Measurement Techniques of Emissions from Ultra Wideband Devices

Transcription

1 2252 IEICE TRANS. FUNDAMENTALS, VOL.E88 A, NO.9 SEPTEMBER 2005 INVITED PAPER Special Section on Ultra Wideband Systems Measurement Techniques of Emissions from Ultra Wideband Devices Jun-ichi TAKADA, a), Shinobu ISHIGAMI, Juichi NAKADA,EishinNAKAGAWA, Masaharu UCHINO, and Tetsuya YASUI, Members SUMMARY This paper describes the measurement techniques of emissions from UWB devices discussed in ITU-R task group (TG) 1/8 to study the compatibility between ultra-wideband (UWB) devices and radiocommunication services. This paper also provides the background idea behind the measurement methods, as the final output of the discussion, i.e. ITU-R Recommendation, will not contain any citations to the references, nor any educational description of the theoretical background. key words: UWB devices, measurement, ITU-R 1. Introduction Various new applications are considered by utilizing the ultra wideband (UWB) radio taking the advantages of low power consumption, high data rate, and fine position accuracy. However, UWB signals are usually transmitted over the frequency spectrum already in use by other systems and applications, such as the radio communications and remote sensing. For the compatibility between the conventional radio systems and the UWB devices, the emission limit of the UWB devices shall be restricted. The permissible emission level has been discussed extensively between the proponents of UWB systems and the potential victims. Therefore, it is essential to define the emission level of the UWB devices by the measurements. However, it is not so straightforward to measure the emission characteristics of the UWB devices as the ordinary carrier modulated signals. In the International Telecommunication Union Radiocommunication sector (ITU-R), task group (TG) 1/8 has been established in July 2002 to study the compatibility between ultra-wideband devices and radiocommunication ser- Manuscript received May 10, Final manuscript received May 18, The authors are with the UWB Technology Institute, National Institute of Information and Communications Technology, Yokosuka-shi, Japan. The author is with the Department of International Development Engineering, Graduate School of Engineering, Tokyo Institute of Technology, Tokyo, Japan. The author is with the Communication System EMC Group, National Institute of Information and Communications Technology, Yokosuka-shi, Japan. The author is with the Advantest Corporation, Meiwa-machi, Gunma-ken, Japan. The author is with the Telecom Engineering Center, Tokyo, Japan. The author is with the Anritsu Corporation, Atsugi-shi, Japan. a) DOI: /ietfec/e88 a vices. TG 1/8 is divided into four working groups (WGs), i.e. characteristics in WG1, compatibility in WG2, spectrum management framework in WG3, and measurements in WG4. The standard measurement techniques of emissions from UWB devices have been discussed in TG 1/8WG4. Presently, the preliminary draft new recommendation (PDNR) on the measurement to be entitled Measurement techniques of emissions from systems using ultra-wideband technology (ITU-R SM.[UWB.MES]) is still under discussion. It should be finalized by the last meeting of TG 1/8 in October This paper describes the measurement techniques presented in the PDNR ITU-R SM.[UWB.MES]. Since the final recommendation has not been approved, the authors refer the Japanese contribution submitted to May 2005 meeting of TG 1/8 [1]. ITU-R recommendation does not contain any citations to the references, nor any educational description of the theoretical background. Therefore, the paper provides the background idea behind the measurement methods, which will not be available on the PDNR. Although the techniques proposed by the authors are more emphasized, all the techniques are reviewed. Numerical examples are limited to the microwave frequency, although the PDNR includes those for the quasi-millimeter and millimeter waves. Section 2 presents two different philosophies of the measurements. Section 3 describes a concept of spectrum emission mask. If the emission mask is deployed, some parameters normally applied to emissions, such as occupied bandwidth and unwanted emissions, do not need to be specified. Section 4 compares the frequency domain and time domain measurement approaches. Section 5 lists the parameters and their units to be measured. Section 6 describes the measurement conditions and environments. Section 7 presents the detailed measurement techniques of the emission in the frequency domain, which Sect. 8 present those in the time domain. Finally, Sect. 9 summarize the paper. 1.1 Glossaries CAF : complex antenna factor DFT : discrete Fourier transform DUT : device under test This paper is written in April Copyright c 2005 The Institute of Electronics, Information and Communication Engineers

2 TAKADA et al.: MEASUREMENT TECHNIQUES OF EMISSIONS FROM ULTRA WIDEBAND DEVICES 2253 EIRP : equivalent isotropic radiated power EMC : electromagnetic compatibility FCC : Federal Communications Commission, USA FFT : fast Fourier transform IBW : impulse bandwidth ITU-R : International Telecommunication Union Radiocommunication sector LNA : low noise amplifier LO : local oscillator PAPR : peak to average power ratio PDNR : preliminary draft new recommendation PRF : pulse repetition frequency QP : quasi-peak PDF : probability density function PSD : power spectral density RBW : resolution bandwidth RMS : root-mean-square TG : Task Group TRP : total radiated power VBW : video bandwidth WG : Working Group 2. Philosophies of Measurements It seems that there are two different philosophies to consider the measurement techniques of the radio signals. One is the very precise measurement of the physical quantity. The main purpose is to get as accurate physical quantities as possible. To accomplish the purpose, it is the case that very time consuming measurement procedures shall be necessary by using very expensive instruments with high accuracy and precision. The other is the regulatory or the compliance measurements. The measurement procedure shall be simple enough to complete within a feasible period by using the off-theshelf instruments. The measured value shall be reasonably meaningful from the view points of compatibility and/or coexistence with other radio systems, but it is not necessary that the measured value is completely identical to the real physical quantity. The latter philosophy is applied through the paper, but the efforts were made to get more accurate values. 3. Spectral Emission Mask The UWB emission is spread over a very large frequency range and overlaps several frequency bands allocated to radio services. Therefore, it is a reasonable approach that the emission level shall be restricted by the spectral emission mask, just like the unwanted emission limit of unintentional emitters for electromagnetic compatibility (EMC). For example, Fig. 1 shows spectral masks of indoor and handheld UWB systems approved by the Federal Communications Commission (FCC) in USA [3]. Note that no other administration than USA has approved the unlicensed use of UWB technology yet for commercial applications. Fig. 1 Emission limits of indoor and hand held UWB systems by FCC. Although the emission limit itself is not the matter of the measurement, the value has the impact on the specification and the configuration of the instruments. 4. Frequency Domain vs. Time Domain Measurement Approaches Chapter 2 of PDNR ITU-R SM.[UWB.MES] [1] describes about two alternative approaches for the measurements. One approach involves a measurement of the time domain characteristics of the signal. The time domain data are transformed into the frequency domain data via the discrete Fourier transform (DFT) or the fast Fourier transform (FFT). The second approach involves a direct measurement of the UWB spectral characteristics in the frequency domain. There are equipment limitations that must be considered with respect to either of these measurement approaches. The state-of-art single-end oscilloscope can measure a single-event waveform up to 12 GHz, while the quantization bits of A/D converter is limited to 8 bits. The sampling oscilloscope can measure a periodic waveform up to 50 GHz, with the quantization bits of A/D converter 14 bits. It is suitable for the peak power emission measurements with wider resolution bandwidth (RBW) that can not be achieved by the spectrum analyzer. Contrary, the relatively narrow dynamic range prevents the measurements of the low level emission outside the UWB bandwidth. The spectrum analyzers are typically designed to measure conventional narrow-band signals. The IF bandwidth of the spectrum analyzer is significantly smaller than the bandwidth of the UWB waveform, and is even smaller than the 50 MHz reference bandwidth for the peak power measurement. It is necessary to convert the narrow band measurement results into the estimate of the power within wider RBW. Note that this value is not the true power value but only the estimate of the power. The conventional spectrum analyzer (Fig. 2) uses a super heterodyne architecture and a sweep frequency local oscillator (LO), and the sample timing for each frequency bin is different. Therefore, the measurement results shall be influenced by the sweep time and the measurement time window of the spectrum analyzer, as well as by the duty ratio and the pulse repetition frequency of the UWB signal. In addition, conventional spectrum analyzer is not so sensitive as to detect UWB signals at the very low emission limits applicable in specific frequency bands.

3 2254 IEICE TRANS. FUNDAMENTALS, VOL.E88 A, NO.9 SEPTEMBER 2005 Fig. 2 Block diagram of super heterodyne spectrum analyzer. Fig. 3 General UWB measurement system. 5. UWB Parameters Three power values are to be measured; peak power, average power, and quasi-peak power. Peak power mainly influences the RF front end of the receiver, e.g. the saturation of the low noise amplifier. Therefore, it is not necessary to evaluate the total peak power in the whole bandwidth, but the peak power within the sufficiently wide bandwidth that corresponds to the RF filter of the victim receiver. Average power is a measure of the interference to the victim system. To roughly estimate the effect of the interference, the interference signal power can be treated as the increase of the noise level. UWB technology is expected to be used in imaging applications in the frequency below 1000 MHz. They utilize an unmodulated pulse train with low pulse repetition frequency (PRF). Typically, PRF is below 1 MHz and the specrum has multiple line peaks. The peak power of a spectral line can be an intereference to a tuned narrowband victim receiver. Therefore, emissions below 1000 MHz are measured by a quasi-peak signal detector, as this type of UWB waveform should be peak-power limited. Note that quasi-peak represents a weighted peak. Equivalent isotropic radiated power (EIRP) is used to evaluate the emission level. It is the product of the power supplied to the antenna and the antenna gain in a given direction relative to an isotropic antenna. EIRP is a direct measure of the emission, but it is a function of both direction and frequency, and it is rather difficult to find the highest value. Instead of the total power in the full frequency band, power spectral density (PSD) is used as a measure of the power contained within a specified segment of spectrum. PSD is expressed as dbm/mhz. UWB bandwidth is another parameter to be determined by the measurement. It is typically defined as 10 db bandwidth [2]. 6. Measurement Conditions Chapter 4 of PDNR ITU-R SM.[UWB.MES] [1] describes the measurement conditions. Figure 3 shows a block diagram of a general UWB measurement system. Measuring receiver can be a spectrum analyzer and/or an EMI receiver for frequency domain measurements, and an oscilloscope for time domain measurements. It is recommended to measure the emission in a highperformance anechoic chamber, as other licensed services may use the same frequency and the measurements may be corrupted due to the saturation of the high-gain low noise amplifier (LNA). A semi-anechoic chamber may be used at the frequency below 1000 MHz, with the correction of 4.7 db from the maximum value of the receiver antenna height pattern. In addition, a reverberation chamber may be employed for the average power measurements. The UWB device under test (DUT) must be oriented with respect to the measurement system to ensure reception of the maximum radiated signal. The use of a nonconductive turntable is suggested to systematically search for the orientation that provides the maximum response within the measurement system. The spherical wave theory [4] suggests that the maximum number of the ripples of the directive emission pattern in 360 degrees rotation N is related to the radius of the sphere enclosing the UWB device r 0 and the maximum frequency f H as N 2π f Hr 0, (1) c where c = m/s is the velocity of the electromagnetic wave in free space. It is a rule of thumb that five to ten samplesperrippleissufficient to find the peak. A separation distance of 3 m is used for measuring conventional wireless devices. However, it may not be possible to measure low level UWB signals. In the case, the distance shall be reduced to 1 m or less, taking care to maintain the far field condition. In fact, there are two far field conditions to be considered. First one is the boundary between the reactive region and the radiative region. The radius of the boundary r r is r r = λ π, (2) where λ = c f is the wavelength for frequency f. Within r r, the reactive energy is bigger than the radiated energy, and the measured result does not coincide with the emission power. Another one is the boundary between the Fresnel region and the Fraunhoefer region. The radius of the boundary r f is r f = 8(r a + r 0 ) 2, (3) λ where r a is the radius of the sphere enclosing the measuring antenna. r f is defined to limit the phase error due to the sizes of the antenna and the DUT within π 8 rad. Therefore,

4 TAKADA et al.: MEASUREMENT TECHNIQUES OF EMISSIONS FROM ULTRA WIDEBAND DEVICES 2255 the influence of r 0 may be negligible as far as the antenna of DUT is not designed with high directivity. Note, however, that r f is bigger than 3 m at 10 GHz if r a is bigger than 0.1 m. For the DUT with a high gain antenna, such as a short range radar, the conducted measurement presented in Sect. 7.6 shall be considered alternatively. The radiated emissions from a UWB device are often too weak to overcome the noise figure of a conventional spectrum analyzer. The EIRP limit due to the noise is discussed in Appendix A. Therefore, it becomes necessary to utilize an LNA at the output of the measurement antenna to reduce the effective noise figure of the overall measurement system. 7. Frequency Domain Measurements Chapter 6 of PDNR ITU-R SM.[UWB.MES] [1] describes several alternative techniques of frequency domain measurements. Three signal detectors are to be used in measuring UWB waveforms. For measuring signal characteristics in the radio frequency spectrum below 1000 MHz, a CISPR quasi-peak (QP) detector [5] is specified. A rootmean-square (RMS) average detector is specified for measuring the average UWB radiated signal amplitude in the frequency spectrum above 1000 MHz. A peak detector is also necessary for determining the peak power amplitude associated with UWB waveforms in the spectrum above 1000 MHz. 7.1 Measurement Uncertainty of Spectrum Analyzers In general, frequency characteristic of spectrum analyzer is not flat. For example, when the level measurement of unmodulated signal in the frequency range below 40 GHz is performed by the spectrum analyzers, the measurement uncertainty is ±3dB or±5.5db for the high performance spectrum analyzers or the off-the-shelf general purpose spectrum analyzers, respectively. In this case, the measurement uncertainty includes errors caused by non-linearity of the logarithmic amplifier and by the gain deviation which depends on the RBW value of the spectrum analyzer. This uncertainty can be reduced to 0.49 db after the calibration by using a power meter and a 4-port coaxial transfer switch. The detail of the uncertainty budget and the calibration method is presented in Appendix B. 7.2 Determination of UWB Bandwidth A peak PSD with the reference bandwidth of 1 MHz is used as an emission level to determine the bandwidth in the following manner. The measurement shall be made using a spectrum analyzer with a 1 MHz RBW and a 3 MHz video bandwidth (VBW). Choice of VBW is discussed in Appendix C. The analyzer shall deploy a peak detector and a maximum-hold trace mode. The frequency point for the highest radiated emission evaluated by the peak PSD in a 1 MHz segment is designated as f M. The 1 MHz segments, below and above f M, where the emission level falls 10 db, are designated f L and f H, respectively. The two recorded frequencies represent the highest f H and lowest f L bounds of the emission. The centre frequency f C and fractional bandwidth µ 10 can be calculated from f L and f H as f C = f L + f H 2 (4) µ 10 = f H f L f C (5) 7.3 QP Detector for PSD at Frequency below 1000 MHz A QP detector specified in CISPR [5] is used to measure PSD below 1000 MHz. However, the drawback of the use of the CISPR QP detector is its long response time. In fact, the output of QP detector does not exceed the output of peak detector described in Sect Therefore, it is suggested that the peak power shall be measured preliminarily. The QP detector is then used only when the peak power exceeds the emission limit. 7.4 Average PSD Average PSD is defined as the maximum EIRP within 1 MHz bandwidth averaged over 1 ms. Three alternative measurement methods are presented. Table 1 compares these three methods. First and second methods use a sample detector and the power sum shall be taken as the post-processing, while third method uses an RMS detector to take the average in the spectrum analyzer. First and third methods use the IF filter with RBW of 1 MHz, while the second method sweeps within 1 MHz bandwidth with narrower RBW to coincide with the rectangular spectral mask with the bandwidth of 1MHz Radiometric Measurements for Low EIRP In certain frequency ranges, the emission limit of UWB devices may be very low. Appendix A describes the noise power of the spectrum analyzer. It is difficult to measure very low EIRP by using a conventional spectrum analyzer, even by using the LNA. Radiometric techniques provide a viable method for measuring such a low level of EIRP. An example setup of the measurement is shown in Fig. 4. In Table 1 Comparison of three measurement methods for average PSD. Sect Sect Sect RBW* 1MHz 10 khz 1MHz VBW** 3 RBW 3 RBW 3 RBW Frequency span 0 1MHz N/A Detector sample sample RMS Sweep time 1ms coupled function (number of bins) 1ms Average PSD power sum power sum each bin Note RBW*: Resolution bandwidth VBW**: Video bandwidth, see Appendix C

5 2256 IEICE TRANS. FUNDAMENTALS, VOL.E88 A, NO.9 SEPTEMBER 2005 Fig. 4 Example of radiometric technique for EIRP measurement in 1 2 GHz. the radiometric techniques, EIRP of the DUT and EIRP of background are measured. After subtracting the latter from the former, the true value of EIRP emitted from DUT can be obtained properly. The detailed scheme is described in Ref. [6] and Appendix 5 of PDNR ITU-R SM.[UWB.MES] [1]. Fig. 5 Measurement setup in the reverberation chamber Total Radiated Power Measurement by Using Reverbration Chamber A reverberation chamber is a metal chamber with rotating metallic blades which are called stirrers. The stirrers have been used to realize a statistically uniform electromagnetic field distribution by changing the boundary condition inside the chamber. DUT is set at the testing area of a chamber, and the radiated power is evaluated by an average receiving power that is measured by carrying out stepping or continuous rotation of the stirrers. Figure 5 shows a measurement setup in the reverbration chamber. Reverberation chambers offer advantages for UWB measurements at higher frequencies: Measurement sensitivity is grater than that for free space measurements, since the energy is contained in the chamber. The probability density function (PDF) of the received signal is substantially independent of the orientation of the test device and receive antenna. Since the radiation power in each frequency is measured, the frequency at which the highest radiated emission occurs f M can be known. The maximum radiation direction cannot be known by a reverberation chamber measurement, and the total radiation power (TRP) is measured instead of EIRP. Upper bound of EIRP can be estimated as the product between TRP and maximum antenna gain. More accurate value can be measured in the subsequent anechoic measurement at the frequency f M. The requirements for a reverberation chamber to be used for emission measurements are described in Sect of the CISPR [7]. In the measurement process, DUT in the chamber is rotated at intervals of 45 degrees, and the examination is repeated four times. The receiving antenna rotation is set at intervals of 90 degrees. TRP of DUT can be measured using the substitution method. The input power P TX is delivered from the reference signal generator to the input of the transmitting antenna, and the receiving power P RX is measured by the receiver connected to the receiving antenna. The receiver is a spectrum analyzer setup in the same manner as the anechoic measurements. In the next step, DUT is fixed at the same position as the transmitting antenna. Then the received power P M of DUT can be measured. TRP of DUT P DUT is obtained as P DUT = P M P TX η TX, (6) P RX where η TX is the radiation efficiency of the transmitting antenna. 7.5 Peak PSD Peak PSD is defined as the maximum EIRP within 50 MHz bandwidth with the observation time window of 0.1 ms. However, most of the spectrum analyzers are not equipped with the IF filter with RBW of 50 MHz. Therefore, the measurement shall be done with narrower RBW, and the result is converted to the 50 MHz reference bandwidth by some means. Two alternative measurement methods are presented, but only the first method is clearly defined. Table 2 compares the parameters for two methods. The first method uses peak detector and the power sum over 50 MHz bandwidth shall be taken as the post-processing. When the addition law of the signal is known to satisfy either power sum law (7) or voltage sum law (8), the appropriate formula is used. Otherwise,voltagesumlaw(8)isusedasitalwaysgives larger value than power sum law (7). Sp P = 10 log 10 N IBW Sp P = 20 log 10 N IBW N n=1 N n=1 10 P(n) 10, (7) 10 P(n) 20, (8) where P dbm is the peak PSD in the span, SpMHz is the frequency span, N is the number of measurement bins in

6 TAKADA et al.: MEASUREMENT TECHNIQUES OF EMISSIONS FROM ULTRA WIDEBAND DEVICES 2257 Table 2 Comparison of two measurement methods for peak PSD. Sect Sect RBW* 1MHz 1MHz VBW** 3MHz 1 MHz (3 MHz recommended) Frequency span 50 MHz not specified Detector peak peak Sweep time 0.1msperbin not specified Peak PSD power sum or voltage sum scaling; rule not specified IBW*** corrected no consideration Note RBW*: Resolution bandwidth VBW**: Video bandwidth, see Appendix C IBW***: Impulse bandwidth, see Appendix D the span, P(n) dbm is the power value of the n-th bin measured by the spactrum analyzer, and IBW MHz is the impulse bandwidth of the spectrum analyzer. The definition and the meaning of the impulse bandwidth is described in Appendix D. The second method has not yet been clearly defined. Instead of measuring the spectrum within 50 MHz reference bandwidth, the maximum peak PSD for 1 MHz RBW is scaled to 50 MHz, but the conversion law is not specified. 7.6 Conducted Measurement The radiation measurement takes a long time to calibrate the measurement environment, as well as to measure it under the condition where directivity and gain of antenna are affected by the chassis, cables, etc. Contrary, the conducted measurement is effective to measure TRP in a short time to know whether the highest emission level exceeds the emission limit. In addition, the measurement sensitivity may not be the major problem in the conducted measurements since the power is fed dorectly from DUT to the spectrum analyzer. EIRP can be obtained as the product between TRP and maximum antenna gain. The conducted measurement is possible only when the antenna terminal is available on the transmitter (DUT). The spectrum analyzer is directly connected to DUT via the coaxial cable to measure the spectral characteristics of TRP. The antenna gain shall be separately measured by varying the direction to find the maximum gain. 8. Time Domain Measurements Chapter 7 of PDNR ITU-R SM.[UWB.MES] [1] describes the time domain measurements and their limitations. 8.1 Desirable Specifications for an Oscilloscope There are two categories of oscilloscopes. One is a singleevent oscilloscope to acquire the waveform by the realtime sampling. The other is a sampling oscilloscope to measure only the periodic waveform by shifting the sample timing. Sampling oscilloscopes exceeds the single-event oscilloscopes in the performance. However, it is necessary to stop time gating of a UWB signal waveform to generate a Table 3 Specifications of state-of-the-art single-event and sampling oscilloscopes. Single-event Sampling Maximum frequency 12 GHz 50 GHz Quantization bits 8 bits 14 bits periodic waveform. The relationship between the quantization bits n and the dynamic range D db is expressed as D = 20 log 10 2 n. (9) For example, to measure an UWB signal with D = 60 db, at least 10bits quantization is required for the A/D converter of the oscilloscope. It is simultaneously required for the analog frontend of the oscilloscope to have a noise floor below the minimum descritization level. Simulation studies have been performed for the measurement error of the noise levels or spurious level, and the results are shown in Appendix E. For comparison, specifications of state-of-the-art single-end and sampling oscilloscopes are listed in Table 3. Comparing the results of Appendix E and Table 3, it is insufficient to use the oscilloscopes for the purpose to measure low emission levels. Therefore, the time domain measurements by using oscilloscopes are only adequate to measure the highest emission limit. As is defined in Sect. 7.5, the reference bandwidth of the peak power is f b =50 MHz, but most of the spectrum analyzers are not equipped with the IF filter with RBW of 50 MHz. Use of an oscilloscope may be one of the effective methods to achieve the wider RBW by the post processing of the measured time domain data. Instead of the IF filter, a digital filter with 50 MHz RBW can be implemented offline. The filter can be implemented either in frequency domain or in time domain. Since the Gaussian filter is usually used as the IF filter of the spectrum analyzer, the Gaussian digital filter is considered in the time domain processing. The frequency transfer function of the Gaussian filter G( f ) with 3 db bandwidth f b is given as ( ) 2 G( f ) = 2 2 f fc f b, (10) where f c is the center frequency of the filter. To find the peak PSD, the measurement time window shall be T w = 0.1 ms. The number of samples necessary for

7 2258 the data processing N s is simply obtained by N s = T w f s, (11) and N s = when the sampling frequency is f s = 50 GHz. For the sampling oscilloscope, this value is the upper bound because T W is replaced by the period of the signal T p when T p < T w Measurement Error due to Jitter Due to the phase noise of the sampling clock, the sample timing of the sampling oscilloscope has some random error called jitter. It is assumed that the timing error of the sampling τ e is according to PDF h(τ e ). When a periodic waveform g(t) is measured by the sampling oscilloscope with the sample timing jitter expressed as h(τ e ), and is averaged by the intrinsic average function of the sampling oscilloscope, the measured waveform s (t) is s (t) = s(t τ)h(τ)dτ. (12) Equation (12) shows that the effect of the jitter is expressed as the convolution of the periodic waveform and the jitter PDF. In the frequency domain, Eq. (12) is rewritten as S ( f ) = S ( f )H( f ), (13) where S ( f ), S ( f ), and H( f ) are the Fourier transform of s(t), s (t), and h(t), respectively. The characteristic function of the jitter behaves like a bandpass filter. For example, when h(τ) is Gaussian distributed with the standard deviation σ,i.e. ) 1 h(τ) = exp ( τ2, (14) 2πσ 2σ 2 its Fourier transform is H( f ) = exp ( 2π 2 σ 2 f 2), (15) and 3 db bandwidth is about 0.22/σ. Considering the analogy to video filter in Appendix C, the acceptable deviation of the jitter for 50 MHz peak measurement is 1.5ns. IEICE TRANS. FUNDAMENTALS, VOL.E88 A, NO.9 SEPTEMBER 2005 F c ( f ) = E( f ) V 0 ( f ), (16) where E( f )andv 0 ( f ) are the complex electric field strength at a specific at a specific reference point of an antenna element and the complex matched voltage of the antenna terminal for the load of the matched impedance Z 0 = 50 Ω. Note that the conventional antenna factor can not be used to perform the reconstruction of electric-field waveform, as the group delay characteristics are not known. Contrary, CAF has the phase information and the effect of the group delay is considered. CAF must be measured for each individual antenna Reconstruction of Electric Field from Measured Time Domain Data Figure 7 shows an example of an apparatus for measuring the electric field waveform radiated by DUT. The waveform observed with an oscilloscope v m (t) is obtained as the convolution of the impulse response of the measuring apparatus from the antenna output to the output of the oscilloscope with the antenna output signal v a (t). EIRP can be converted from the electric field intensity by considering distance assuming the far field condition. Figure 8 shows the equivalent circuit of the waveform measuring apparatus shown in Fig. 7. In the figure, S denotes the S-matrix of a pre-amplifier and cables, and S S is the S-matrix of an oscilloscope. Γ a and Γ s denote the reflection coefficients of a receiving antenna and of the input port of the oscilloscope, respectively. Now, S 12 of the pre-amplifier, i.e. a transmitting S-parameter from the output port to the input port of the pre-amplifier, is assumed to be zero. S 22S and S 12S can also be assumed as zero because V m is not a real signal but a digitized numeric output by the oscilloscope. As a result of S-parameter analysis under the above-mentioned conditions, the electric field in the frequency domain E( f ) is expressed as E( f ) = (1 S 11Γ a )(1 S 22 S 11s ) F c ( f )F [v m (t)] S 21 S 21s 8.2 Post-Processing of Time Domain Data Complex Antenna Factor When an antenna receives a plane wave of frequency f,as shown in Fig. 6, a complex antenna factor (CAF) F c ( f )is defined as [9] Fig. 7 Electric field waveform measuring apparatus. Fig. 6 Definition of CAF. Fig. 8 Equivalent circuit of the waveform measuring apparatus.

8 TAKADA et al.: MEASUREMENT TECHNIQUES OF EMISSIONS FROM ULTRA WIDEBAND DEVICES 2259 = F c( f ) K F [v m(t)] (17) where F denotes the Fourier transform, and is implemented as DFT or FFT. K is the calibration coefficient of the system, and it is unity in the ideal case. The output waveform of the Gaussian filter (Eq. (10)) e f (t) is expressed as [ ] e f (t) = F 1 Fc ( f )G( f ) F [v m (t)]. (18) K Finally, EIRP p(t) is obtained from the electric field as p(t) = 4πr2 e 2 f (t) ( e f (t)r ) 2 =, (19) Z f 30 where Z f = 120 πω is the characteristic impedance of free space, and r is the distance between DUT and the receiving antenna. The peak PSD is the maximum value of p(t). Different from the frequency domain measurement Sect. 7.5, this time domain measurement gives the true value of the peak PSD. More detailed discussion and a measurement example are available on Ref. [10] Use of Spectrum Analyzer as Frequency Converter for Time Domain Measurement It is very expensive to execute the baseband sampling of the UWB signal by using the high performance oscilloscope. In fact, the full band measurement is not necessary to obtain the peak PSD. Therefore, a wideband frequency downconverter can help mitigation of the required specification of the oscilloscope. In Fig. 9, a spectrum analyzer is used as a frequency downconverter, and the oscilloscope samples the IF output of the spectrum analyzer. The first IF in Fig. 2 can be output from the spectrum analyzer. The center frequency of the first IF is relatively high, ranging from 60 to 500 MHz. Thus, the bandwidth of the signal is considerably wider than the reference bandwidth of 50 MHz for peak PSD. It is a practical idea to use this IF signal to measure in the bandwidth wider than RBW of spectrum analyzers. This first IF signal is measured by an oscilloscope. Since it samples the IF signal and not RF signal, sampling rate of oscilloscope can be much lower. The post processing is very similar to Sect to get the peak PSD. By changing the RF center frequency of the spectrum analyzer, the whole bandwidth can be covered by the appratus. The following requirements shall be fulfilled by the apparatus shown in Fig. 9: Fig. 9 Time domain measurement apparatus using a spectrum analyzer as a frequency down converter. The passband amplitude response of the spectrum analyzer should be flat. The passband phase response should be linear. The conversion gain from the RF input level to the IF output level shall be obtained by the calibration for each of the center frequencies. Input bandwidth of the oscilloscope shall be sufficiently high to sample the IF signal. 9. Conclusion This paper has described the measurement techniques of the UWB emission, based on the discussion in ITU-R TG 1/8. In addition, the paper has provided the background idea behind the measurement methods, which will not be presented on the ITU-R Recommendation. Since the final draft has not been approved, some change may occur from this paper. If there are major changes in the PDNR, the authors intend to submit a supplement of this paper. Acknowledgements The authors have worked together in the microwave masurement working group, National Institute of Information and Communications Technology (NICT) UWB Consortium. They have also contributed to the ITU-R TG 1/8WG4. They submitted 21 contribution documents and dispatched several members to every meeting. Mr. T. Yasui has been the chairman of WG4. The authors thank all the members in NICT UWB Institute and UWB Consortium for their helpful discussions, suggestions, and encouragement. Data presented in Appendix E have been provided by Dr. Hitoshi Sekiya of Anritsu Corp. References [1] Japan, Proposed modification to the PDNR SM.[UWB.MES], ITU-R, SG 1, TG 1/8, Contribution Document 1-8/284-E, May [2] Working document toward a preliminary draft new recommendation ITU-R SM.[UWB.CHAR] Characteristics of ultra-wideband technology, ITU-R, SG 1, TG 1/8, Contribution Document 1-8/256- E, Annex 1, Dec [3] Radio frequency devices, Part 15, Federal Communications Commission Rules, Dec [4] J.E. Hansen, Spherical Near-Field Antenna Measurements, Peter Peregrinus, London, [5] Specification for radio disturbance and immunity measuring apparatus and methods Part 1-1: Radio disturbance and immunity measuring apparatus Measuring apparatus, CISPR , IEC, Nov [6] M. Uchino, Radiometric measurement of equivalent isotropic radiated power of UWB devices, IEICE Trans. Commun. (Japanese Edition), vol.j87-b, no.6, pp , June [7] Specification for radio disturbance and immunity measuring apparatus and methods Part 1-4: Radio disturbance and immunity measuring apparatus Ancillary equipment Radiated disturbances, CISPR , IEC, May 2004.

9 2260 IEICE TRANS. FUNDAMENTALS, VOL.E88 A, NO.9 SEPTEMBER 2005 [8] H. Sugama and Y. Yamanaka, EMI measuring equipment above 1 GHz Characteristic evaluation of spectrum analyzers, IEICE Technical Report, EMCJ99-86, [9] S. Ishigami, H. Iida, and T. Iwasaki, Measurements of complex antenna factor by the near-field 3-antenna method, IEEE Trans. Electromagn. Compat., vol.38, no.3, pp , July [10] S. Ishigami and Y. Yamanaka, Reconstruction of electric-field waveform radiated from UWB device by using the complex antenna factor, Proc. International Symposium on Electromagnetic Compatibility (EMC SENDAI 04), pp.41 44, June Appendix A: Noise Level of Spectrum Analyzer Appendix 3 of PDNR ITU-R SM.[UWB.MES] [1] describes conversion from the noise level of the spectrum analyzer to EIRP. This section describes an alternative way to derive the noise EIRP. The noise power of the receiver N dbw is expressed as N = 10 log 10 kt B + F, (A 1) where k = J/K is Boltzmann s constant, T K is the temperature of the receiver, B Hz is the receiver noise bandwidth, and F db is the noise figure of the receiver. For a receiver with 1 MHz bandwidth, i.e. specified RBW, the noise power is expressed as N [dbm/mhz] = F, (A 2) where T = 290 K is assumed. Note that typical noise figure values of spectrum analyzers are in the range of db. Received power P r dbm is expressed by Friis transmission formula as P r = P te + 20 log 10 λ 20 log 10 (4πd) + G r, (A 3) where P te dbm is EIRP of the DUT, λ m is the wavelength, d m is the distance between DUT and the receiver antenna, and G r db is the receiver antenna gain. Typical gain values of the receiver antennas are in the range of 2 to 4 dbi including the cable loss. Therefore, the typical noise power is equivalent to the EIRP of 58 to 50 dbm/mhz. If 10 db SNR is necessary for the measurement, the minimum EIRP to be measured is in the range of 48 to 40 dbm/mhz. Therefore, the use of LNA is necessary to measure the low level EIRP. Appendix B: Uncertainty Budget of Measurement for Spectrum Analyzers Table A 1 shows the estimate of the measurement uncertainty in the general purpose and the high-performance spectrum analyzers that can measure up to 22 GHz. The total uncertainty is obtained as the square root of the squared sums of the error factors. Figure A 1 shows a level calibration system for spectrum analyzers using a 4-port coaxial transfer switch, a signal generator, an RF power meter, and a spectrum analyzer. In the calibration mode Fig. A 1(a), the coaxialtransfer switch links the reference signal generator to the spectrum analyzer. A CW reference signal with 0 dbm power is Table A 1 Uncertainty budget of spectral measurement below 22 GHz without calibration. General purpose High performance Calibration source 1 0dB 0dB Frequency response 2 ±5dB ±2.5dB Reference level ±0.4dB ±0.2dB Log linearity ±1.0dB ±0.5dB RBW switching ±0.3dB ±0.2dB Total uncertainty ±5.5dB ±3.0dB 1 : The uncertainty of the calibration source is included in the uncertainty of reference level. 2 : The frequency response is assumed to have a uniform distribution. (a) Calibration mode (b) Measurement mode Fig. A 1 Calibration and measurement setup using 4-port coaxial transfer switch. fed from the signal generator to the spectrum analyzer, and the indicated value P SA is recorded. Next, the coaxial transfer switch is turned into the measurement mode to link the signal generator to the RF power meter and to link the input port to the spectrum analyzer. Then the spectrum analyzer measures UWB signal under test, and the indicated value is P M. Simultaneously the RF power meter measures the RF power of the unmodulated carrier fed from the signal generator, and the indicated value P PM is recorded. The calibrated power value P DUT is then given as P DUT = P MP PM. (A 4) P SA Uncertainty budgets after the above calibration are summarized in Table A 2. Expanded uncertainty after the calibration that is defined as 95% confidence interval is 0.49 db below 22 GHz. Appendix C: Choice of Video Filter As shown in Fig. 2, a super heterodyne spectrum analyzer has two filters, i.e. an IF filter and a video filter. Two

10 TAKADA et al.: MEASUREMENT TECHNIQUES OF EMISSIONS FROM ULTRA WIDEBAND DEVICES 2261 Table A 2 Uncertainty budget of spectral measurement below 22 GHz after calibration. Sources of uncertainty X i Estimate Uncertainty U(X i ) PDF Standard uncertainty u i (y) Lsm mw 0.02% Gaussian 0.05% Pm mw 0.02% Gaussian 0.05% Pmc mw 0.29% rectangular 0.29% Pcal mw 0.69% rectangular 0.69% Kb % rectangular 1.45% Mu % U-shaped 4.07% Muc % U-shaped 0.45% Msp % U-shaped 4.07% Lsw % rectangular 0.40% Combined standard uncertainty u c (y) 6.01% Expanded uncertainty U = k u c (y), k = % Expanded uncertainty in db 0.49 db Sources of uncertainty Lsm : Indicated value on the spectrum analyzer Pm : Indicated value on the RF power meter Pmc : Indicated value on the RF power meter connected to the reference signal generator Pcal : Output power of the reference signal generator Kb : Sensor calibration factor of the RF power meter Mu : Mismatch loss between the input port and the RF power meter Muc : Mismatch loss between the reference signal generator and the RF power meter Msp : Mismatch loss between the input port and the spectrum analyzer Lsw : Insertion loss of the 4-port coaxial transfer switch Fig. A 2 Frequency response of IF Filters (RBW = 1MHz). measurement parameters are related to these filters, i.e., the bandwidth of the IF filter is RBW, and that of the video filter is VBW. Figure A 2 shows the relation between VBW and the peak power recorded by the spectrum analyzer when RBW is fixed to 100 khz. The figure suggests that the peak power measurements should be made with VBW at least 3 times of RBW. Otherwise, the video filter attenuates the spectrum component at the band edge, which results in the decrease of the measured value. Care must be taken in the measurement, as the default choice of the coupled VBW in a conventional spectrum analyzer usually the same value as RBW. Appendix D: Impulse Bandwidth Spectrum analyzers are usually calibrated for sinusoidal inputs to yield accurate results. However, they are not guaranteed to yield reproducible results for other type of inputs such as an impulse waveform generated by an UWB device. It is necessary to know the response to an impulse waveform. The impulse bandwidth (IBW) represents this response [5]. Fig. A 3 Experimental setup for measurements of impulse bandwidth. IBW is not usually indicated as a specification of a spectrum analyzer. Therefore, this value is necessary to be measured. Figure A 3 shows the measured frequency response curves of the IF filters in the spectrum analyzers adjusted for a nominal RBW of 1 MHz. The horizontal axis denotes the frequency normalized by the RBW, and the vertical axis does the attenuation of the IF filter. It is shown that 6dB or 20 db bandwidth is different in each model even though nominal 3 db bandwidth is 1 MHz in all the analyzers. IBW is defined as B imp = A(t) max, (A 5) 2G 0 IS where A(t) max is the peak value of the envelope at the IF output of the analyzer with an impulse area IS = V(t)dt (A 6) applied at the analyzer input, and G 0 is the gain of the circuit

11 2262 IEICE TRANS. FUNDAMENTALS, VOL.E88 A, NO.9 SEPTEMBER 2005 Fig. A 4 Peak power vs. VBW for 100 khz RBW. at the center frequency. The experimental setup for measurements of IBW is shown in Fig. A 4 [8]. VBW should be set at least three times larger than RBW. The waveform of the rectangular pulse with the repetition frequency f p, the pulse width t,and the pulse voltage V p is measured by an oscilloscope, and the impulse area IS is calculated from Eq. (A 6). There are two alternative methods to measure IBW. 1. The peak value of the envelope A(t) max is obtained from the RMS average value A(t) rms as (a) Measurement error of noise power. A(t) max = 2A(t) rms. (A 7) A(t) rms is measured at a frequency very close to the peak of the spectrum of the pulse. 2. When PRF is lower than the impulse bandwidth B imp,a peak-detection spectrum-analyzer indicates a constant peak value. However, when PRF is higher than the impulse bandwidth B imp, a peak value becomes equal to average value, and is proportional to PRF. The PRF of the intersection of these two responses is the impulse bandwidth. Both of the measurement methods almost show the same measured values. Note that B imp /B 3 is about 1.5inacaseof a Gaussian type BPF. Appendix E: Limitation of the Time Domain Measurements by Using Oscilloscope Simulation studies have been performed for the measurement error of the noise level or spurious level. Either band-limited noise within 960 to 1610 MHz or 1200 MHz spurious signal is added to the impulse UWB signal which has a spectrum of χ 2 function. In this study, the peak to average power ratio (PAPR) of a UWB signal is assumed to be 20 db. Figure A 5 show the noise and the spurious levels vs. the number of quantization bits of an oscilloscope. In Fig. A 5(a), noise PSD within 960 to 1610 MHz is varied from 50 dbm/mhz to 90 dbm/mhz. In Fig. A 5(b), power of 1200 MHz line spectrum is varied from 50 dbm to 90 dbm. For example, the emission limit between 960 and 1610 MHz is 75.3dBm/MHz in the FCC mask (Fig. 1). Then, 12 bits quantization is necessary to obtain the results Fig. A 5 domain. (b) Measurement error of spurious power. Measurement error of noise and spurious measurements in time within 1 db error for the noise measurements, while 17 bits quantization is necessary for the spurious measurements. Jun-ichi Takada received B.E., M.E., and D.E. degrees from Tokyo Institute of Technology (Tokyo Tech), Japan, in 1987, 1989, and 1992, respectively. From 1992 to 1994, he has been a Research Associate at Chiba University, Chiba, Japan. Since 1994, he has been an Associate Professor at Tokyo Tech. His current research interests are wireless propagation and channel modeling, ultra-wideband radio, and applied radio instrumentation and measurements. Since 2003, he has been a part time Specialty Researcher in the UWB Institute, National Institute of Communications and Information Technology (NICT), Japan, where he contributes to the propagation measurement and modeling, as well as the standardization of the UWB emission measurement in ITU-R. He is a member of IEEE, ACES, and ECTI Association Thailand.

12 TAKADA et al.: MEASUREMENT TECHNIQUES OF EMISSIONS FROM ULTRA WIDEBAND DEVICES 2263 Shinobu Ishigami received the B.E., M.E., and D.E. degrees from University of Electro- Communications (UEC), Tokyo, Japan, in 1990, 1992, and 1997, respectively. From 1992 to 1999, he was a research associate in UEC. He joined the National Institute of Information, Communications Technology (NICT, former Communications Research Laboratory), in He is now a senior researcher of Communication system EMC group in NICT. He is a member of the IEEE, the IEEJ. Juichi Nakada received the B.S. degrees in Physics from Toyama University in He has been with ADVANTEST CORPORATION, Tokyo, Japan. His interests include wireless device evaluation and test and measurement instruments for next generation wireless communication systems. Tetsuya Yasui received the M.E. degree in electric engineering from Waseda University, Tokyo, Japan, in He joined the Ministry of Posts and Telecommunications (MPT, presently MIC), Japan in From 1986 to 2002 he engaged in the approval of mobile communication business, the frequency management, ITU work, the planning of space satellite R & D, the planning of national ICT R & D and the licensing of wireless access systems. Since 2002 he has been with National Institute of Information and Communications Technology, as Research Center Supervisor engaged in international collaboration and standardization for new generation mobile communication project and UWB technology. Since 2003 he has been appointed as the chairman of WG4 (measurement method) of TG1/8 (UWB) in ITU-R. Eishin Nakagawa received his B.E. degree from Chubu Institute of Technology in He joined NEC Radio & Electronics Ltd. in 1977 and engaged in development of radio communication equipment. He moved to Telecom Engineering Center in 2001 and has been engaged in development of the measuring methods required for the technical regulations conformity certification of radio equipment. measurement. Masaharu Uchino received his B.E. and M.S. degrees in electrical engineering from Tokyo Denki University in 1978 and 1980 respectively. He has been working for Anritsu Corporation since From 1996 to 1998, he worked at the Electromagnetic Compatibility Research Laboratories Co., Ltd., Sendai. He received D. Eng. degree in electromagnetic compatibility from Tohoku Gakuin University, Sendai, in His current interests are in the area of precise frequency control and ultra-wideband