Optimize Benefits Of Subharmonic Mixers

Software design tools can help simply the challenge of creating subharmonic mixers that provide the full advantages of their unique characteristics.

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.

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The 3D plot in Fig. 2 clearly shows how output power varies with junction capacitance and series resistance. One axis is the junction capacitance and the other is series resistance and output power. The spike at the center of the output power is the precise point at which the diodes are perfectly balanced (and maximum rejection of even harmonics occurs). Performance drops off precipitously as series resistance and junction capacitance increase.

The upconverting subharmonic mixer configuration that was modeled in this article features an LO frequency of 14 GHz, RF output of 28 GHz, intermediate-frequency (IF) input of 100 MHz, a bandpass filter at the LO frequency, phased properly with the diode pair, output stubs for rejection at the LO and 3LO frequencies, a stub for rejection of 3LO frequencies at the input port, and an LC filter in the LO arm terminated at the RF arm in order to screen IF input signals.

Creation of several mixer components is aided by Ansoft Designer's Solver on Demand technology. This is a particularly important capability when creating microstrip coupled-line steps and transitions, which are traditionally limited to the tried-and-true shapes that have served designers for decades.

Ansoft Designer's embedded planar EM solver also aids creation of several mixer components. Using the unique Solver-on-Demand technology, engineers can directly define and model circuit components based on arbitrary geometries and insert these components into their larger networks. This allows engineers the freedom of utilizing novel, better-performing topologies without paying the price of uncertain model behavior. This is a particularly attractive feature when creating optimum performing microstrip coupled-line steps and transitions that traditionally are based on abrupt step changes due to modeling concerns.

Since abrupt steps are undesirable at high frequencies, a designer wishing to create a nonstandard topology normally must rely on a tedious trial-and-error process. In addition, the orientation step in the layout process can also be time-consuming. With the Solver-on-Demand technology, designers can quickly create custom shapes represented by accurate models, eliminating the need to rely on circuit equivalents. Solution caching in Ansoft Designer automatically embeds planar EM solutions for all instances of that Solver-on-Demand component in a network with the results of a single EM simulation, reducing the overall simulation time.

An edge-coupled filter provides an excellent example of this capability. A coupled-line filter always has a critical junction, the effect of which must be minimized usually through a step that often still results in an undesirable discontinuity. Since the Solver-On-Demand technology allows any type of component or model to be created, a custom taper can be used instead of a step.

By defining a microstrip coupled line with a custom width and separation that is layout compatible (allowing it to transition into the next section), the effect of this junction on performance can be significantly minimized. Since Ansoft Designer is netlist based, a circuit equivalent can be created that contains common components for faster analysis. This netlist fragment is part of the component definition. The netlist shows four step discontinuities with a coupled line in between to produce a multiple-line netlist for association with the component. The layout and netlist assigned to a component is shown in Fig. 3. The components were used for the design of an LO filter (Fig. 4).

Design of the stubs is also critical to ensure that the LO signal remains in the diodes and is not present in the output signal, which should be purely 2LO±RF. At the mixer input, the 3LO signal will tend to be reflected back and must be restricted, while at the output strong signals will exist at LO and 3LO, which must also be reflected back into the diode. The stubs for these frequencies allow the angle of reflection to be optimized.

Via-hole modeling is an important consideration in the stub design, and Ansoft Designer incorporates many 2D and 3D arbitrary-shaped via holes (Fig. 5). Its cosimulation capability allows custom via holes to be defined with no restrictions on layers or geometry. Spiral inductors on 5-mil alumina are used in this design, and while they have low quality factor (Q), any adverse effect is mitigated by the fact that higher-order products are handled by the stubs and filter.

After the stubs have been created, the circuit can be assembled with the stubs in the intended locations, along with the SMT or chip capacitor, diodes, LO filter, and grounded stub for the DC return (Fig. 6). Various basic performance measurements can then be performed, including output power versus LO input power, output power versus RF input power, output power versus frequency, and harmonic output. The harmonic output measurement reveals that even harmonics are not present, verifying that diode balance is excellent. A 3D plot of IF power versus length and width dimensions shows how varying these parameters affect circuit performance, a comparison that would be difficult to interpret in a 2D plot (Fig. 7).

Ansoft Designer allows the completed mixer to be inserted into another circuit or even a system-level circuit to provide accurate system simulation. The software's system tool automatically extracts the necessary parameters from the mixer circuit for use in the system simulation.

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