.4e} {:12.4e} {:12.4e}\n".format(frequency, z, g))Plotting this gives:[ Specified attachment is not available ]Am I missing something in the analysis?Quote from: TimFox on June 28, 2021, 05:06:46 pmThis is an interesting idea. I'll take a look at the 465A and similar.

author: matthuszagh
author_rank: Frequent Contributor
timestamp: June 28, 2021, 07:18:07 pm
content: Quote from: Alex Eisenhut on June 28, 2021, 05:53:37 pmI'm using an HP 8566 that goes down to 100 Hz. However, I may at some point get an audio SA that goes even lower (e.g., the HP 3585A that goes down to 20 Hz). In any event, 10 Hz is probably a bit overkill. Still, I would like the same device to work if I get the capability to go a bit lower than 100 Hz.

author: TimFox
author_rank: Super Contributor
timestamp: June 28, 2021, 07:36:33 pm
content: You should see a notch at 2 MHz, but that resonance will have infinite Q, since you don’t include ESR in your capacitor models. Your plot misses the zero width notch.I made some tests on small leaded capacitors (C0G, X7R, and one polystyrene) from 1.0 nF to 100 nF, where I tried to maintain the total length (two axial leads plus body) at 0.75 inch (because of the PS unit). I found a reasonably constant series inductance, 12 to 15 nH, over a range of self-resonance between 4.7 and 40 MHz. Apparently, for leaded capacitors, the ESL is really a function of length, which I have seen in articles about capacitors.

author: matthuszagh
author_rank: Frequent Contributor
timestamp: June 28, 2021, 07:48:29 pm
content: Quote from: TimFox on June 28, 2021, 07:36:33 pmGood point. I've now added 0.1 ohm ESR to each. Here's the new code and new plot (confined to between 1 kHz and 10 MHz to make the notch visible).Code:import numpy as npdef impedance(c, l, f):omega = 2 * np.pi * freturn 1j * omega * l + 1j / (omega * c)cs=100e-9ls=1e-9rs=0.1cl=1.8e-3ll=60e-9rl=0.1fname="data/gain.dat"freq = np.geomspace(1e3, 1e8, 1001)zs = impedance(cs, ls, freq) + rszl = impedance(cl, ll, freq) + rlztotal = 1 / (1 / zs + 1 / zl)gain = 20 * np.log10(np.abs(1 / (1 + ztotal / 50)))with open(fname, "w") as f:f.write("{:^12} {:^12} {:^12}\n".format("f", "z", "g"))for (frequency, z, g) in zip(freq, np.abs(ztotal), gain):f.write("{:^12.4e} {:12.4e} {:12.4e}\n".format(frequency, z, g))The notch should still be insignificant (see the factor of 10^(-2) scaling on the gain axis). Any problems with this analysis?I'd heard the part about the ESL being a function of lead length. That's useful/interesting.

author: TimFox
author_rank: Super Contributor
timestamp: June 28, 2021, 08:02:48 pm
content: That value of ESR with the 100 nF capacitor at 2 MHz, below its SRF with your ESL value, gives a Q of only 12 or so at 2 MHz. I have seen much stronger effects in my pre-retirement with an accidental resonance of a 1 nF C0G and a 100 nF X7R (both radial leaded parts) at around 64 MHz (above the SRF of the 100 nF part). These details are crucial.Anyway, if you build your network between BNC connectors in a metal box, you can check for problems using the tracking generator in your SA.

author: TimFox
author_rank: Super Contributor
timestamp: June 28, 2021, 10:27:57 pm
content: Were you serious about the AVX FFLR6C0188KJE capacitor? It costs $156.75 in singles, is 235 mm long with the capacitor between two terminals 50 mm apart, with 116 mm diameter, and is mounted with an M12 x 16 mm male thread at the opposite end of the cylinder. With the terminals 50 mm apart, the ESL will be much worse than 1 nH.

author: matthuszagh
author_rank: Frequent Contributor
timestamp: June 28, 2021, 10:57:52 pm
content: Quote from: TimFox on June 28, 2021, 10:27:57 pmI am. It's expensive, but if it does the trick then it's worth the money to me (assuming there's not an equivalent/better alternative that's cheaper). The 1 nH was my estimate for the ESL of the small SMT 100nF cap (I actually expect it would be a bit lower). The ESL (as stated in the datasheet) for the huge 1.8 mF cap is 60 nH. I used that value to simulate the performance.

author: TimFox
author_rank: Super Contributor
timestamp: June 28, 2021, 11:31:35 pm
content: Interesting. I was trying to estimate the self inductance of the large unit based on my earlier measurements with 19 mm total length, and guessed 50 nH for the 50 mm terminal spacing. The SMT capacitor will have long leads to reach the big unit’s terminals, and will have roughly the same inductance, not 1 nH, in the circuit.However, with the overall length (235 mm), there are bound to be other modes to the impedance at higher frequencies.To get a useful measurement at 60 Hz, you definitely should use a higher impedance amplifier (100 k to 1 M) and a 100 to 470 nF polypropylene capacitor. I would recommend starting with the capacitor and a small series resistor to a 220 k or so shunt resistor with a switch to ground for precharging. You can use the MCL dc blocks at higher frequency, with appropriate transient suppression, if the voltage rating suffice.

author: jonpaul
author_rank: Super Contributor
timestamp: June 29, 2021, 07:32:31 am
content: Bonjour a tous, Remarkable this post has generated such detailed and tangential responses, especially the exotic series caps !1/ the PSU noise is easily checked with a TRMS wideband meter rather than a SA. We used the Fluke 8920-21-22A TRMS 20 MHz.For LF noise and ripple easy to use the Tek AA501 audio analyzer.2/ OP should better define the noise test intended, eg BW, type of PSU, power level.3/ For OP, is regulatory compliance eg CISPR, IEC, etc. required? Check the standard applicable most EMI starts at 150 kHz or 500 kHz.4/ Many 1M probes with 50 Ohm output exist to make the C block simpler.5/ Protection of SA input cannot be with any TAZ, it must be low capacitance double anode. We used the General Semi /Vishay low capacitance TAZ.6/ The larger the capacitance of the DC block, the more energy on initial connector to the DC source.Energy = 1/2 C * V exp 2.Depending on the C and initial DC voltage you can STILL blow up the SA input or mixer!7/ Most noise is above 20 kHz for SMPS PSU, as Fsw is 40kHz>>>25 MHz.8/ For linear PSU, the ripple is 120 hZ and harmonics, just use a scope.9/ No need for costly or exotic caps! The series L and shunt C are fine if built carefully in a small Pomona box.We used 100 nF for HF SA and a pair of tants opposed ~ 68 MF to make a non-polar DC block no problems.Hope this info is of use. Open to comments and corrections (:-:)Bon Soirée et Bon Chance!Jon(EE since 1968..an Internet dinosaur!)

author: matthuszagh
author_rank: Frequent Contributor
timestamp: June 29, 2021, 06:43:09 pm
content: Quote from: jonpaul on June 29, 2021, 07:32:31 amThanks for the recommendations! I will look into those.Quote from: jonpaul on June 29, 2021, 07:32:31 amI did specify some of this in the original post. 10 Hz - 1 GHz, 1uV to 0.3 V (both RMS). The upper limit isn't so critical since anything higher than a few mV can be seen on a scope. The lower voltage range, though demanding, should be easy with an SA, assuming I can feed it a DC-free signal. This is why I chose an SA.This device is meant to be general across linear and switching supplies. This would cover the mostly low-frequency noise of linear supplies, and the switching noise + several harmonics of switching supplies (the high harmonics explain the high upper frequency bandwidth limit).Quote from: jonpaul on June 29, 2021, 07:32:31 amNo compliance needed, this is for personal use.Quote from: jonpaul on June 29, 2021, 07:32:31 amI can look at this in more detail, but wouldn't this create significant attenuation?Quote from: jonpaul on June 29, 2021, 07:32:31 amThanks for the recommendation! Can you clarify what TAZ is? When I googled your words, I foundhttps://www.vishay.com/docs/85908/vbus03b1-sd0.pdf.Is this what you're referring to? I'm completely new to low-capacitance voltage suppression devices, so could use whatever help people can offer here.Quote from: jonpaul on June 29, 2021, 07:32:31 amAgreed, and this was mentioned above. I'm very grateful people mentioned this as I hadn't considered it. This is what I'm hoping the voltage suppression device between the capacitor and SA RF input will provide.I'm concerned that the charge up time of a very large capacitor will be longer than the acceptable transient time of one of these devices. But, I'll need to run some calculations to see.Quote from: jonpaul on June 29, 2021, 07:32:31 amSure, but I want to support linear PSUs too.Quote from: jonpaul on June 29, 2021, 07:32:31 amI disagree. Many linear PSUs are quieter than the vertical resolution supported by most (including my) scopes. Additionally, I'd like to see the spectral noise contributions and my scope doesn't have a very good FFT. I could use a preamplifier, but that's another (maybe more complicated) device. Still, I'd prefer to have the spectral content provided by my SA.Quote from: jonpaul on June 29, 2021, 07:32:31 amAgain, depends on the input resistance of the test instrument. For 50ohm input with a low cutoff frequency, 100n is nowhere near adequate. A 1M input is a different story.Worth reiterating here that my two main concerns with the device I posted are:1. the capacitor charge up2. high frequency behavior of such a large capacitorI'm hopeful I can solve the first one. I'm not sure there will be a way around the 2nd one. This may prevent me from using this device, and I may have to go to a high input impedance amplifier configuration. I need to do some more analysis before I decide to order and test the large cap.It's also worth noting that I think I can circumvent a lot of the problems of the 2nd issue with a small parallel cap. It was previously noted that the lead length of the large cap will also impose a constraint on the small cap, making it have 60nH of series inductance and thus killing its high frequency behavior. I'm not sure that's right. I can place the small cap on a PCB with a microstrip tx line and with coaxial cable to the input and output connectors. I have yet to perform the sim, but I think this should present a low-impedance path for high frequencies. To reiterate, just because the large cap has a large inter-lead spacing, I don't think the small cap also needs to have a large interlead spacing. Please correct me if I'm wrong here.

author: TimFox
author_rank: Super Contributor
timestamp: June 29, 2021, 07:14:21 pm
content: Placing a SMT capacitor in the middle of a proper transmission line is the usual method to optimize its behavior against frequency.However, when you place a much larger capacitor in parallel with that smaller capacitor, the leads to the large capacitor increase its inductance, which will then resonate with the value of the small capacitor.

author: rfclown
author_rank: Frequent Contributor
timestamp: June 29, 2021, 07:22:33 pm
content: Quote from: jonpaul on June 28, 2021, 05:53:03 amI second this suggestion. My go-to DC blocks are BLK-89-S+, 100kHz-8GHz. Only $20. It is rated 50V.I think you are complicating things by trying to cover 10 Hz - 1 GHz with one solution. I would use something like a BLK-89-S+ for 100kHz and up, and something different for 10 Hz - 100 kHz. For that I'd probably just use a scope AC coupled. You said you didn't like your scope's FFT, but also noted that your SA doesn't go below 100 Hz. If you need a different instrument for lower frequencies, why does the DC block need to cover both cases? You could even use a sound card for the lower frequencies.

author: matthuszagh
author_rank: Frequent Contributor
timestamp: June 29, 2021, 07:44:19 pm
content: Quote from: rfclown on June 29, 2021, 07:22:33 pmI expect you're probably right. I'd still like to use just one device if at all possible, but it's looking increasingly like 2 separate devices makes the most sense.

author: rfclown
author_rank: Frequent Contributor
timestamp: June 29, 2021, 09:57:40 pm
content: If it's two instruments, they can be running simultaneously. A 50 ohm connection going to the SA, and a higher impedance probe for the lower frequency going to whatever you settle on for measuring that. Greatly simplifies the DC block problem.

author: jonpaul
author_rank: Super Contributor
timestamp: June 30, 2021, 06:20:02 am
content: Rebonjour matthuszagh:1/Active FET Probes for HF high Zo>>50 Ohm are avail for scopes and SA, from TEK and HP, 1X BW ~ GHz eg TEK P6201.2/ 1...10 GHz SA will not have the BW or RES for LF use. We use TEK 7L5 for LF SA use.3/ Linear PSU testCheck 1970s...1900s app notes from HP, Nat Semi, Linear Tech, Jim Williams re linear PSU and low noise PSU design. Especially recommend the huge HP app note on power supply measurements.They have 50/60-100/120 Hz ripple that depends on the load and bus caps, plus noise from load and regulator.Ripple is much greater than the noise, eg 5-25 V ripple and 1-100 mV noise.No fancy SA needed to test linear PSU, just scope and TRMS meter.4/ 10 GHz and LF tests are different instruments and setups, so two different DC blocks5/ transient protection: Double anode (bi directional) 5V low capacitance TAZ = Transient Absorbing ZenerVishay:https://www.vishay.com/docs/49639/49639.pdfProtekSMDA05LCCSMP6LLC05-2Phttp://www.protekdevices(parts we used and tested for broadcast/digital audio line protections)Zener series L is important, place the TAZ very close to the input/connector needing protection.J'espère que mes notes seront bonnes pour vos utiliser.Bon ChanceJon(designed SMPS and electronic ballasts in epoch 1970s..1990s)

author: matthuszagh
author_rank: Frequent Contributor
timestamp: July 01, 2021, 02:53:38 am
content: Quote from: jonpaul on June 30, 2021, 06:20:02 amThe P6201 looks like a nice device! I ended up ordering one, as this seems to accomplish much of what I'm looking for.Quote from: jonpaul on June 30, 2021, 06:20:02 amI've come across the Jim Williams notes. I need to spend more time reading them; they seem very interesting.Do you by any chance have a link to the HP app note on power supply measurements? Or any other information that could help me to track it down? This sounds really interesting. I didn't have any luck with a quick search on hparchive.com.Quote from: jonpaul on June 30, 2021, 06:20:02 amThanks for the recommendations!Quote from: jonpaul on June 30, 2021, 06:20:02 amVraiment!

author: jonpaul
author_rank: Super Contributor
timestamp: July 01, 2021, 05:41:00 am
content: Bonjour matthuszagh1/ P6201: BEWARE, the FET probes input V is limited and the FET will have a gate damage if the spec is exceeded even for nanoseconds. We have these but use with great caution and precautions.So, I have NEVER used P6201 for PSU testing.Beware that a P6201 on Ebay may have gate damaged, the seller will not know.It MUST have all the tips, attenuators, etc to be useful. We have found them useable IF like new and never used.The active FET probes require power 15/-15 V PS from a TEK Lemo 4 pin probe power connector (found on some old scopes and separate PSU 1103)You can DIY the PSU, but the Lemo connector is not so easy.Check the P6201 manual and TEKwiki.https://w140.com/tekwiki/wiki/P62012/ Jim Williams: Old friend and Analog Guru, since 1970s. Used to see him at Lin tech, Ham fleas and AES conventions, also with his friend Bob PEASE.JW books and articles are a goldmine of info.https://en.wikipedia.org/wiki/Jim_Williams_(analog_designer)Sad days when they both passed away a decade ago...3/ Hewlett Packard: The old HP/Agilent App notes are another goldmine of info: These decades old notes are classics, and will clear most of your questions.https://www.hpmemoryproject.org/ressources/resrc_an_01.htmin particular see the excellent Agilent AN 372-1 "Power Supply Testing"http://ridl.cfd.rit.edu/products/manuals/Agilent/power%20supplies/CD1/Literature/5952-4190.pdfJe vous souhaite un bon apprentissage des techniques d'alimentation électronique !Bonne journée,Jon