RF power measurement in. three-mixer method

Transcription

1 RF power measurement in D-band using downconverter calibrated by three-mixer method Katsumi Fujii a), Toshihide Tosaka, Kaori Fukunaga, and Yasushi Matsumoto National Institute of Information and Communications Technology Nukui-Kitamachi, Koganei-shi, Tokyo , Japan a) katsumi@nict.go.jp Abstract: To establish an RF power standard above 110 GHz, we have developed a measurement system using a down-conversion mixer with a spectrum analyzer, and the conversion loss of the mixer was determined by the three-mixer method. This method measures the three transmission S-parameters S ) of three mixer pairs using a vector network analyzer. The experimental results proved that the three-mixer method can determine the conversion loss of mixers with an uncertainty of 0.5 db level of confidence: 95%) in the entire range of the D-band. Using this uncertainty and the uncertainty of the spectrum analyzer, the proposed system can measure the RF power in the D-band with an uncertainty of less than 2.0 db. Keywords: D-band, reciprocal mixer, RF power standard, conversion loss Classification: Microwave and millimeter wave devices, circuits, and systems References [1] Hewlett Packard, Improving Network Analyzer Measurements of Frequency-Translating Devices, Application Note , [2] T. Tosaka, K. Fujii, K. Fukunaga, and Y. Matsumoto, Measurement of frequency conversion losses with 3-mixer method for traceable mmwave power measurement method in D-band, IRWWM-THz, Sept Accepted). 1 Introduction With the progress of wideband and high-speed wireless communication systems such as a real-time transmission system for HDTV high definition television), the demand for the establishment of an RF power standard for millimeter waves, i.e., electromagnetic waves above 110 GHz, has been rapidly 1096

2 growing. The present de jure international RF power standard, however, has only been established up to 110 GHz, and there is no calibration service for higher frequencies. To promote the use of millimeter wave wireless communication, we have developed an RF power measurement system for calibration in the D-band whose frequency range is from 110 GHz to 170 GHz. The system uses a down-conversion mixer with a spectrum analyzer to measure the IF signal power. When the conversion loss of the mixer is accurately calibrated by the three-mixer method, values obtained by the spectrum analyzer can be corrected. The effectiveness of this method was verified by comparing the conversion loss calibrated by the three-mixer method with the loss determined by the well-established substitution method using a conventional RF power meter that is traceable to the national standard at 110 GHz. Then, the conversion losses up to a frequency of 170 GHz, were calibrated. Finally, we estimated the uncertainty of the newly developed measurement system. 2 Power-measuring system A down-conversion method using an external mixer with a spectrum analyzer, which measures the IF power emitted from the mixer, has been commonly used to measure the RF power. The input RF power in the D-band, P in,can be obtained using the following equation: P in [dbm] = P meas. [dbm] + C [db], 1) where P meas. is the power indicated by the spectrum analyzer and C is the conversion loss of the down-conversion mixer. Since a de jure RF power standard above 110 GHz has not been established, the RF power in the D-band is measured by the down-conversion method. In this method, the accuracy of calibration of the conversion loss of the mixer determines the total uncertainty. 3 Three-mixer method The conversion loss of mixers can be determined by the three-mixer method [1, 2]. Figure 1 a) shows the measurement setup used to determine the conversion loss. When two of the three mixers are connected in series with a band-pass filter for a target frequency in the D-band, the transmission S- parameter S ) is measured using a network analyzer. Assuming that mixer X is a reciprocal mixer, the three different combinations of the three mixers give three equations that can be used to calculate the conversion loss of each mixer in terms of S in db as follows see Appendix A): C X [db] = 1 2 C Y [db] = 1 2 C Z [db] = 1 2 S YX) + S XZ) S YZ) S F ) ) S YX) S XZ) + S YZ) S F ) ) S YX) + S XZ) + S YZ) S F ) ) 2) 3) 4) 1097

3 Fig. 1. Three-mixer method and internal structure of mixers under calibration. where S YX), S YZ) and S XZ) are the transmission S-parameters in db for the, of the filter, three mixer pairs including the transmission S-parameter, S F) which is measured previously. Subscripts 1 and 2 indicate port numbers of the network analyzer used. Superscripts YX), YZ) and XZ) indicate the mixer pairs. For example, in the case of the pair YX), mixer X is applied to an up-converter and mixer Y is applied to a down-converter. To yield the Eqs. 2) 4), mixer X must be a reciprocal mixer, which means the conversion loss as the down-converter is equal to that as the up-converter. 4 Conversion loss measurements The conversion losses of the three mixers were calibrated by the three-mixer method using a conventional vector network analyzer that is traceable to the national standard with PC-3.5 coaxial connectors. To suppress unwanted signals such as the leakage of the LO signal and spurious signals, the bandpass filter for the measuring frequency is inserted between the RF ports of 1098

4 two mixers as shown in Fig. 1 a). Figure 1 c) shows the internal structure of the mixers. The main part of each mixer is a single balanced mixer that is driven as a subharmonic mixer. A diplexer with an amplifier was connected to the LO port. The IF frequency was fixed to 4 GHz. The port has a band-pass filter to suppress unwanted signals. The input power was set to about 20 dbm to avoid the saturation of the main mixers. The 6 db fixed attenuator was connected to the RF port of the mixer to improve the reflection coefficient, S 11. The conversion losses were determined at 110 GHz, 120 GHz and 170 GHz as shown in Fig. 2. The red circles indicate the conversion loss of mixer Y which was only driven as a down-converter. The error bars indicate expanded uncertainties of 0.5 db level of confidence: 95%). The conversion loss at 110 GHz was compared with the value obtained by a conventional method that uses a conventional power meter with a waveguide taper transition from WR-10 W-band) to WR-06 D-band), for which the insertion loss was known. The green square in Fig. 2 shows the value obtained by the conventional method. The difference between the two methods was 0.3 db. This result proves that the three-mixer method is effective for practical use. In addition, another measurement, as shown in Fig. 1 b), was performed. The RF ports of two mixers were connected with a network analyzer with millimeter wave expanders for the D-band, and the IF ports were connected via a coaxial cable whose characteristics were known. The conversion loss obtained in this configuration is shown as a blue solid line in Fig. 2, and the blue dotted lines indicate the expanded uncertainties level of confidence: 95%). The deviations at 110 GHz, 120 GHz and 170 GHz are below 0.7 db, providing that the experimental results obtained by the different measurements are in good agreement. Fig. 2. Calibration results for the conversion loss of mixer Y. 1099

5 5 Uncertainty of RF power measurement The uncertainty of RF power measurement in the D-band is obtained by combining the uncertainty derived from the determined conversion loss determination and the uncertainty of the spectrum analyzer itself. Table I shows the budget of uncertainty for RF power measurement. The conversion loss is calibrated with an uncertainty of 0.5 db level of confidence: 95%). The accuracy of the spectrum analyzer according to the specification is 1.6 db worst case). The mismatch between the IF port of the mixer and the input port of the spectrum analyzer with the coaxial cable is less than 0.1 db measured). Thus, the maximum expanded uncertainty of RF power measurement is 2.0 db level of confidence: 95%). Table I. Uncertainty of RF power measurement in the D- band by the proposed system. 6 Conclusion To establish an RF power standard above 110 GHz, a measuring system has been developed that consists of a down-conversion mixer and a spectrum analyzer. The conversion loss of the mixer in the D-band is determined by the three-mixer method with an uncertainty of 0.5 db, and the total uncertainty of RF power measurement is estimated to be 2.0 db level of confidence: 95%) by adding the uncertainty of the spectrum analyzer. In the future work, we will evaluate the linearity of the mixer to improve the accuracy of the RF power measurement to realize a calibration service for frequencies above 110 GHz. Acknowledgments The authors would like to thank Dr. K. Shimaoka, Dr. M. Horibe, Mr. M. Kinoshita and Ms. R. Kishikawa of the National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology AIST/ NMIJ) for their helpful advice. 1100

6 A. S-parameter between two mixers The configuration and signal flow graph of the method used to measure the transmission S-parameter S ) between two mixers with a filter is shown in Fig. A.1. The measurement data of S YX) is expressed in terms of the S-parameter of each component as S YX) = S X) 12 SF) SY) ) 1 S X) ) 11 SF) 11 1 S Y) 11 SF) 22 S X) 11 SY) 11 SF) SF) 12 A.1) where the superscripts indicate mixer numbers. This equation is rewritten in db as follows: S YX) = S X) 12 + SF) + SY) M in db, A.2) where M is the mismatch factor between the two mixers and the filter and is expressed as, { M = 20 log 10 1 S X) ) 11 SF) 11 1 S Y) ) 11 SF) 22 S X) } 11 SY) 11 SF) SF) 12 in db. A.3) The value of M can be corrected by measuring the S-parameter of each component. However, in this paper, M is included in the budget of uncertainty of the mixer calibration because the value of M is smaller than any other factors of uncertainty. Fig. A.1. Signal flow graph of the three-mixer method. 1101