Scale Model Tests

SIS quasiparticle tunnel junction mixers have a rather large geometric capacitance that has been traditionally tuned out using high quality non-contacting backshort and E-plane tuners [7]. The reliance on waveguide tuners alone has two major disadvantages. The large capacitance and small normal state resistance of the SIS junction typically places a severe demand on the waveguide tuners and results in a relatively small frequency band over which an adequate match can be achieved. In addition, the process of tuning with waveguide tuners becomes very complicated and time consuming from the point of view of an astronomer using the full array. To improve the junction match to the embedding impedance of the waveguide circuit and to increase the instantaneous bandwidth of the mixer, a variety of inductive tuning circuits fabricated along with the junction have been used [14]. For such designs, a knowledge of the waveguide embedding impedance is essential. Scaled model tests are a traditional technique to obtain the embedding impedance of a probe in a waveguide. In this section, we describe the results of scaled model tests of the half-height 345 GHz mixer block presented in Section 2. In the next section, we discuss finite element analysis (FEA) methods to obtain embedding impedances.

 

Figure 4: Schematic of the Scaled Model Test Jig. The backshort drive was constructed with a captured pin and a precision screw to ensure smooth and repeatable motion inside the waveguide.

 

Figure 5: (a) Scale model test results. Real and imaginary parts of embedding impedance of half-height waveguide in combination with RF choke and probe in a suspended microstrip configuration for a backshort distance of 0.2 mm at 345 GHz. (b) Input match scaled back to the frequency of operation for an impedance of $40+j20~\Omega$looking into the tuned junction.

We constructed a half-height scale model (see Figure 4) with a center frequency of 5 GHz (scale factor of $\sim 68$) and measured three different RF choke structures to determine a favorable embedding impedance. Acetyl was used as the substrate to approximate fused quartz. The real and imaginary parts of the embedding impedance for one of the choke structures is shown in Figure 5(a) for a backshort distance of 0.2 mm. Shown in Figure 5(b) is the input match of this waveguide-probe combination to a tuned junction with an effective impedance of $40+j20~\Omega$. This impedance value is being considered for one of the baseline designs of the 345 GHz array junction to be fabricated by JPL [15]. As can be seen from Figure 5, this combination of embedding impedance and junction design is able to provide a broadband match for the desired band of the array.