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Design of a Novel Balun for a Star Configuration Double-balanced Mixer at S-band
A double-balanced mixer (DBM) has some advantages over other types of mixers. Ring and star configurations are available for the construction of a DBM. Between the two, the star configuration is preferred for high frequency operation. The star configur...
The merits of a doubly balanced mixer (DBM) include high dynamic range, good port-to-port isolation and rejection of even-order spurious responses.1 The ring configuration, using two trifilar wound ferrite transformers, is most commonly used for DBMs at low frequencies. The application of a transformer DBM is, however, limited to low frequency because of the resonance caused by the winding capacitances and inductances. Baluns (that is parallel-line baluns), replacing the ferrite transformers, are used for DBMs at higher frequencies. However, the lack of a center tap in baluns requires IF bypass inductors and DC blocking capacitors to extract the IF energy. A star configuration DBM has an inherent DC coupling structure and may have an advantage, considering the complexity of the IF decoupling in the realization of the ring configuration.
The traditional star mixer is nonplanar and requires a suspended structure. Recent studies2,3,4 of star mixers have successfully overcome the nonplanar structure problem. However, they are implemented on coplanar waveguide (CPW), and four diodes cannot be connected to a common point, although some are implemented on microstrip. A narrow strip that encircles the structure center point is used to connect four discrete diodes. A surface-mount star quad-diode package is still difficult to place in these structures.
In this article, a novel planar dual balun is proposed and implemented in microstrip, which consists of two identical Marchand baluns using coupled lines. The two dual baluns provide an easy way of mounting a packaged star quad-diode in this star mixer design and the narrow strip ring can be removed which otherwise would degrade the mixer performance. The performance of the proposed star mixer is compared with that of a commercial transformer DBM. It is found that the performances are almost equal.
Figure 1 shows the dual balun. Port 1 is the unbalanced input port, and ports 2 and 3 and ports 4 and 5 are the two balanced output ports. Two identical Marchand baluns using coupled microstrip lines are connected to the input port 1. The electrical lengths of each coupled line are all a quarter wavelength at the 2 GHz center frequency. The unbalanced signal applied at port 1 is then split equally and comes out as two balanced signals between ports 2 and 3 and between ports 4 and 5. It should be noted that a simple quarter wavelength coupled line that behaves like a coplanar strip (CPS) line as the even-mode impedance approaches infinity does not work as a balun near the center frequency. Considering the existence of an even-mode voltage equal to the odd-mode voltage at point A, the even-mode voltage may form a standing wave peak at the coupled line ends. Due to the Marchand balun properties, the even-mode signals at the balanced ports are completely suppressed.
The circuit is designed using an FR4 (r = 4.6, h = 1.6 mm) substrate. For each Marchand balun, the balanced impedance between ports is set as 100 Ω. The impedance looking into each Marchand balun from point A should be 100 Ω for maximum power transfer. The even- and odd-mode impedances Zoe, Zoo of the Marchand balun are set as 133 and 39.8 Ω, respectively, for maximum power transfer. An infinite even-mode impedance is desirable, but impossible to achieve in practice. The even-mode impedance is set as high as possible for the given substrate characteristics to provide broad bandwidth. The ratio of Zoe/Zoo = 3.3 is higher than 3.0, the value recommended by S. Maas.1
Figure 2 shows the EM simulation results using Agilent ADS™ compared with the measured results for the dual Marchand balun. To obtain the amplitude and phase imbalances at the balanced ports, each transmission characteristic of the ports for the unbalanced input excitation is measured using a 8510C vector network analyzer (VNA) with the other ports terminated in 50 Ω loads. The amplitude and phase imbalances are then calculated using the measured data. The amplitude imbalance was within approximately 0.2 dB and the phase imbalance was within 180°±1.5° over the frequency band from 1.5 to 2.5 GHz.
Figure 3 shows the configuration of the double-balanced star mixer using the suggested balun and the photograph of the fabricated mixer on epoxy glass. The Agilent HSMS-286R device in a low cost surface-mount package is used as the star quad-diode. The PCB has two metallization layers and the bottom is used as the ground. The size of the substrate is 55 x 59 mm. The baluns were bent to minimize the length for the connection to the star quad-diode. Placing the two arms as far apart as possible (except in the connection region) minimizes the undesired coupling. Jumper wires are used for the crossing of the two dual baluns. The IF output is drawn from the center of the star diode package and the IF port is placed at right angle to the LO to minimize its coupling to the IF port.
Figure 4 shows the measured conversion loss versus LO power. The LO and RF frequencies are 2.0 and 1.8 GHz, respectively. The RF power is set as –30 dBm and the LO power is varied from –5 to 10 dBm. The conversion gain increases with LO power and becomes saturated at approximately 6 dB above an LO power greater than 6 dBm. Fixing the LO power at 6 dBm, the RF frequency was swept from 1.5 to 2.5 GHz. The worst conversion loss is about 7.1 dB and falls rapidly outside the frequency band of 1.5 to 2.5 GHz. Although the amplitude and the phase imbalances are small outside the frequency band, the input mismatch is believed to be the cause for the conversion loss drop.
Figure 5 shows the IF output spectrum for the RF two-tone input with power of –14 dBm and separated by 0.5 MHz. The center frequency of the RF two-tone is 1.8 GHz. The LO frequency and power are set as 2 GHz and 6 dBm, respectively. The measured third-order intermodulation distortion (IMD3) is –43.67 dBc. The output IP3 (third-order intercept) is obtained by extrapolating the measured data for the two-tone RF power swept from –50 to +2 dBm. The output IP3 results in approximately 5 dBm.
Table 1 shows a comparison of the performance of the proposed star mixer with that of a commercial transformer DBM (ADE-18W) from Mini-Circuits. The performances are almost equal, which show that the mixer successfully operates as a DBM. The operating bandwidth of the RF and LO are slightly narrower than that of the ADE-18W. The bandwidth of the mixer can be improved by broadband synthesis of the Marchand balun.5 It should be noted that the suggested mixer provides the possibility for higher frequency operation, but the transformer DBM does not.
In this article, a novel dual balun structure, suitable for a doubly balanced star mixer, is suggested. The measured phase imbalance was within 180°±1.5° and the amplitude imbalance was within ± 0.2 dB over a 1.5 to 2.5 GHz band. A star mixer at S-band is implemented using the suggested balun with a packaged star quad-diode on an FR4 substrate. The mixer was successfully operated and has a conversion loss of approximately 6 dB. Its performance is comparable to a commercial DBM with transformer and is believed applicable for higher frequency DBMs. The bandwidth of the suggested mixer could be broader through broadband synthesis of the Marchand balun. The mixer is also believed suitable for a MMIC with a backside via process.
This work was supported by KOSEF under the ERC Program through the MINT Research Center at Dongguk University.
- S.A. Maas, Microwave Mixers, Second Edition, Artech House Inc., Norwood, MA, 1993.
- S.A. Maas, “A Broadband Planar, Doubly Balanced Monolithic Ka-band Diode Mixer,” IEEE Transactions on Microwave Theory and Techniques, Vol. 41, No. 10, October 1993, pp. 2330–2335.
- Y.I. Ryu, K.W. Kobayashi and A.K. Oki, “A Monolithic Broadband Doubly Balanced EHF HBT Star Mixer with Novel Microstrip Baluns,” IEEE Microwave and Millimeter-wave Monolithic Circuit Symposium Digest, 1995, pp. 119–122.
- C.Y. Chang, “Ultra Broadband Doubly Balanced Star Mixers Using Planar Mouw’s Hybrid Junction,” IEEE Transactions on Microwave and Theory Techniques, Vol. 41, No. 6, June 2001, pp. 1077–1085.
- C.L. Goldsmith, et al., “Synthesis of Marchand Baluns Using Multiplayer Microstrip Structure,” International Journal of Microwave and Millimeter-wave Computer-aided Engineering, Vol. 2, No. 3, John Wiley & Sons Inc., Somerset, NJ 1992, pp. 179–188.
- G.D. Vendelin, A.M. Pavio and U.L. Rohde, Microwave Circuit Design Using Linear and Nonlinear Techniques, John Wiley & Sons Inc., Somerset, NJ 1990, pp. 562–570.
- R.B. Mouw, “Broadband Hybrid Junction and Application to the Star Mixer,” IEEE Transactions on Microwave Theory and Techniques, Vol. 16, No. 11, November 1968, pp. 154–161.