Subharmonic mixers are attractive for millimeter-wave systems, at frequencies where signal generation is expensive. Such mixers are often used in applications above 30 GHz, such as digital microwave radios, and even called upon in support of scientific applications working as high as 200 GHz. While the design of subharmonic mixers is relatively difficult, optimizing their performance can be greatly facilitated by the use of electronic-design-automation (EDA) tools. To illustrate how these tools can benefit the design process, a subharmonic mixer with an RF output of 28.1 GHz was created using the Ansoft Designer™ suite of software tools.

Subharmonic mixers benefit from the capability of working with a local-oscillator (LO) frequency that is a fraction of the frequency of incoming RF signals. For example, a system operating at 38 GHz with a second-subharmonic mixer requires an LO of only 19 GHz, which is considerably easier to design and less expensive to obtain than an LO operating closer to the nominal RF. To operate with even lower-frequency LOs, higher-order subharmonic mixers can also be used. In this same example, a fourth-subharmonic mixer would require an LO of only 9.5 GHz. Subharmonic mixers inherently provide 50-to-60-dB rejection of the even harmonics of the LO signal at the RF output, which reduces the amount of LO rejection (filtering) that must designed into in the circuit. A typical subharmonic mixer spectrum is shown in Fig. 1.

Although offering obvious cost benefits, subharmonic mixers have been traditionally difficult to design. They have inherent characteristics that must be accommodated in order to achieve the desired performance. For example, subharmonic mixers have higher conversion loss than their single- or double-balanced counterparts, and the higher-order spurious responses that they produce must be minimized. Nevertheless, through careful design aided by electromagnetic (EM) simulation, conversion loss can be controlled to only slightly more than that of balanced mixer designs, and their spurious responses can be well characterized, allowing the significant benefits of this type of mixer to be realized to their fullest.

Subharmonic mixers use an antiparallel diode pair to generate a nonlinear conductance waveform at twice the frequency of the LO signal. Since the LO frequency is one-half the RF, isolation between the RF and LO ports is simple to achieve. The matching of these diodes is essential for optimization of the circuit, since attenuation of even harmonics is determined in large measure by diode balance.

The higher-order spurious responses can be dealt with via nonlinear simulation in order to thoroughly characterize harmonic content and achieve accurate harmonic balance. Since the spectral location of the responses must be found, the harmonic-balance software engine can be set to address them. In order that all of these low-level harmonic responses are properly identified, the simulation software must have an extremely low noise floor.

A mixer for study was chosen to be on a 5-mil-thick alumina substrate, using a flip-chip-mounted diode pair and spiral inductors. Selecting the diodes requires an analysis of their responses as functions of voltage and other parameters. Such mixer diodes are widely available in surface-mount-technology (SMT) beam-lead, and flip-chip versions and, since they are obtained from adjacent sections of the wafer, their characteristics are generally quite well matched. SPICE parameters for such diodes are generally available from most manufacturers. Modeling data for all package types can be imported into the simulation software, parameterized if desired, and two- and three-dimensional (2D and 3D) layouts can be realized. For this example, SMT Schottky diodes from Skyworks Solutions (Woburn, MA) were selected, accompanied by SPICE parameters for modeling purposes.

Ansoft Designer makes it simple to create nonstandard components, in this case an antiparallel diode pair. While diode manufacturers generally supply five or six key parameters, only three characteristics—series resistance, junction voltage, and junction capacitance—have a substantial effect on diode balance, so the remainder of the parameters can be considered constant from diode to diode in the SPICE model. Each diode is given different value of each critical parameter so that the variations can illustrate their effect on reduction of the 2LO signal. Since the netlist is SPICE compatible, a custom diode can be built and any type of layout can be incorporated.

The arbitrary parameter sweep function within Ansoft Designer can be used to sweep the properties of diodes, and 3D visualization can be employed to show output power as a function of junction capacitance and series resistance (Fig. 2). A very simple circuit created in Ansoft Designer can be used for the analysis, and variables are set up for junction capacitance, series resistance, and forward voltage. The first diode uses these variables, and the accompanying diode in the pair can be characterized as a "percent difference" from the first. The percent difference in each parameter is then varied and plotted in both 2D and 3D formats.