Mixer Block Design

Figure 2 shows a schematic side view of the designed mixer block. The first section is the horn block and consists of a diagonal feedhorn [2] that transitions from a full-height rectangular waveguide. The full-height rectangular waveguide is then transformed to a half-height waveguide through a three-section transformer. Diagonal feedhorns have been chosen over corrugated feeds, because of their relative ease of construction using split-block techniques. Although their Gaussian coupling efficiency is $\sim 13\%$ smaller than corrugated horns, diagonal horns have been shown to be a good candidate for use in submillimeter focal-plane arrays [3]. We follow the design outlined in [3] by making a direct transition from rectangular to diagonal feed. The half-opening angle of the feedhorn is $10.2^\circ$, with a slant length of 8.26 mm. The analytical designs of the full-height to half-height transformer and the rectangular to diagonal horn transition were verified using Ansoft's High Frequency Structure Simulator (HFSS) [4]. The half-height waveguide dimensions are 0.7by 0.175 mm.

 

Figure 2: Schematic side-view of the mixer block.

 

Figure 3: Layout of the 4 to 6 GHz IF matching network.

The second section is the junction block, which also houses the IF matching network. The fused quartz substrate carrying the SIS junction sits in a suspended microstrip line configuration, which is parallel to the E-field of the waveguide. Using the suspended strip configuration considerably eases tolerances in mounting the junction on the mixer block. The junction substrate is designed to be $0.309\times 0.078 \times 3.48$ mm. The dimensions of the suspended microstrip line channel are derived from the successful CfA designs [5, 6]. The channel has a $0.222\times 0.038$ mm airgap behind the junction substrate and a similar airgap above the substrate in the horn block (also see Figure 7). When the junction is placed in the channel, it is oriented such that the junction lies within the waveguide. The IF output and DC bias inputs are made through the matching network circuit. The matching network is orthogonal to the junction substrate. The magnetic field for suppressing Cooper pair tunneling is brought into the mixer via magnetic field concentrators embedded in the junction block [7]. The ground side of the junction will be held in place with silver paint, and the ``hot'' side of the junction will be connected to the IF matching network by wire-bonding.

The designed normal state resistance of the junction is expected to produce an IF output impedance of $> 100 ~\Omega$. The IF output impedance of the mixer is transformed to 50 $~\Omega$ through a 4-6 GHz IF matching network. A prototype IF matching network has been designed, built and tested. Figure 3 shows the layout of the matching network that was designed using HP's Microwave Design System circuit simulator program [8]. The value of the DC bias resistors and chip capacitors used for RF chokes are shown in the layout. A 5.6 pF capacitor is used as DC block. The substrate used in the microstrip design is Rogers Duroid 6002 with a thickness of 30 mils and $\epsilon_R=2.94$ [9]. The overall dimensions of the matching network is $1.25\times 0.5$ inches. The IF output of the SIS junction is wire-bonded to the $\sim 160 ~\Omega$ line to the left of the layout. The DC bias traces shown in the bottom portion of the layout are part of a a 4-wire SIS bias circuit. IF output at $50 ~\Omega$ is brought out in the lower right of the layout in Figure 3 through an SMA connector (see Figure 2). The fabricated matching networks were tested against predictions using a special purpose fixture constructed for this purpose. For an input impedance range of 100 to 180$~\Omega$, the match to a $50 ~\Omega$ output is found to be better than -10 dB throughout the 4-6 GHz band. The IF output of each mixer then passes through an isolator before entering the first amplifier. Low-noise 4-6 GHz amplifiers have been ordered from Miteq [10]. The amplifier specifications are a noise temperature requirement of $\sim 5$ K, a gain of $\sim$30 dB, and a power dissipation of $\sim 50$ mW.

The fabrication of waveguide structures at submillimeter wavelengths tends to be difficult and expensive. For higher frequencies, wet etching or laser micro-machining [11] methods may be required. At lower frequencies, conventional machining has been successful. The so-called ``split-block'' technique has often been used [12]. In an effort to keep the cost of machining down, the array mixer blocks will be fabricated using this approach. The mixer blocks will be machined at the University of Massachusetts (UMass), in return for which UMass astronomers will receive a proportional amount of observing time on the HHT. A new numerically-controlled precision milling machine has been constructed at UMass using Aerotech positioners [13] that will allow the fabrication of waveguide components to a few microns of accuracy at low cost.