Spurious modes can occur in printed circuit boards (PCBs) in spite of the best-laid plans. These modes support extra, unwanted signals, in addition to the intended signals, that can wreak havoc on a PCB and its application, causing interference and degradation of the intended signals. Although minimizing spurious modes in PCBs is largely a result of careful design practices, the choice of PCB material can have some bearing on the final spurious mode behavior, especially at higher frequencies. Understanding how these spurious modes originate can help in keeping them under control, especially on PCBs operating at millimeter-wave frequencies.
At RF, microwave, and millimeter-wave frequencies, numerous transmission-line technologies are fabricated on PCB materials, stripline and microstrip are two popular transmission-line methods at higher frequencies. The transmission-line structures propagate electromagnetic (EM) waves in different ways, with stripline supporting transverse-electromagnetic (TEM) wave propagation while microstrip supports quasi-TEM propagation. Quite simply, the mechanical structures of these transmission lines are different, with stripline employing a metallic conductor surrounded by dielectric material while microstrip fabricated the conductor on the top of a dielectric layer with a ground plane on the bottom of the dielectric layer. Coaxial cables, where the conductor is also surrounded by dielectric material, also operate in a TEM propagation mode like stripline.
Spurious waves can be surface waves that propagate through a high-frequency PCB or they can be produced by resonant effects within circuits fabricated on a PCB. Microstrip transmission lines offer very little design freedom for minimizing spurious mode propagation. In terms of physical changes to the PCB, using a thinner microstrip PCB material can diminish the amount of spurious mode propagation in a high-frequency circuit, and this is one of the reasons that thinner circuit materials are used at higher-frequencies.
Of course, many of the PCBs designed with microstrip transmission lines must also make a transition to coaxial cables at a launch point, and this represents a transition from the TEM mode of the cable to the quasi-TEM mode of the microstrip transmission lines. But simply because a PCB has been fabricated with microstrip transmission lines and circuitry does not mean that other modes cannot propagate on that PCB; spurious signals represent one of these other propagation modes. These unwanted spurious or “parasitic-mode” signals can interfere with the desired quasi-TEM-mode signals of the microstrip transmission lines and circuitry.
The quality of the signal launch to a microstrip PCB can affect the amount of spurious mode suppression. For example, EM waves propagating from a coaxial connector to a microstrip PCB will not only make a transition from the TEM mode of the connector to the quasi-TEM mode of the microstrip, but the EM waves from the connector to the microstrip will also make a transition from the polar orientation of the cable and connector to the planar orientation of the microstrip. Even the most ideal coaxial-connector-to-microstrip PCB can suffer stray electrical reactances as a result of the transition of the propagating EM waves across an interface that will have some mechanical variations. Even minor impedance mismatches at the connector-microstrip transition can result in signal reflections and radiation at the transition. In addition, variations between the signal path and the ground return path in the transition area can lead to EM wave skew and additional “interruptions” in the intended propagation path and additional sources for spurious mode propagation.
A grounded coplanar-waveguide (GCPW) launch, which is also known as conductor-backed coplanar waveguide (CBCPW), is capable of a fairly smooth transition to a microstrip transmission line, with minimal spurious signal generation. When even more spurious mode suppression is required, for example at millimeter-wave frequencies, GCPW or CBCPW transmission lines can be used on the PCB in place of microstrip transmission lines. This provides more design freedom to minimize spurious mode generation, with a tradeoff being in added design complexity.
GCPW circuits are often used at millimeter-wave frequencies rather than microstrip transmission lines for better suppression of spurious modes at those higher frequencies. The physical configuration of these circuits helps suppress the resonances that can lead to spurious signals. In addition, the use of grounding viaholes in GCPW circuits can help suppress the propagation of resonance modes between the signal and ground planes. The pitch of these viaholes is important, and related to the wavelength of the operating frequency. The pitch of the viaholes should be 1/8 wavelength or less of the highest intended operating frequency for the circuit.
For a PCB, particularly based on microstrip transmission lines and at higher frequencies, resonances in a circuit and its transmission lines can lead to unwanted spurious signals. Resonances can develop between the transmission line’s signal conductor and the PCB ground plane, with resonances occurring between opposite edges of the signal conductor and paving the way for spurious signal propagation. Such resonances can generate their own EM waves in a circuit or transmission line, especially in microstrip circuits at higher frequencies.
The resonances occur according to the dimensions of the transmission-line conductor and the wavelength of the frequency of interest for the circuit. For example, if the physical width of a microstrip conductor is equal to ½ or ¼ the wavelength of the circuit’s operating frequency, resonances will occur. These resonances can lead to EM waves that can interfere with the intended quasi-TEM waves that are meant to propagate through a microstrip circuit. As with the pitch of the grounding viaholes in the GCPW circuits, a design goal that can help avoid the generation of circuit-based resonances (and their accompanying spurious modes) in microstrip circuits is to make certain that no transmission line or circuit features are greater than 1/8 wavelength of the intended operating frequency.
What does the choice of PCB material or PCB material characteristics have to do with spurious mode rejection? The quest for increased spurious mode rejection typically becomes more difficult at higher frequencies, notably at millimeter-wave frequencies, and is not highly dependent on the choice of PCB material, although the dielectric constant (Dk) of a circuit material is one parameter that can have an impact on spurious mode rejection. When a circuit material with higher Dk value is selected, it results in shorter wavelengths for a given operating frequency, which in turn can affect the target size of the microstrip transmission lines when trying to ensure that these transmission lines and circuit features are no greater than 1/8 wavelength of the intended operating frequency.
Although the thickness of a PCB material can be a concern at higher frequencies, such as millimeter-wave frequencies, the particular conductor width (as noted earlier) is more of a concern at these higher frequencies (with their smaller wavelengths). Still, thinner circuit laminates can help minimize spurious modes at millimeter-wave frequencies, and thinner laminates are also beneficial for reducing radiation losses in higher-frequency circuits. A tradeoff in selecting thinner PCB materials is that they tend to have higher losses than thicker circuit materials. Fortunately, advances in modern circuit materials, such as the lower insertion loss exhibited by RO4000® LoPro™ laminates from Rogers Corp., make it possible to achieve good spurious mode suppression at higher frequencies without necessarily compromising circuit loss performance.
Download the ROG Mobile appto access Rogers’ calculators, including the popular Microwave Impedance simulation tool, literature, technical papers, and the ability to order samples of the company’s high performance printed circuit board materials.
Do you have a design or fabrication question? Rogers Corporation’s experts are available to help. Log in to the Rogers Technology Support Hub and “Ask an Engineer” today.
Make your reservations now for the 2014 International Printed Circuit & APEX South China Fair at the Shenzhen Convention & Exhibition Center, December 3-5, 2014. Stop by and see us in booth #2F31.
We’ll also be presenting as part of the technical program. Sharon Young, Market Development Manger – Asia, will present on December 3, 2014, 13:30-14:30pm, Hall 2:
“How to Choose PCB Material When Facing High Power and Temperature. How Lamination Selection Contributes to Thermal Management of Microwave Circuit Performance.”
In this video, John Coonrod discusses why there are so many different dielectric constants (Dk) that are used in the microwave printed circuit board industry.
Send us questions/comments by tweeting us @Rogers_ACM!
RF/microwave power applied to a printed-circuit board (PCB) will generate heat. A key to designing a practical circuit on a given PCB material is to understand how different circuit material properties can impact the heating patterns on an RF/microwave PCB, and to work within the limits of a high-frequency circuit material.
For any RF/microwave PCB, the heat generated by a circuit element, such as a high-power transistor, will follow a general heat-flow model, flowing from the source of the heat to a heat sink or cooler part of the PCB. The properties of the circuit material will have a great deal to do with how the heat flows across the PCB and how heating patterns develop.
The heating patterns on a PCB are related to the current density patterns on that PCB, and the way that current is transformed to heat within a PCB might be best described by the PCB material’s thermal conductivity (TC). The TC, which is typically measured in watts of power per meter of material per degree Kelvin (W/m/K), is often used to compare different rates of energy loss as heat through different materials. Quite simply, higher TC values mean better heat flow through a material. Since PCBs are comprised of different materials, such as copper conductors with very high TC value and dielectric materials with very low TC values, heat flow through a PCB will occur at different rates, resulting in heating patterns such as at where the edges of conductors meet dielectric substrates.
High TC values usually denote materials engineered for high-power applications, where heat flows more efficiently through the material. PCB materials with lower values of TC will exhibit less heat flow through the material, with an increase in circuit temperature.
As a comparison, copper is a good electrical conductor and a good thermal conductor, with an extremely high TC value (typically 400 W/m/K). Heat energy will flow quickly and easily along copper conductors according to standard heat-flow models, with thermal energy moving from an area of higher temperature to an area of lower temperature, such as a heat sink. The dielectric materials used in PCBs have very low TC values and behave more like thermal insulators. For example, polytetrafluoroethylene (PTFE) circuit materials provide good dielectric properties but have a much lower value of TC (typically 0.20 W/m/K), so that much less thermal energy will flow through the material compared to copper. For a circuit material to generate less heat for the same power, it must exhibit a higher value of TC, such as RT/duroid® 6035HTC circuit material from Rogers Corp., with a typical TC value of 1.44 W/m/K.
Dielectric constant (Dk) is another circuit material parameter that can impact the heating patterns through a PCB. Dk is an important circuit material parameter in terms of determining the dimensions of different circuit structures. It also has a great deal to do with the heat flow across a high-frequency PCB, since a material’s Dk helps determine circuit dimensions for a given characteristic impedance. For typical 50-? microstrip circuit designs, for example, PCB materials with lower Dk values support the use of wider conductors for a desired signal frequency compared to circuit materials with higher Dk values.
Additional material parameters that can impact the heating patterns across a PCB include dissipation factor (and how it relates to the loss of the material), copper conductor surface, and even the thickness of the PCB material. As noted earlier, the dielectric material content of a PCB is more like a thermal insulator than a thermal conductor, so a thinner circuit material offers a shorter heat-flow path than a thicker circuit material, resulting in less heat buildup in the dielectric material.
To minimize heat at higher power levels, a PCB material with smaller dissipation factor (Df) is better since this low dielectric loss translates to RF/microwave circuits with lower insertion loss. Lower circuit insertion loss means less heat produced at higher RF/microwave power levels. Copper conductor surface roughness is also related to circuit material loss, with smoother copper conductors usually yielding lower conductor loss, and lower heat produced for a given amount of RF/microwave power handled by a circuit. Copper conductor roughness tends to yield frequency-dependent effects, with some differences in heating at different frequencies.
One additional circuit material parameter, a circuit’s maximum operating temperature (MOT), can be useful for comparing PCB materials intended for high-power use. This is a measure of the maximum temperature at which a circuit can be operated for an indefinite period of time.
To better understand the thermal behavior of typical PCB materials, heat rise testing was performed on a number of different microwave PCB materials, using 50-? microstrip transmission lines to conduct high-power test signals and measure the resulting temperature rises through the PCB materials. Differences in the materials included many of the key material parameters already noted, such as dielectric thickness, copper conductor surface roughness, dielectric constant, insertion loss, and TC.
A worst-case example involved transmission lines fabricated on FR-4 circuit material with Dk value of 4.25, Df of 0.0200, low TC of 0.25 W/m/K, and relatively high insertion loss of 0.37 dB/in. of microstrip transmission line at 3.4 GHz. A best-case example involved RT/duroid 6035HTC circuit material with Dk of 3.60, low Df of 0.0013, much higher TC of 1.44 W/m/K, and low insertion loss of 0.11 dB/in. For 85-W test signals applied at 3.4 GHz, the difference in heat rise for the two materials was startling. For the worst-case material, the temperature rise was +74°C; for the best-case material, with the same test signal, a temperature rise of only +14°C occurred.
Ideally, the heat flow through a PCB construction will follow established heat flow models and the heat produced by an applied source or an on-circuit active device will channel from an area of high temperature to an area of much lower temperature. The various PCB material parameters, such as TC and Df, help provide some insight into how various heating patterns will develop across different PCB materials at higher RF/microwave power levels, and how those PCB material parameters can be consulted when choosing a circuit material for higher power levels.
Do you have a design or fabrication question? John Coonrod and Joe Davis are available to help. Log in to the Rogers Technology Support Hub and “Ask an Engineer” today.
There are a number of test methods to determine the dielectric constant of circuit materials used in the microwave or high frequency industry.
In this video, “Common Test Methods for Measuring Dielectric Constant,” you will learn about the most common test methods like Clamped Stripline Resonator Test, Split Post Dielectric Resonator, Full Sheet Resonance (FSR), and Microstrip Differential Phase Length Method.
For additional information and technical tools, join us at rogerscorp.com/techub