SY-EPC-LPC Converter-Concepts / EMC

Introduction

Input conducted Noise

Output conducted Noise

Introduction

This page gives measurement methods and limits to be generally applied to a LPC converter, regarding noise emitted by the converter (strong focus on conducted noise vs radiated noise). CERN applies specific gabarit in line with its core activity, and proposes pragmatic measurement methods.

All Sides: Conducted/Radiated Noise All Sides: Conducted/Radiated Noise

Some fundamental measurements should always be performed using a high frequency current probe. These measurements can easily inform on the radiated emissions as well as the conducted ones. One concerns the input/output current cables, when the other one refer to the direct radiated noise emitted by the D.U.T. They are quick and don't request high-cost additionnal material (except the spectrum analyser).

Current Probe Measurement Current Probe Measurement

Measuring the current leaving/entering a D.U.T can give better information than a standard dissymmetrical measurement, since, for example it can give access to the real common mode current. Moreover, measurement can be done up to hundreds of MHz and radiated emissions can roughly be deduced without a test with antenna and anechoid chambers.

Moebius Probe Measurement Moebius Probe Measurement

A test using a very low-cost Moebius probe can give some interesting information on the radiated noise on a D.U.T, indicating up to the area which is the most polluted if the case. It is then a nice measurement for a designer which want to improve the radiated performances of his D.U.T.

AC Input Side: Conducted Noise AC Input Side: Conducted Noise

Noise conducted on AC lines can affect any other user powered by the same mains network. This requirement is strongly defined in the standards an is a basic requirement for any system delivered to CERN. A computer is measured vs level proposed here

CERN Compliance Matrix CERN Compliance Matrix

Meas Type	Differential Noise	Dissymmetrical Noise
Frequency Domain	Validation Criteria: → See Gabarit/Levels below
Time Domain	Validation Criteria: → No measurement required on time domain

Recommended measurement methods vs frequency range

CERN Frequency Reference Gabarit/Levels CERN Frequency Reference Gabarit/Levels

CERN gabarit is historically based on IEC-478-3 Curve C (replaced by IEC-61204-3), with extension down to 2kHz.

[2kHz..10kHz] range is out of standard EMC equipement range (impedance and measurement capability) and should be treated following CERN recommandations.
Quasi-Peak Measurement required, even if peak could be used (saving time) initially to point at possible issues to be later studied more carefully. A Peak measurement below limit confirms entirely the compliance with Quasi-Peak limit. The more noise signal "duty cycle" reduces (its frequency of apparition) the more Quasi-Peak Measurement result differs strongly from Peak one. Converter noise (by nature) typically generates less than 6dB of difference between Q-Peak and Peak.

Using CERN level CERN level - CERN level vs CISPR11 CERN level vs CISPR11 -
Frequency Domain measurement Test Set Up .vsd
CERN Input Side Gabarit .xls
Predefined Plot versus standards .xls

Values for spectrum analyser are given below.

Frequency Range	[9kHz..150kHz]	[150kHz..30MHz]	[30MHz..300MHz]
Resolution Bandwidth	200 Hz	9 kHz	120 kHz

Resolution Bandwidth to be set on Spectrum Analyser vs frequency range

Measurement method Measurement method

This measurement is well documented in standards, with some freedom (high power constraints) described below.

LISN provides "dyssimmetrical" measurement, from ground to active measured lines, getting common and differential mode.
LISN provides known & balanced impedance (50 Ohms line to ground f>fo) on all lines (3x for 3-phases) giving meas. reproducibility.
LISN provides also reduction of existing noise on power network not to pollute the measurements.
A 1500 Ohms EMC probe measurement is possible when LISN is not available ...
- Connect generally the ground part of the probe in less than a 30 cm area from the measurement point.
- Line Impedance is not guaranted anymore (being then given by network), jeopardizing measurement reproducibility.
- No decoupling exists between other clients on network and D.U.T. A measurement is mandatory with D.U.T OFF to identify the initial quality of the network.
- Sensitivity of the measurement is highly reduced (30..35dB attenuation from the probe, due to its natural high impedance required not to un-balance measured line).
- A 1500 Ohms EMC probe probe takes 1-2 hours to be built home-made
  
  home-made 1500 Ohms 9khz-30Mhz probe
  
  home-made 1500 Ohms 9khz-30Mhz probe schematics
  
  home-made 1500 Ohms 2khz-100khz probe schematics (specific low frequency CERN measurement probe)
  
  home-made 1500 Ohms 9khz-30Mhz probe attentuation measurement
A frequency domain measurement can be done using modern oscilloscope ...
- Nyquist criteria is taken into account, with correct matching between sampling rate and max frequency in signal measured
  → f_max in signal of interest < 0.5 x (Samples / s) Hz (if not, aliasing to be expected)
- Sufficient number of points allows decent frequency range FFT measurement (signal frequency is "contained" in oscilloscope memory)
  → f_min measured safely > 2/ [ Nb.Samples / (Samples / s) ] Hz
- Adequate passive filters can be placed to limit anti aliasing phenomena, if frequency range dynamic is too high
- Adequate vertical sensitivity is used (some mV of noise are not correcly computed if a too large (1V/div) setting is used).
- AC coupling is used correclty when spying low frequencies
- Simplified Procedure:
  
  Choose the highest sampling rate + maximum memory size oscilloscope you own
  Exemple: TDS3014 100MHz: 1.25GS/s + 10ksamples.memory
  
  Identify maximum frequency f.signal.max-freq in measured signal.
  (this can easily be done sampling at highest sampling rate of the oscilloscope)
  Exemple: 50kHz switching converter : f.signal.max-freq = 20MHz
  
  Caclulate the minimum safe sampling rate Ch.sampling.rate to be set on oscilloscope
  → Ch.sampling.rate > 2(min) x f.signal.max-freq samples/sec
  → Set it adjusting Channel horizontal time scale
  → Exemple: Ch.sampling.rate = 50 MS/s → Channel horiz. scale = 20µs
  
  Caclulate the minimum safe measurable frequency
  → f.signal.min-measurable-freq = Ch.sampling.rate / Nb.Acquisition.Points x 2
  → Exemple: f.signal.min-measurable-freq = 50 MS/s / 10kS x 2 = 10 kHz
  
  Adjust channel vertical sensitivity to optimum, keeping signal maximum in windown
  
  Make FFT Result analyze:
  → Safe frequencies measured range = [f.signal.min-measurable-freq .. Ch.sampling.rate]
  → Never touch Channel horizontal scale setting for zooming on FFT, use FFT one.
  → Exemple: Frequencies measurables in range = [10kHz..50MHz].
  Measuring lower frequency would require or:
  - a lower sampling rate with the addition of external additionnal sharp low-pass filter to avoid aliasing phenomena.
  - better oscilloscope with higher nb of points memory type
- Typical Settings on TDS3014 Oscilloscope:
  
  1500 Ohms 2k..100kHz CERN EMC probe with 100kHz low pass filter activated, on 50 Ohms selected Input
  Time Scale = 400µs (2.5 MS/s)
  Measurement Range = [2kHz..100kHz] (flat part of the pass-band)
  Measurement Gain = 35dB (56.2x) in range [2kHz..100kHz] only
  Minimum measurable signal amplitude = 110dBµV (0.3Vrms) on 500mV amplitude channel

Meas. / Freq.	[2kHz..9kHz]	[9kHz..30MHz]
Frequency Domain	Oscilloscope + FFT Module + Single-Ended Probe → to cope with range of [2kHz..9kHz]	Spectrum Analyser + LISN
Frequency Domain		Spectrum Analyser + Standard 1500 Ohms Probe → Accepted if input current too high for standard LISN

Recommended & practical measurement methods vs frequency range

Using LISN LISN - 1500 Ohms Probe 1500 Ohms Probe - Oscillo + Single-Ended Probe Oscillo + Single-Ended Probe
Frequency Domain measurement Test Set Up .vsd

Output Side: Conducted Noise Output Side: Conducted Noise

Output side (user side) is a key parameter for general CERN applications, since many often connected to magnet controled at the level of ppm with ultra precise electronics, potentially sensitive to noise.

CERN Compliance Matrix CERN Compliance Matrix

CERN pay high attention to noise below 1MHz, especially in differential mode, even if randomly distributed and with very low ocurrence, or coming from specific conditions: 0V crossing in 4 quadrant power converters, a fan or contactor switched on... For this reason, time-domain measurement are required, in addition to frequency-domain measurement, to catch these unexpected signals.

Meas Type	Differential Noise	Dissymmetrical Noise
Frequency Domain	Validation Criteria: → See Gabarit/Levels below
Time Domain	Validation Criteria: Trigger set @ V.treshold (150mVpk if Vout < 50V) → No trig expected after 10 sec, in any operating conditions (static or dynamic)

Recommended & practical measurement methods vs frequency range

CERN Reference Gabarit/Levels CERN Reference Gabarit/Levels

CERN gabarit is based on IEC-478-3 Curve C, extended for low frequencies (magnet current ripple constraints).

[10Hz..10kHz] range is out of standard EMC equipement range (impedance and measurement capability) and should be treated with adequate material (bandwidth issue on 1500 Ohms probe for example, with a 35dB rejection for frequency higher than 10kHz usually).
A time domain reference level V_peak-peak measured with 100 MHz bandwidth capability should always be performed in parallel to this test.
Reference gabarit is given (strong line) for a DC output voltage of less than 50V. Dashed line shows an hypothetic level for a higher range / purpose / topology application. This limit level is defined from final user requirements.

Measurement Using Peak Peak - Quasi-Peak Quasi-Peak

CERN Output Side Gabarit Peak-preferred (<50V dc output) .xls

Values for spectrum analyser are given below.

Frequency Range	[9kHz..150kHz]	[150kHz..30MHz]	[30MHz..300MHz]
Resolution Bandwidth	200 Hz	9 kHz	120 kHz

Resolution Bandwidth to be set on Spectrum Analyser vs frequency range

Measurement method Measurement method

Differential Mode Measurement Differential Mode Measurement

An oscilloscope running on a battery can measure differential mode noise very easily. Use of modern proble or a basic additionnal capacitor to cut the DC voltage can be used (1uF..100uF) in series with simple 50-ohms cable can be used.

Some spectrum analyser with differential input exists, but FFT module available with modern oscilloscope fits well the frequency range.
Avoid using differential probes, with common mode rejection ration not sufficient when reaching high frequencies.
Better use a batterry powered oscilloscope than a insulation transformer on AC powered oscilloscope. (coupling capacitor)
Always keep in mind that an ultra-basic measurement with a Fluke voltmeter (Series 79 or similar), used in AC.mode gives you interesting indication expressed in true rms value, in the range [45; 1 0000] Hz, regarding the noise being measured.

Output differential mode measurement methods .vsd

Meas. / Freq.	Vout	[10Hz..2kHz]	[2kHz..9kHz]	[9kHz..30MHz]
Frequency Domain	< 42V_Peak	Battery Powered Oscilloscope + Single-ended Probe + Oscilloscope FFT Module
	> 42V_Peak	100MHz Oscilloscope + FFT Module + 100MHz High CMRR / Bandwidth Differential Input Probe
Time Domain	NA	100MHz Oscilloscope + 100MHz High CMRR / Bandwidth Differential Input Probe

Recommended & practical measurement methods: frequency range vs Frequency / Time Domain

"Common Mode" or dyssimmetrical Measurement

This "dyssimmetrical" measurement gets both common and differential mode (not a pure "common mode" meas.) and has to be performed on all outputs. (common mode often referred as asymetrical, when differential mode is symetrical noise).

For practical reason, dyssimmetrical measurement on output side is done at the level of the standard DC connections (bolts), with ground reference connected locally to the busbar (30cm area). Electrical high frequency conductiviy of the rack is assumed compliant.
Measurement has to be made on both polarities even if often identical (high frequency diff. capacitors usually being used).
A 1500 Ohms EMC probe or a standard modern oscilloscope probe provide sufficient impedance and is not intrusive.
A 1500 Ohms EMC probe measurement is possible when LISN is not available ...
- Connect generally the ground part of the probe in less than a 30 cm area from the measurement point.
- Line Impedance is not guaranted anymore (being then given by network), jeopardizing measurement reproducibility.
- No decoupling exists between other clients on network and D.U.T. A measurement is mandatory with D.U.T OFF to identify the initial quality of the network.
- Sensitivity of the measurement is highly reduced (30..35dB attenuation from the probe, due to its natural high impedance required not to un-balance measured line).
- A 1500 Ohms EMC probe probe takes 1-2 hours to be built home-made
  
  home-made 1500 Ohms 9khz-30Mhz probe
  
  home-made 1500 Ohms 9khz-30Mhz probe schematics
  
  home-made 1500 Ohms 2khz-100khz probe schematics (specific low frequency CERN measurement probe)
  
  home-made 1500 Ohms 9khz-30Mhz probe attentuation measurement
A frequency domain measurement can be done using modern oscilloscope ...
- Nyquist criteria is taken into account, with correct matching between sampling rate and max frequency in signal measured
  → f_max in signal of interest < 0.5 x (Samples / s) Hz (if not, aliasing to be expected)
- Sufficient number of points allows decent frequency range FFT measurement (signal frequency is "contained" in oscilloscope memory)
  → f_min measured safely > 2/ [ Nb.Samples / (Samples / s) ] Hz
- Adequate passive filters can be placed to limit anti aliasing phenomena, if frequency range dynamic is too high
- Adequate vertical sensitivity is used (some mV of noise are not correcly computed if a too large (1V/div) setting is used).
- AC coupling is used correclty when spying low frequencies
- Simplified Procedure:
  
  Choose the highest sampling rate + maximum memory size oscilloscope you own
  Exemple: TDS3014 100MHz: 1.25GS/s + 10ksamples.memory
  
  Identify maximum frequency f.signal.max-freq in measured signal.
  (this can easily be done sampling at highest sampling rate of the oscilloscope)
  Exemple: 50kHz switching converter : f.signal.max-freq = 20MHz
  
  Caclulate the minimum safe sampling rate Ch.sampling.rate to be set on oscilloscope
  → Ch.sampling.rate > 2(min) x f.signal.max-freq samples/sec
  → Set it adjusting Channel horizontal time scale
  → Exemple: Ch.sampling.rate = 50 MS/s → Channel horiz. scale = 20µs
  
  Caclulate the minimum safe measurable frequency
  → f.signal.min-measurable-freq = Ch.sampling.rate / Nb.Acquisition.Points x 2
  → Exemple: f.signal.min-measurable-freq = 50 MS/s / 10kS x 2 = 10 kHz
  
  Adjust channel vertical sensitivity to optimum, keeping signal maximum in windown
  
  Make FFT Result analyze:
  → Safe frequencies measured range = [f.signal.min-measurable-freq .. Ch.sampling.rate]
  → Never touch Channel horizontal scale setting for zooming on FFT, use FFT one.
  → Exemple: Frequencies measurables in range = [10kHz..50MHz].
  Measuring lower frequency would require or:
  - a lower sampling rate with the addition of external additionnal sharp low-pass filter to avoid aliasing phenomena.
  - better oscilloscope with higher nb of points memory type
- Typical Settings on TDS3014 Oscilloscope:
  
  1500 Ohms 2k..100kHz CERN EMC probe with 100kHz low pass filter activated, on 50 Ohms selected Input
  Time Scale = 400µs (2.5 MS/s)
  Measurement Range = [2kHz..100kHz] (flat part of the pass-band)
  Measurement Gain = 35dB (56.2x) in range [2kHz..100kHz] only
  Minimum measurable signal amplitude = 110dBµV (0.3Vrms) on 500mV amplitude channel

Using Oscilloscope Oscilloscope - Spectrum Analyser Spectrum Analyser
Dyssimmetrical Measurement .vsd

Meas. / Freq.		[10Hz..2kHz]	[2kHz..9kHz]	[9kHz..30MHz]
Frequency Domain 2 possibilities accepted	1 →	Oscilloscope + Single-ended Probe + FFT Module
	2 →	Oscilloscope + Single-ended Probe + FFT Module		Spectrum Analyser + Standard 1500 Ohms Probe
Time Domain		100MHz Oscilloscope + 100MHz Single-ended Probe

Recommended & practical measurement methods: frequency range vs Frequency / Time Domain

Introduction

All Sides: Conducted/Radiated Noise All Sides: Conducted/Radiated Noise

Current Probe Measurement Current Probe Measurement

Moebius Probe Measurement Moebius Probe Measurement

AC Input Side: Conducted Noise AC Input Side: Conducted Noise

CERN Compliance Matrix CERN Compliance Matrix

CERN Frequency Reference Gabarit/Levels CERN Frequency Reference Gabarit/Levels

Measurement method Measurement method

Output Side: Conducted Noise Output Side: Conducted Noise

CERN Compliance Matrix CERN Compliance Matrix

CERN Reference Gabarit/Levels CERN Reference Gabarit/Levels

Measurement method Measurement method

Differential Mode Measurement Differential Mode Measurement

"Common Mode" or dyssimmetrical Measurement

Measurements examples Measurements examples

Noise level comparison converter vs 10m-far distance load

Time vs Frequency Domain measurements Time vs Frequency Domain measurements