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.
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Bandpass filters are essential to many RF/microwave circuits and systems. They eliminate unwanted signals and noise, and can work with both receivers and transmitters. Bandpass filters can be assembled in a variety of ways, using lumped-element discrete inductors and capacitors at lower frequencies and semiconductor technologies for tiny monolithic filters at higher frequencies. Still, the most popular RF/microwave bandpass filters may be the ones based on microstrip transmission lines with distributed circuit elements on printed-circuit-board (PCB) substrates. With the right circuit material, microstrip bandpass filters can provide excellent performance in small circuits. This first of two blogs on RF/microwave bandpass filters will review some of their basic performance parameters and how they relate to PCB material characteristics, with a focus on one material in particular, RT/duroid® 6010.2LM circuit material from Rogers Corp. As a followup, the next blog will explore how bandpass filters perform on other circuit materials.
A bandpass filter is defined by a center frequency within a passband, channeling all signals within that passband with minimal loss while rejecting signals at frequencies above and below the passband with as much attenuation as possible. In contrast, a lowpass filter passes all signals below a given cutoff frequency, rejecting signals above that frequency, and a highpass filter passes signals above a cutoff frequency and attenuates signals below it. A band-reject filter suppresses signals within a designed bandwidth passing signals outside the rejection band with minimal loss. A bandpass filter can be described by various performance parameters, including center frequency, passband, passband insertion and return loss, upper stopband, lower stopband, and attenuation within the stopbands.
Transitions from a bandpass filter’s passband to its upper and lower stopbands can be extremely rapid or more gradual, typically described by different filter response types including Butterworth, Chebyshev, and Bessel filters. Each type of bandpass filter exhibits some form of tradeoff. For example, a Butterworth filter is typically characterized by flat amplitude response across its passband, sacrificing sharpness in the transitions from passband to stopbands. Chebyshev filters achieve sharp transitions, at the cost of higher amplitude ripple in the passband than a Butterworth filter. Bessel filters offer linear passband phase response, giving up some stopband attenuation compared to the other two filter types.
The performance of a PCB filter is highly dependent on the circuit substrate material. The choice of material can limit center frequency, passband loss, and other key filter parameters. For many filter designers, the choice of material starts with a laminate’s dielectric constant. For a distributed-element filter such as a microstrip bandpass filter, the size of the transmission lines and distributed filter elements is inversely proportional to the square root of the PCB material’s dielectric constant; in short, PCB materials with higher dielectric constants make it possible to design and fabricate smaller filters for a given frequency. RF/microwave filter designers have long favored PCB substrates with dielectric constants of 10 or higher to create filter circuits with relatively small dimensions for a given center frequency/wavelength.
Since the dimensions of microstrip and other PCB filters are determined by the dielectric constant of a circuit material, it is important that the value of dielectric constant used for a particular material is very accurate. The dielectric constant of any PCB material can vary, so it is critical that these variations remain within the dielectric-constant tolerance range cited for a particular material by its manufacturer, such as 10.2 ± 0.25. Whether a filter’s dimensions are calculated manually or with the help of a computer-aided-design (CAD) program, even small errors in the value of the dielectric constant used in the calculations will result in unwanted changes in designed wavelength/frequency and shifts in center frequency and passband.
Dissipation factor or dielectric loss is another important circuit material parameter for bandpass filters. Quite simply, low values of dissipation factor indicate materials capable of achieving low insertion loss. For a bandpass filter, low PCB dissipation factor also means high filter quality factor (Q), which translates into the potential for a filter with low passband insertion loss and sharper transitions from passband to stopbands.
When designing and fabricating RF/microwave PCB-based bandpass filters, variations in dielectric constant should be minimized whenever possible. A circuit material parameter known as moisture absorption can play a large role in the stability of the material’s dielectric constant under certain environmental conditions, notably under high humidity. Ideally, a PCB material’s moisture absorption should be as low as possible. A material with a high value of moisture absorption can suffer variations in dielectric constant and dissipation factor that far exceed the tolerance ranges specified by the manufacturer. The dielectric constant of the material will change with even a small amount of moisture absorption, resulting in unexpected performance variations in bandpass filter center frequency, passband, and passband insertion loss.
Filter designers choose PCB materials with high dielectric constants in order to minimize the dimensions of their RF/microwave filters. A popular dielectric-constant value for such materials is 10.2, typically for materials based on polytetrafluoroethylene (PTFE). Although a filled PTFE substrate has excellent electrical properties, it can be guilty of moisture absorption on the order of 0.25%. Although this is a relatively small value compared to most PCB materials, a PCB material with this value of moisture absorption can exhibit significant changes in dielectric constant and dissipation factor under conditions of high humidity, possible causing a filter to exceed its performance limits for passband loss or suffer a shift in center frequency and passband from expected values.
RT/duroid 6010.2LM microwave laminate from Rogers Corp. is a composite that blends ceramic filler with PTFE for stable performance and low moisture absorption. The material achieves small bandpass filter dimensions, by merit of its high dielectric constant of 10.2 in the z direction and 10 GHz, with tolerance of ±0.25, and features dissipation factor of only 0.0028 for low passband insertion loss. Its moisture absorption is a fraction of that for many filled PTFE substrates, at typically only 0.01% (compared to 0.25% for other filled PTFE substrates).
A bandpass filter fabricated on this material will have the same dimensions as a filter formed on filled PTFE with dielectric constant of 10.2. However, it will not suffer variations in dielectric constant and dissipation factor, with their resulting variations in filter performance, in environments in which humidity may change dramatically. In fact, the improvements possible with this material compared to PTFE for bandpass filters are detailed in a study available for free download from the Rogers’ web site, “The Benefits of Selecting RT/duroid 6010LM for Band Pass Filter Applications.”
RT/duroid 6010.2LM laminates can be specified with various thicknesses and cladding options and well suited for a wide range of RF/microwave bandpass filters. As the next blog will show, however, other circuit materials with lower dielectric constants and different parameters are available in support of repeatable, high-performance RF/microwave bandpass filters.
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.