How to measure frequency spectrum of a power supply line - Page 1
Forum section: electronics / testgear

author: davorin
author_rank: Unknown
timestamp: June 19, 2015, 10:40:11 am
content: What input/protection circuit is necessary to measure the frequency spectrum of a power supply with an analyzer like the R&S CRTU or the SA44B?Just a simple capacitor in series and a 50R load resistor?

author: Dave
author_rank: Super Contributor
timestamp: June 19, 2015, 12:50:40 pm
content: Uhm... What exactly are you trying to measure?

author: davorin
author_rank: Unknown
timestamp: June 19, 2015, 12:53:58 pm
content: Well...energy spectrum in the range of 30MHz to 1GHz of a switching power supply....to see if there are any peaks are caused by poorly designed power supply or PCB design...

author: Dave
author_rank: Super Contributor
timestamp: June 19, 2015, 01:04:30 pm
content: Then yes, you need a DC blocking capacitor, if the SA isn't AC coupled itself and can accept the DC bias voltage of whatever your power supply is putting out. The 50 ohm load should be the SA input itself.You should measure the spectrum with various loads. A loaded power supply will put out much more crap than an unloaded one.

author: davorin
author_rank: Unknown
timestamp: June 19, 2015, 01:08:49 pm
content: Well it is already a load on the power supply...Basically a controller where a WLAN modul can be plugged in.The module is powered from the controller, but has an internal DC switcher as well...So I want to see what's on the 12V rail feeding the module or whatever is coming back from the module....so to compare the spectrum to the EMC testing...

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: June 19, 2015, 02:52:56 pm
content: Frankly, measuring interference on a DC power line isn't really a job for a Spectrum Analyzer. Your PSU output won't be a properly terminated 50ohms transmission line, which is what a SA expects, and the low 50ohms impedance of the SA will distort your measurement beyond recognition.A DC PSU also doesn't require a multi-GHz analyzer as most interference on the DC power line will be very well below 500MHz. The better suited tool for this task is therefore a DSO with sufficient bandwidth (i.e. 500MHz) and good FFT, and proper set of passive or better active probes.However, if you want to measure RF emissions (i.e. free air emissions) then the CRTU (together with a suitable measurement antenna) will do fine.

author: davorin
author_rank: Unknown
timestamp: June 19, 2015, 02:59:00 pm
content: Yes I know....the suitable antenna is the problem ;-)

author: Kleinstein
author_rank: Super Contributor
timestamp: June 19, 2015, 03:08:33 pm
content: A DC blocking network is a good idea, but this should have something like 50 Ohm AC output impedance and some protection a against spikes when connecting it to the supply. So this could be a capacitor with 50 Ohm in series and an additional pair of 1N4148 (or similar) anti parallel to ground. The DC path should be given by the analyzer, otherwise something like 10 K to GND are needed.

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: June 19, 2015, 03:18:46 pm
content: You also really need to think about what exactly you want to measure.If it's interference on the DC power line then you really only want to look at lower frequencies (way below 500MHz) anyways. Why? Because you can't reliably measure high frequency distortions in your setup. The power wiring will act as antenna, and depending on load, and a lot of external factors (like for example the RF environment or how the cables are positioned) they will very likely pick up (and re-emit!) a wide range of crap. That's why you have to have proper input filtering in your consumer (i.e. your WiFi module) anyways, which ensures that what comes through to the components is (mostly) DC.If you want to measure RF emissions of your PSU then connecting an SA to the DC lines will give you nothing that has any relation to reality. Simple as that. If you want to measure RF emissions then you need an antenna. If all you want to do is some general checks then any wideband antenna should do (you can often find various antennas in the up to 2.4/2.7GHz range on ebay for essentially peanuts). Unless you calibrate your setup it won't give you exact power levels, but at least it will show you any areas of concern. However you still need to perform the measurements in an area which is pretty RF silent.

author: davorin
author_rank: Unknown
timestamp: June 19, 2015, 03:28:48 pm
content: The only antennas I've found so far are way too expensive....or peanuts is defined as Euro 900 and more (o;Already looked at some broadband antenna designs to build my self.....but mostly the exact details are not disclosed...sometimes a youtube video is all you get:

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: June 19, 2015, 03:42:56 pm
content: Quote from: davorin on June 19, 2015, 03:28:48 pmMore like 47,35€http://www.ebay.de/itm/Log-Periodic-LPDA-outdoor-Antenna-800-2500Mhz-mobile-booster-repeater-amplifier-/281724062483?pt=LH_DefaultDomain_3&hash=item4198100b13Just as an example. There are lots of similar cheap antennas for the frequency range of the CRTU, which are more than sufficient for a very simple RFI assessment.

author: davorin
author_rank: Unknown
timestamp: June 19, 2015, 03:51:07 pm
content: Well...then I need a second antenna covering 30MHz to 800MHz then (o;

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: June 19, 2015, 04:03:18 pm
content: Quote from: davorin on June 19, 2015, 03:51:07 pmMany around (i.e. used with radio scanners) for even lower prices.http://www.ebay.de/itm/PROCOMM-SP-800-SPIDER-BROADBAND-DISCONE-SCANNER-BASE-ANTENNA-30-1200MHz-/151656953941?pt=LH_DefaultDomain_0&hash=item234f757855http://www.ebay.de/itm/Super-Wide-Band-Double-Fold-Over-Telescopic-Antenna-AH-W100RX-SMA-70M-900mhz-/221299966138?pt=LH_DefaultDomain_0&hash=item33868180ba

author: davorin
author_rank: Unknown
timestamp: June 19, 2015, 04:08:32 pm
content: Thanks :-)Guess searching for EMC antennas revealed only the expensive ones (o;

author: Melt-O-Tronic
author_rank: Frequent Contributor
timestamp: June 19, 2015, 04:34:09 pm
content: I've thought about this myself. If I'm interpreting the question correctly, you want to calculate exactly the filtering you need to kill non-transient noise and ripple?You definitely need a DC block, but I believe the 50 (or 75) ohm impedance of a spectrum analyzer is a bad match for the signals you're trying to detect if you probe directly. So either a high impedance probe with a low impedance output (differential probe?) or an EMI probe like the Beehive type is probably better.

author: davorin
author_rank: Unknown
timestamp: June 19, 2015, 04:37:50 pm
content: Well another option might be those near field probes....they seem to be quite easy to build....

author: G0HZU
author_rank: Super Contributor
timestamp: June 21, 2015, 02:26:57 pm
content: Quote from: Wuerstchenhund on June 19, 2015, 02:52:56 pmEvery EMC test house that has tested my designs over the last 25 years has used some form of (50 Ohm) spectrum analyser to measure conducted RF emissions on DC power supply lines. Usually from a few kHz up to 10MHz or sometimes 30MHz and sometimes even higher.A specific/simple impedance stabiliser is fitted on DC power lines to permit coupling/use of the spectrum analyser and to achieve consistent results wrt source/load impedances.

author: georges80
author_rank: Frequent Contributor
timestamp: June 21, 2015, 04:43:46 pm
content: Um... I assume you want to measure conducted noise on the power leads to/from your power supply (switcher) and the 12V input to it?A LISN is what you want - it sits between the 12V source and your power supply and has all the necessary 'stuff' to tap in and provide you a 50 ohm match to your SA to then measure/see the noise coming back along the power leads from your power supply AND NOT noise that may be coming from your 12V source.cheers,george.

author: G0HZU
author_rank: Super Contributor
timestamp: June 21, 2015, 04:58:38 pm
content: QuoteYes, for 'conducted' emissions fron the DUT's power connection, a network that provides impedance stabilisation is needed, although for DC stuff the term LISN often gets shortened to ISN (Impedance Stabilisation Network). I'm not an EMC engineer but I think the network (complete with 50R analyser) is designed to present a reasonably predictable 50R load impedance at the end of the DUT's power cable across the band of interest.As opposed to the (L)ISN providing any kind of 50R match to the analyser. The idea is that the DUT cable terminations will see a fairly stable 50R load across frequency but sadly, the analyser will not see a 50R source impedance feeding it across the band of interest (unless you also fitted an external inline attenuator at the analyser port of the ISN).

author: georges80
author_rank: Frequent Contributor
timestamp: June 21, 2015, 05:21:16 pm
content: ISN eh? I've only seen LISN even for DC, so I'll believe you that some folk call it an ISN.Anyhow, I've used a DC LISN (tekbox clone) very successfully to find switching supply issues. I had a chance to use it 'back to front' on the output of a AC/DC switcher to measure the noise coming from the power supply INTO a high speed digital design. The collected data matched very well with conducted noise that the compliance lab measured with their AC LISN on the source. It helped us choose an AC/DC switcher that would not fail us during our test (our product did well, it was our AC adapter that was letting us down).So, a DC LISN and an SA is a very helpful tool for examining and resolving conducted emissions either into or out of the DC power lines.Basically we used the DC LISN 'backwards' to determine our AC adapter conducted emissions INTO our product and then 'forwards' to determine conducted emissions back OUT of our product. Very useful tool.cheers,george.

author: G0HZU
author_rank: Super Contributor
timestamp: June 21, 2015, 07:16:11 pm
content: QuoteI guess it doesn't matter much and I think many people simply use the term LISN for both typesHowever, I was told (possibly wrongly?) that a LISN is a 'Line' Impedance Stabilisation Network for making conducted emissions measurements on the cable connection carrying line (or mains) voltages to the DUT. This means it has to conform to all kinds of safety regs because a mains voltage LISN (by design) has a deliberate high leakage path to earth via some of its capacitors. So the risk of receiving a fatal shock from touching it is very real if it isn't installed safely and correctly. Also, the components in it have to be suitably rated for these voltages. A (L)ISN for DC use is often shortened to just ISN because it never gets connected to AC line voltages and it doesn't need such stringent design and safety requirements for safe earth paths etc. But it may well be designed to carry very high supply currents. I think there is also a class of ISN that gets used solely for making measurements on small signal and data lines rather than power cables and the stabilisation network is designed slightly differently to suit the higher impedance of these cable types.However, I googled several companies that make these things and the term LISN seems to get used to cover most or all of them. AC or DC, low or high voltage.Either way, I fully agree that they can be very useful tools when looking for conducted emissions from PSUs or other electronic items