Reverberation Chambers for EM Applications
Christopher L. Holloway John Ladbury, Galen Koepke, and Dave Hill National Institute of Standards and Technology Electromagnetics Division Boulder, Colorado 303-497-6184, email: [email protected] OPEN AREA TEST SITES (OATS)
Problems: Ambients Reflections Scanning Interference Positioning TEM
Problems: High Frequencies Reflections Test Volume Positioning GTEM
Problems: Test Volume Uniformity Along Cell Positioning ANECHOIC CHAMBER
Problems: Low Frequencies Reflections Positioning REVERBERATION CHAMBER
Why use reverberation chamber? The Classical OATS Measurements
The classic emissions test and standard limits (i.e, testing a product above a ground plane at a specified antenna separation and height) have their origins in interference problems with TV reception. Emissions Test Standard Problem
One problem with the emissions test standard is that it is based on an interference paradigm (interference to terrestrial broadcast TV) that is, in general, no longer valid nor realistic today. In a recent report, the FCC indicated that 85 % of US households receive their TV service from either cable, direct broadcast satellite (DBS), or other multichannel video programming distribution service, and that only a small fraction of US households receive their TV via direct terrestrial broadcast.
Coupling to TV antennas designed to receive terrestrial broadcast may no longer be an issue. EMC Environment Today
• In recent years, a proliferation of communication devices that are subject to interference have been introduced into the marketplace.
• Today, cell phones and pagers are used in confined offices containing personal computers (PCs). Many different products containing microprocessors (e.g., TVs, VCRs, PCs, microwave ovens, cell phones, etc.) may be operating in the same room.
• Different electronic products may also be operating within metallic enclosures (e.g., cars and airplanes). The walls, ceiling, and floor of an office, a room, a car, or an airplane may or may not be highly conducting.
• Hence, emissions from electric devices in these types of enclosures will likely be quite different from emissions at an OATS. In fact, the environment may more likely behave as either a reverberation chamber or a free space environment. Where Should We Test?
• Thus, would it not be better to perform tests more appropriate to today's electromagnetic environment?
• Tests should be Shielded, Repeatable, Simple, Inexpensive, Fast, Thorough, … Commercial Solutions…
• Stirrer
• Turntable Reverberation Chamber
Probe(s) Receive Antenna D-U-T Transmit Antenna
Motor Control
Signal Generator Control and Monitoring Receive Instrumentation: Amplifier Instrumentation for Spectrum Analyzer Directional coupler Device-under-test Receiver, Scopes Power Meter(s) Probe System etc. etc. Fields in a Metal Box (A Shielded Room)
Frozen Food
•In a metal box, the fields have well defined modal field distributions.
Locations in the chamber with very high field values Locations in the chamber with very low field values Fields in a Metal Box with Small Scatterer
In a metal box, the fields have well defined modal field distributions.
Small changes in locations where very high field values occur Small changes in locations where very low field values occur Fields in a Metal Box with Large Scatterer (Paddle)
Large changes in locations where very high field values occur
Large changes in locations where very low field values occur
In fact, after one fan rotation, all locations in the chamber will have the same maxima and minima fields. Stirring Method TIME DOMAIN
Paddle
Click to play paddle rotation 750MHz Field Variations with Rotating Stirrer Reverberation Chamber: All Shape and Sizes
Large Chamber Small Chamber
Moving walls Reverberation Chamber with Moving Wall Original Applications • Radiated Immunity • Antenna efficiency components • Calibrate rf probes large systems • RF/MW Spectrograph • Radiated Emissions absorption properties • Shielding • Material heating cables • Biological effects connectors • Conductivity and materials material properties Wireless Applications
• Radiated power of mobile phones
• Gain obtained by using diversity antennas in fading environments
• Antenna efficiency measurements
• Measurements on multiple-input multiple-output (MIMO) systems
• Emulated channel testing in Rayleigh multipath environments
• Emulated channel testing in Rician multipath environments
• Measurements of receiver sensitivity of mobile terminals
• Investigating biological effects of cell-phone base-station RF exposure Fundamentals • A Reverberation Chamber is an electrically large, multi-moded, high-Q (reflective) cavity or room. • Electromagnetic theory using mode or plane-wave integral techniques provides several cavity field properties (mode density, Q and losses, E2, etc.) • Changing boundary conditions, antenna or probe location, or frequency will introduce a new cavity environment to measure (sample) • The composite effects of multiple cavity electromagnetic environments are best described by statistical models Sampling Considerations
• How many samples do we need? time / budget constraints acceptable uncertainties standards may dictate • Sampling rates may not be identical equipment-under-test instrumentation probes • How long to dwell at each sample? Sampling Considerations
• How are samples generated? change boundary conditions change device-under-test and antenna locations change frequency (bandwidth limits) • Samples must be independent Changes are ‘large enough’ • Measurement time proportional to number of samples (time = $$) Sampling Considerations
Techniques to Generate Samples • Mechanical techniques Paddle(s) or Tuner(s) • stepped (tuned) • continuous (stirred) Device-under-test and antenna position Moving walls (conductive fabric, etc.) • Electrical techniques (immunity tests) Frequency stirring • stepped or swept • random (noise modulation) • Hybrid techniques Measurement Procedures
Calibrate and Initialize system
Generate new environment Establish Fields in Chamber i.e. move paddle move antennas change frequency, etc. Measure response of EUT, probes, receive antenna, etc.
In reverberation chambers you cannot hide emission problems. Reverberation chambers will find problems. Unethical Practices • The current measurement methodologies of a product can easily miss radiation problem of a products. • That is, energy propagation in a direction that a received antenna would miss. • Not to suggest that companies would be unethical, but we have been told the individuals have setup products for emission measurements in such a manner to ensure that emission problems would not be detected. Unethical Practices • Secondly, not to suggest companies are unethical once can, but we have also been told that some individual have lists of OATS around the world with their corresponding ambient noise sources. • Thus, if one had a product that has an emission problem at frequency “x”, and it is known that one particular OATS at some site in the world as a ambient noise problem at the same frequency “x”, then a product could be tested and possible certified on the OATS, in which the product emission problems would not showing up. One Possible Solution • These two possible ways of hiding emission problems cannot be accomplished in a reverberation chamber. • In reverberation chambers you cannot hide emission problems. • Reverberation chambers will find problems. • If reverberation chambers are to be used, new standards are needed. EMISSION LIMITS •Devices and/or products are tested for emissions to ensure that electromagnetic field strengths emitted by the device and/or product are below a maximum specified electric (E) field strength over the frequency range of 30 MHz to 1 GHz. •These products are tested either on an open area test site (OATS) or in a semi-anechoic chamber. •Products are tested for either Class A (commercial electronics) or Class B (consumer electronics) limits, Class A equipment have protection limits at 10 m, and Class B equipment have protection limits at 3 m. 6.0E-4 5.0E-4 Class A: based on 10 m separation 4.0E-4 Class B: based on 3 m separation 3.0E-4 | (V/m) max |E 2.0E-4 1.0E-4 0.0E+0 0 200 400 600 800 1000 Fr equency (MHz) Total Radiated Power for Reverberation Chambers Holloway et al., IEEE EMC Symposium -40 -45 -50 (dBm) total P -55 Class A: based on 10 m separation -60 Class B: based on 3 m separation Class A: average limit Class B: average limit -65 0 100 200 300 400 500 600 700 800 900 1000 Frequency (MHz) Emission Measurements Relate total radiated power in reverberation chambers to measurements made on OATS with dipole correlation algorithms. Spherical Dipole -44 Rever b Cham ber OATS -48 -48 Rever b Cham ber -52 Anechoic Chamber E-fiel d (dBV/ m ) -52 -56 m] / -56 -60 E-fiel d [ dBV 200 300 400 500 600 700 800 900 1000 Frequency (Hz) -60 -64 200 400 600 800 1000 Frequency (Hz) Emissions Measurements of Devices Total Radiated Power Comparison Max Field Comparison Shielding Properties of Materials The conventional methods uses In most applications, materials are exposed normal incident plane-waves, i.e., to complex EM environments where fields are Coaxial TEM fixtures. incident on the material with various polarizations and angles of incidence. E i E t ⎛ P ⎞ SE = −10Log ⎜ t ⎟ 10 ⎜ P ⎟ ⎝ i ⎠ However, these approaches determine SE Therefore, a test methodology that better for only a very limited set of incident represents this type of environment would wave conditions. be beneficial. Nested Reverberation Chamber Sample Nested Reverberation Chamber C.L. Holloway, D. Hill, J. Ladbury, G. Koepke, and R. Garzia, “Shielding effectiveness measurements of materials in nested reverberation chambers,” IEEE Trans. on Electromagnetic Compatibility, vol. 45, no. 2, pp. 350-356, May, 2003. ⎛ P P P P ⎞ SE −= 10Log ⎜ ,, sinr ,, nsor ,, nsinrQ ,, sintx ⎟ 3 10 ⎜ ⎟ ⎝ P ,, nsinr P ,, sor P ,, sinrQ P ,, nsintx ⎠ Sample Nested Reverberation Chamber 80 Material 1 70 Material 2 Material 3 60 Material 4 90 50 Material 2: with coating 80 Material 2: with no coating 40 SE (dB)SE 70 30 a 60 20 50 10 (dB)SE 90 40 a 0 80 30 110 Frequency (GHz) 70 20 Different Materials 60 10 50 110 Frequency (GHz) SE (dB)SE 40 Edge Effects 30 a SE3: Chamber A SE : Chamber B 20 3 SE1: Chamber A 10 SE1: Chamber B 0 110 Frequency (GHz) Different Chamber Sizes Loading Effects: Rat in a Cage Research Goal Descritize in to FDTD Grid Take Real System • To provide a way to gain intuition into loaded chamber responses to better help measurements • Ultimately correlate Simulate measured and modeled data, and use models to predict what we cant measure. • Develop a fast and efficient numerical code to explore multiple chamber topologies. (FDTD) Predict Stirred Fields Computationally to give us insight into chamber measurements in a loaded environment Measurements of Fake Rats Lossy Body Configuration Large Lossy Body Distributed Lossy Bodies Periodic Lossy Bodies Measuring Shielding for “small” Enclosures reverb chamber Pin Pout “small” enclosure SE= -10 Log(Pin / Pout): with frequency stirring Problem with “small” Enclosures: Measuring the Fields Inside •The “rule-of thumb” for antennas positioning in a chamber is ½ wavelength from the wall. This is because the tangential component of the E-field is zero are the wall. •However, Hill (IEEE Trans on EMC, 2005) has recently shown that the statistics for the normal component of the E-field at the wall are the same for the field in the center of the chamber. Thus, small monopole (or loop) probes attached to the wall can be used to determine the power in the center of a “small” enclosure. Comparison with Different Reverberation Chamber Approaches port 3 port 2 port 2 port 1 port 1 port 4 Mode-Stirred with a Horn Antenna: SE => S31 Mode-Stirred with a Monopole Antenna: SE => S41 port 3 port 2 port 2 port 1 port 1 port 4 Frequency Stirring with a Horn Antenna: SE => S31 Frequency Stirring with a Monopole Antenna: SE => S41 Comparison with Different Approaches 20 20 18 18 mode_stirring_horn 16 mode_stirring_horn 16 freq_stirring_horn freq_stirring_horn 14 14 mode_stirring_monopole mode_stirring_monopole freq_stirring_monopole 12 12 freq_stirring_monopole 10 10 SE (dB) SE (dB) 8 8 6 6 4 4 2 2 0 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Frequency (MHz) Frequency (MHz) (a) open aperture (b) half-filled aperture 20 20 18 18 mode_stirring_horn 16 freq_stirring_horn 16 14 mode_stirring_monopole 14 freq_stirring_monopole 12 12 10 10 SE (dB) 8 SE (dB) mode_stirring_horn 8 6 freq_stirring_horn 6 4 mode_stirring_monopole 4 freq_stirring_monopole 2 2 0 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Frequency (MHz) Frequency (MHz) (c) narrow slot aperture (d) generic aperture Different Probe Lengths and Locations 30 50 Position 1, probe length=1.3 cm 45 Position 2, probe length=1.3 cm Position 1, probe length=2.5 cm 25 40 Position 2, probe length=2.5 cm probe location 1 a=0.95 cm probe location 2 35 20 probe location 3 30 probe location 4 a=0.5 cm 25 15 SE (dB) SE SE (dB) 20 10 15 10 5 5 0 0 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000 22.533.54 Frequency (MHz) Frequency (GHz) Antenna Measurement: NIST Reverberation Chamber and Experimental Set-up Dual-Ridged Horn used as receive antenna Antenna Under Test (AUT) Two paddles used Experimental Results to Compared to Horn Antenna Results presented here are for relative total radiated power (RTRP) referenced to either a Horn or to a small Loop. For a Horn we have: P Total Radiated Power = AUT PHorn where PAUT is the received power in the chamber when transmitting on the antenna under test (AUT), and PHorn is the received power in the chamber when transmitting on the horn. All antennas were connected to a 50 ohm cable, so results include both mismatch and efficiency (ohmic loss) effects. If we assume the horn is the “best” antenna available, then these measurements will show how “well” an antenna is performing. Ten-Layer Spherical 612 MHz Structure 8 mm Loop for 612 MHz antenna 612 MHz antenna: loop with ten-layer spherical material Ten-Layer Spherical 612 MHz Structure 0 -5 -10 20 -15 15 Comparison to 8 mm Loop Comparison to 32 mm Loop 10 -20 Ten-Layer Spherical Structure Total Radiated Power Relative to Horn (dB) 8 mm Loop for 612 MHz Antenna 32mm Loop Antenna 5 -25 500 550 600 650 700 750 800 850 900 950 1000 Frequency (MHz) 0 Comparison to Horn -5 Total Radiated Power Relative Loops (dB) to -10 500 550 600 650 700 750 800 850 900 950 1000 Frequency (MHz) Comparison to Loops Magnetic Antenna New Magnetic Antenna Comparison to Horn 5 Loop Antenna 0 -5 -10 -15 -20 -25 -30 -35 -40 Magnetic EZ-Antenna: Ground Plane-Center of Chamber Magnetic EZ-Antenna: No Ground Plane-Center of Chamber Total RadiatedPower Relativeto (dB) Horn -45 Magnetic EZ-Antenna: On Chamber Wall 3 cm Loop antenna -50 200 250 300 350 400 Frequency (MHz) Can NOT Measurement Directivity 0 Near-Field Antenna Tests nor-xscan w/o material nor-xscan w metamaterial -10 -20 -30 P (dBm) -40 -50 -60 -70 0 20 40 60 80 100 120 140 160 180 200 Location Multipath Environments Multipath Environments Extensive measurements have shown that when light of sight (LOS) path is present the radio multipath environment is well approximated by a Ricean channel, and when no LOS is present the channel is well approximated by a Rayleigh channel: N = AE LOS π c )2cos( + ∑ n π tffAtf ++ φnnc ])(2cos[ n=1 The Amplitude of E is either Rayleigh or Ricean depending if a LOS path is present. Urban Environment Rural Environment Ricean K-factor direct component k-factor: k = or = kLogK )(10 scattered components K= - ∞ dB K=1 dB (Rayleigh) K=4 dB K=10 dB Ricean K-factors and rms Delay Spreads Besides chancing the K-factor, we need to vary its decay rate. Standardization of Wireless Measurements Can we use a reverberation chambers for a reliable and repeatable test facilities that has the capability of simulating any multipath environment for the testing of wireless communications devices? If so, such a test facility will be useful in wireless measurement standards. Reverberation Chambers are Natural Multipath Environments Typical Reverberation Chambers Set-up Antenna pointing away from probe (DUT) a metallic walls Paddle DUT Transmitting Antenna a A Rayleigh test environment Can we generate a Ricean environment? Chamber Set-up for Ricean Environment Antenna pointing toward (DUT) metallic walls Paddle DUT Transmitting Antenna We will show that by varying the characteristics of the reverberation chamber and/or the antenna configurations in the chamber, any desired Rician K-factor can be obtained. Reverberation Chamber Ricean Environment It can be shown: see Holloway et al, IEEE Trans on Antenna and Propag., 2006. 3 V D K = 2 λ Q r 2 Note: •We see that K is proportional to D. This suggests that if an antenna with a well defined antenna pattern is used, it can be rotated with respect to the DUT, thereby changing the K-factor. •Secondly, we see that if r is large, K is small (approaching a Rayleigh environment); if r is small, K is large. This suggests that if the separation distance between the antenna and the DUT is varied, then the K-factor can also be changed to some desired value. •Next we see that by varying Q (the chamber quality factor), the K-factor can be changed to some desired value. The Q of the chamber can easily be varied by simply loading the chamber with lossy materials. Also, if K becomes small, the distribution approaches Rayleigh. Thus, varying all these different quantities in a judicious manner can result in controllable K-factor over a reasonably large range. Measured K-factor for Different Antenna Separation 1.0E+02 0.5 m 1 m separation 1.0E+01 2 m separation 1.0E+00 1.0E-01 K-factor 1.0E-02 1.0E-03 1.0E-04 1000 2000 3000 4000 5000 6000 7000 Frequency (MHz) Each set of curves represents a different distance of separation. The thick black curve running over each data set represents the K-factor obtained by using d determined in the anechoic chamber. Measured K-factor for Chamber Loading 1.0E+03 1.0E+02 6 pcs absorber 2 pcs absorber 0 pcs absorber 1.0E+01 K-factor 1.0E+00 1.0E-01 1.0E-02 1000 2000 3000 4000 5000 6000 7000 Frequency (MHz) The thick black curve running over each data set represents the K-factor obtained by using d determined in the anechoic chamber. Measured K-factor for Different Antenna Rotations 1.0E+02 0 degrees 1.0E+01 30 degrees 1.0E+00 90 degrees K-factor 1.0E-01 1.0E-02 1.0E-03 1000 2000 3000 4000 5000 6000 7000 Frequeency (MHz) The thick black curve running over each data set represents the K-factor obtained by using d determined in the anechoic chamber. Each data set was taken at 1 m separation and with 4 pieces of absorber in the chamber. Measured K-factor for Different Antenna Polarizations 1.0E+03 co-polarized 1.0E+02 45 degree polarization 1.0E+01 1.0E+00 cross-polarized K-factor 1.0E-01 1.0E-02 1.0E-03 1.0E-04 1000 2000 3000 4000 5000 6000 7000 Frequency (MHz) The thick black curve running over each data set represents the K-factor obtained by using d determined in the anechoic chamber. Each data set was taken at 1 m separation and with 4 pieces of absorber in the chamber. Simulating Propagation Environments with Different Impulse Responses and rms Delay Spreads S21 Measurements: Loading the Chamber Impulse Responses and Power Delay Profiles Power Delay Profile: 2 PDP τ = th )()( where h(t) is the Fourier transform of S21( ω) Loading the Chamber rms Delay Spreads One characteristic of the PDP that has been shown to be particularly important in wireless systems that use digital modulation is the rms delay spread of the PDP: ∞ 2 ∫ −τ o )()( dttPt τ = 0 rms ∞ ∫ )( dttP 0 where τo is the mean delay of the propagation channel, given by ∞ ∫ )( dtttP τ = 0 o ∞ ∫ )( dttP 0 rms Delay Spreads vs Threshold Levels Impulse Responses and rms Delay Spreads (200 MHz band filter on S21 data) rms Delay Spreads from Q measurements Q 2 2 )(ln)ln( αεααα+−− 22 1−+ ααα))ln(( 2 τ rms = − ω 1 α )( +− K 1 α +− K ))(( 2 where K is the k-factor and α threshold. Thus, once we have Q, we can estimate τrms Impulse Responses and rms Delay Spreads for Different Ricean K-factors BER Measurements - setup Agilent 4438C Vector Signal Generator Reverberation chamber External trigger Agilent 89600 Vector Signal Analyzer GPIB connection to control VSG Firewire connection to control VSA BER Measurements BER for a 243 ksps BPSK signal BER for a 786 ksps BPSK signal BER Measurements Demodulated 768ksps BPSK signal in I-Q diagram How Well Can we Simulate a Real Environment? Power Delay Profile in an oil refinery. How Well Can we Simulate a Real Environment? BER measurement in a laboratory. Wireless Measurements 1. Reverberation chambers represent reliable and repeatable test facilities that have the capability of simulating any multipath environment for the testing of wireless communications devices. 2. Such a test facility will be useful in the testing of the operation and functionality of the new emerging wireless devices in the future. 3. Such a test facility will be useful in wireless measurements standards. Summary • Reverberation chamber measurements are thorough and robust. • Proper sampling techniques reduce measurement uncertainties • Statistical models help minimize the number of samples required Summary • Reverberation chambers capture radiated power (total within the measurement bandwidth) • Results are insensitive to EUT placement in the chamber • Results are independent of EUT or antenna radiation pattern • Enclosed system free from external interference Rome’s Old Chamber Rome’s New Chamber Rome’s New Chamber Reverberation Chamber Standards • International Standard IEC 61000-4-21: Testing and measurement techniques – Reverberation chamber test methods