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. (www.rogerscorp.com), makes it possible to achieve good spurious mode suppression at higher frequencies without necessarily compromising circuit loss performance.
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