F Planar Image Separating D Balanced Sis Mixers

res of SIS receivers now in the range 2-4 times the ), the overall sensitivity of radio astronomy sly degraded by atmospheric noise, and sometimes by cillator source. In spectral line measurements, wanted (image) sideband can be eliminated by using an o reduce local oscillator noise, balanced mixers can ealize image separating and balanced mixers using RF circuits, but they are difficult to fabricate and ow practical to include the necessary signal and LO and cold loads with the SIS mixer on the same quartz mage separating or balanced mixer can be fabricated S mixer fabrication process with one or two additional n of single-chip balanced and image separating mixers uits are designed using a modified form of coplanar a convenient range of characteristic impedances while acent circuit elements. It is hoped ultimately to ing and balanced designs to make a balanced image le chip. The Virtues of Image Separating Mixers While most mixer receivers respond to both upper and lower sidebands, the majority of applications require only a single sideband response. Signals and noise received in the unwanted (image) sideband degrade the overall system sensitivity. At the NRAO 12-m telescope at Kitt Peak in Arizona, the antenna temperature at the zenith is typically 60 K at 230 GHz. In spectral line measurements with a double-sideband SIS receiver, the image noise contributes ~30% of the overall system noise, thereby doubling the integration time required to attain a given sensitivity. There are three ways to eliminate the image response of a broadband mixer receiver: (i) A filter can be inserted in front of the mixer, which terminates the mixer reactively at the image frequency. This is difficult in widely tunable receivers. (ii) A tunable four-port diplexer with a cold image termination can be used. This can be done quasioptically, e.g., using a Martin-Puplett interferometer as a sideband diplexer, but has a limited IF fractional bandwith and is cumbersome at millimeter wavelengths. (iii) A phasing type of image separation mixer can be used, as will be discussed below. Types of Image Separating Mixer At microwave frequencies, the usual realization of an image separating mixer, shown in Fig. 1, uses a quadrature hybrid to couple the LO to two identical (balanced) mixers with a B/2 phase difference. The signal power is divided equally between the mixers with zero phase difference, and the IF outputs of the two mixers are connected to an IF quadrature hybrid. The down-converted upper and lower sideband signals appear seperately at the two output ports of the IF hybrid. The in-phase and B/2 couplers in the signal and LO paths can be interchanged without losing image separation. Fig. 1. A common configuration for an image separating mixer, consisting of LO and IF quadrature hybrids, and an RF in-phase power splitter. A 100 GHz image separating mixer, using a waveguide magic-T and an adjustable phase shifter in the LO path to one mixer, has been described in [1], see Fig. 2. At shorter wavelengths, the signal and LO phasing can be done quasi-optically, as described in [1] and [2]. A quasi-optical image separating scheme is shown in Fig. 3, in which a crossed-grid power splitter [3] acts as an in-phase beamsplitter for the input signal, and splits the circularly polarized LO beam into two linearly polarized beams with B/2 phase difference. Inclined-grid couplers couple typically 1% of the LO power into each mixer, with 99% of the signal. Even at 250 GHz, such a quasi-optical scheme is physically cumbersome, and requires a large cryostat if several receivers are to be attached to the same refrigerator. Fig. 2. The 100 GHz image separating Fig.3 A possible quasi-optical image separating mixer using mixer of [1] based on a waveguide a 45E signal polarization selector, a crossed-grid signal and magicST. LO splitter, and inclined-grid LO couplers. It is important to note that, in all image separating mixers, noise from the termination on the fourth port of the signal input coupler is down-converted and appears at the IF output ports. In the present work, a scheme similar to that of Fig. 1 is used, but with the signal and LO ports interchanged, so the signal enters through a quadrature hybrid, and the LO through an in-phase power splitter. The RF quadrature hybrid, LO power divider, LO couplers, and SIS mixers are all fabricated on the same quartz substrate. Choice of Transmission Line Medium To avoid the need for very thin quartz substrates, the circuit is designed with thin-film ground plane, dielectric layer, and wiring-layer conductors all on the same side of a thick quartz substrate. The dimensions of coplanar transmission lines are kept substantially smaller than the substrate thickness to prevent the fields penetrating appreciably through the substrate. The extermal RF source and IF load impedances are near 50 ohms. The characteristic impedances required in the RF quadrature hybrid and the matching circuit of the SIS mixer range from 3 to 116 ohms. The lower values are readily obtained with superconducting microstrip lines, while coplanar waveguide (CPW) can be used for the higher impedances. In the range from about 10 to 60 ohms, microstrip lines with thin-film dielectrics are too narrow to use, while CPW requires very narrow gaps between center conductor and ground plane. We therefore lower the characteristic impedance of CPW by using periodic capacitive loading. A capacitively loaded coplanar waveguide (CLCPW) can be regarded as a standard CPW with periodic capacitors to ground. The equivalent circuit of a section of CLCPW is shown in Fig. 4; with 570 nm SiO, the characteristic impedance of this CLCPW is 63 ohms. For the CLCPW's used in this work, simulation using Sonnet em [4] indicates that the inductors L and L can be ignored if the reference plane 1 2 is chosen at the center of the bridge, as shown. Additional important advantages of CLCPW over standard CPW are that the periodic capacitors act as ground bridges, and: (i) greatly reduce coupling between adjacent components, and (ii), prevent odd-mode gap resonances in long CPW lines. Fig. 4. A length of capacitively loaded coplanar waveguide (CLCPW), and equivalent circuit. Dimensions are in microns. An Integrated Image Separating SIS Mixer for 200-300 GHz The mixer is on a 2 x 1 mm quartz substrate, mounted in a block with separate waveguide inputs for the signal and LO, as shown in Fig. 5. Coupling from the waveguide to the mixer substrate is by broadband probes and suspended-stripline on smaller quartz substrates. Connections between the probes and the main substrate are by thin Au ribbon. IF and bias connections are by short wire bonds. Fig. 5. The image separating mixer, showing the signal and LO waveguides, suspended stripline coupling probes, and the main substrate. Fig. 6. Main substrate of the image separating mixer, showing the main components. An enlarged view of the substrate is shown in Fig. 6. The main components are: (i) a 3 dB quadrature hybrid at the signal input, (ii) a 17 dB LO injection coupler in front of each mixer, (iii) an in-phase power splitter in the LO path, and (iv) two SIS mixers. Noise from the resistive termination on the fourth port of the input hybrid is downconverted to appear at the IF output ports of the mixer. Amplitude & Phase Requirements The image rejection obtainable in the image separating mixer depends on the amplitude and phase balance of the two quadrature hybrids and the mixers. The signal flow through the circuit is depicted in Fig. 7. The quantities c , c , 1 2 and t , t , are the coupled port and through port scattering parameters (s and 1 2 21 s ) of the input hybrid (c , t ) and IF output hybrid (c , t ). Using the 31 1 1 2 2 notation in the figure, the amplitudes at IF ports A and B are: Fig. 7. Signal flow through the image separating mixer. For simplicity, the mixers are assumed to have unit conversion gain. V and V are the complex amplitudes of the incident USB and LSB signals. U L VA ' VU t1t2 c2 t2 % c1 t1 % VL t ( 1 t2 c2 t2 % c ( 1 t ( 1 VB ' VU t1t2 1 % c1 t1 c2 t2 % VL t ( 1 t2 1 % c ( 1 t ( 1 c2 t2 c2 t2 % c ( 1 t ( 1 c2 t2 % c1 t1 1 % c1 t1 c2 t2 1 % c ( 1 t ( 1 c2 t2 t1 ' t2 ' 1 %2 c1 ' c2 ' 1 %2 e &j B