This post authored by John Coonrod originally appeared on the ROG Blog hosted by Microwave Journal.

Phase noise has long been a key parameter in high-frequency components, such as oscillators and frequency synthesizers, and high-frequency systems, such as radar and communications receivers. Much has been written about ways to minimize phase noise in different types of oscillators and frequency synthesizers. But printed-circuit-board (PCB) materials are often overlooked in the quest for low phase-noise performance. The choice of PCB material can contribute a great deal to the ultimate single-sideband (SSB) phase-noise performance possible from a circuit design. Understanding the key PCB material parameters that relate to phase noise can help when specifying a circuit laminate for the “quietest” phase noise possible for a given frequency.

PrintIn many systems, phase noise starts with a crystal oscillator or other reference frequency source. In the time domain, phase noise is also known as jitter, and high levels of jitter can degrade bit-error-rate (BER) performance in digital systems. Crystal resonators with high quality factor (Q) are capable of producing stable frequency signals with low phase noise or minimal variations in phase. But crystal oscillators and other types of frequency sources rely on more than just resonators, and the active devices used for amplification and the passive circuit elements used for filtering can all contribute to phase noise under different conditions. And all of these components together, when mounted on a PCB, can depend on the characteristics of the PCB material for acceptable phase-noise performance.

What are the circuit-material parameters to compare when sizing up different candidate laminates for a low-phase-noise application? Quite simply, dielectric constant (Dk) and temperature behavior are the two areas where any circuit material must excel if it is to help minimize phase noise. Any particular Dk value is not as important as is the consistency of the Dk across a board and from board to board in production requirements.

Variations in Dk across a single circuit board result in unwanted impedance variations in transmission lines, interfaces, and other parts of a circuit. For an oscillator intended to be a low-phase-noise design, for example, excessive variations in dielectric constant can present unwanted impedance variations to the active device in an oscillator, such as a high-electron-mobility-transistor (HEMT) device, which can result in higher phase noise. In the search for a low-phase-noise circuit material, selecting a material with a tight Dk tolerance is an essential starting point. Materials from Rogers Corp. that offer tight Dk tolerance include RO4835™ hydrocarbon ceramic laminates with a Dk that is controlled within ±0.05 of 3.48 through the z-axis (thickness) of the material at 10 GHz. Such a tight Dk value makes it possible to maintain tight control of impedance throughout a high-frequency circuit, to avoid the impedance mismatches that can increase phase noise.

Since phase noise is also sensitive to heat and temperature variations, controlling heat and temperature is essential for any PCB material intended for low-phase-noise applications. In connection to the requirement for tight Dk tolerance, a candidate circuit material should suffer very little change in Dk as a function of temperature, which is characterized for different circuit laminates by their thermal coefficient of Dk specifications. For the RO4835 circuit material, for example, the thermal coefficient of Dk is +50 ppm/°C measured in the z-axis of the material at test temperatures from -100 to +250°C. This indicates that the Dk will indeed rise with temperature, but only in small, tightly controlled amounts.

A candidate circuit material for low-phase-noise applications should exhibit the highest possible thermal conductivity (along with tight Dk tolerance) in order to effectively dissipate heat that might be generated within a circuit, such as by transistors in the oscillator or by other active devices in the circuit. Inadequate thermal conductivity will result in heat buildup within a circuit, either from internal or external sources (such as input signals), which can increase phase noise.

In comparing circuit materials for thermal conductivity, values vary widely and typically represent a tradeoff with other material parameters. The RO4835 circuit material, for example, is characterized by thermal conductivity of 0.66 W/m/K measured at +80°C. This value is considered quite good; however, it can be somewhat limited in terms of dissipating heat when compared, for example, to RT/duroid® 6035HTC circuit material from Rogers Corp. with a thermal conductivity of 1.44 W/m/K. That higher value indicates that RT/duroid 6035HTC has almost three times the capability of RO4835 laminate to dissipate heat and prevent the “hot spots” that can increase phase noise. The RT/duroid 6035HTC material has a Dk value of 3.50 in the z-axis at 10 GHz that is tightly controlled within ±0.05, making it a viable PCB candidate for low-phase-noise circuit applications for its tight Dk tolerance.

Other circuit materials are available with tighter control of Dk, such as Rogers RT/duroid 5880 laminate, with Dk of 2.20 controlled within ±0.02 in the z-axis of the material at 10 GHz. But juggling tradeoffs is part of the process in selecting a circuit material for low phase noise. While RT/duroid 5880 laminate features outstanding Dk tolerance, it sacrifices in other areas of concern to lowering phase noise. With a thermal conductivity of only 0.2 W/m/K, it lacks the thermal capabilities of the other two materials to dissipate heat and minimize the effects of heat and circuit hot spots on phase noise.

Phase noise can be critical to the performance of many different systems, disrupting the flow of digital modulation in communications systems and degrading the accuracy of received target information in radar systems. Phase noise can be influenced by many different factors in a circuit, including achieving optimum bias conditions for the active devices used in the oscillators within the circuit. Phase noise can also be impacted by a choice of PCB materials and perhaps a single word to remember when connecting PCBs to phase noise is “stability.” PCB materials that can minimize phase noise (or at the very least not add to it) achieve excellent stability, in terms of dielectric constant with temperature and efficient heat transfer for minimizing thermally related noise. Maintaining consistent flow of heat through a circuit board helps to achieve the stability required for con

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stant Dk and the steady thermal conditions needed for low phase noise. Of course, any choice of PCB material represents a balancing of different parameters, since tradeoffs are always necessary, but a good starting point is in search for PCB materials with tight Dk tolerance and well controlled thermal characteristics.

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This post authored by John Coonrod originally appeared on the ROG Blog hosted by Microwave Journal.

Circuit design engineers have long relied upon the basic physics of printed-circuit boards (PCBs) and how capacitors and inductors can be formed from simple patterns and structures on a PCB. For example, a PCB provides parasitic capacitance because the metal signal plane layer and the metal ground plane layer underneath are parallel to each other, separated by a dielectric material layer. Of course, the characteristics of the metal layers, such as the thickness and type of conductive metal used, and the mechanical and electrical traits of the dielectric material, including its relative permittivity or dielectric constant (Dk), can contribute quite a bit to the final performance levels and consistency that can be achieved for a particular circuit design. A number of PCB material traits, in particular the consistency of the dielectric constant, can go a long way towards achieving consistent and reliable PCB capacitors and inductors especially at RF and microwave frequencies.

PrintCircuit designers are constantly being asked to make their circuits smaller, whether they are for commercial cellular telephones or for portable military radios. Integrated-circuit (IC) designers have been doing their part to contribute to the size reductions, packing more and more active components onto chips, including amplifiers, microprocessors, and oscillators. But when added to PCBs, even these tiny chips require the support of essential passive components, such as capacitors and inductors, for such functions as impedance matching to help transfer signals from the chips to other components. And circuit designers are constantly faced with the need to produce reliable capacitors and inductors on their circuits. Understanding the role of the PCB material in building those capacitors and inductors can be a huge help.

PCB material characteristics that strongly impact the performance and consistency of embedded capacitors and inductors include Dk and Dk consistency or tolerance. Because capacitors and inductors that are created from different structures using, for example, microstrip or stripline circuit traces, can occupy more than a small portion of a section of PCB, it is essential that a candidate circuit material for those passive components and their associated circuitry maintain good Dk control in order to achieve consistent capacitance and inductance from different circuit structures.

When working in microstrip, for example, the amount of transmission-line area above the ground plane will determine the amount of capacitance for a particular circuit structure. Even a 90-deg. change in a transmission line can add capacitance to a circuit. Such simple structures as meander lines have been used to add small amounts of  inductance to a circuit. Spiral circuit patterns have long been used to add inductance to a circuit, with the inductance changed by varying the trace width and the spacing between the traces in the spiral circuit pattern. Such an inductor is characterized by a self-resonant frequency (SRF), which will increase when the inductor trace width is increased and the spacing between traces is decreased.

The amount of capacitance in an embedded capacitor will depend on such factors as the distance from the signal plane to the ground plane—essentially the thickness of the dielectric material between them—and the physical size of the capacitance structure. The coupling between arms on the signal plane can also add to a microstrip circuit’s capacitance. These embedded capacitors and inductors are typically referred to by designers as part of a circuit’s resistor-inductor-conductor-capacitor (RLGC) parameters and are included in computer circuit models to predict the performance and behavior of transmission-line circuits. Computer models assume that such things as the dimensions of the various RLGC structures on a PCB that are entered into the software program are the same dimensions that have been fabricated on the actual PCB. Variations between the sets of values can result in variations in such things as capacitance and inductance values from computer-predicted expectations

Of course, such computer models also consider the contributions of PCB materials when predicting such things as the capacitance in a 90-deg. bend in a circuit’s transmission line. Circuit computer models work with such circuit material parameters as Dk and even Dk tolerance, or the deviations in Dk with temperature and frequency. Models can focus on different parameters with some models, for example, ignoring the losses contributed by the PCB’s conductors and the dielectric material but concentrating on the Dk consistency to accurately predict the values and expected performance of circuit elements such as inductors and capacitors based on the physical dimensional tolerances of circuit structures.

Numerous reference guides are available for calculating the values of microstrip capacitors and inductors based on dimensions etched onto different PCBs, such as the ARRL Handbook from the American Radio Relay League. For microstrip inductors, for example, it provides calculations for microstrip inductors using such variables as transmission-line strip width, distance to the ground plane, and Dk of the PCB material

The choice of PCB material can play a significant role in the performance levels to be expected from these etched capacitors and inductors, with the material properties impacting capacitance value and consistency and such inductor characteristics as quality factor (Q), SRF, and self-inductance factor. A PCB material’s Dk value will determine the size of different circuit structure for a desired operating frequency, but it is the tolerance to which the Dk value is held that greatly impacts the consistency and performance of capacitors and inductors etched onto the copper layer of the PCB material.

As an example, RO4835™ circuit material from Rogers Corp. is a glass-reinforced hydrocarbon and ceramic dielectric material with good z-axis (through the thickness) stability that makes it a candidate for multilayer circuits. It exhibits a z-axis Dk value of 3.48 at 10 GHz with impressive Dk tolerance within ±0.05 of 3.48 across the circuit board. In terms of fabricating passive circuit elements such as capacitors and inductors, this means that they will maintain their capacitance and inductance values with frequency and with temperature, with minimal variations in capacitance and inductance caused by changes in PCB Dk value.

As impressive as that Dk tolerance, Rogers Corporation offers even more tightly controlled circuit materials, such as its RO3003™ ceramic-filled polytetrafluoroethylene (PTFE) circuit material. Its Dk value of  3.00 at 10 GHz through its z-axis is held to a tolerance of ±0.04 across the material and from board to board. And RT/duroid® 5880 is a glass-microfiber-reinforced PTFE composite material with a Dk of 2.20 that is maintained to an extremely tight tolerance of ±0.02 across the material. This low-loss material supports circuit applications across a wide range of frequencies, well into the millimeter-wave frequency range.

Screen shot 2014-08-08 at 1.33.54 PMDesigners may choose a PCB material for a particular Dk value, but the Dk tolerance may not be known or may not be tightly controlled, resulting in etched circuit elements such as capacitors and inductors that may not provide the performance or consistency expected or predicted by commercial software circuit-design programs. A PCB material with tightly controlled Dk tolerance can provide the assurance of achieving the circuit values expected for such passive elements as capacitors and inductors and for maintaining circuit performance with frequency and time.

ROG Mobile App

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.

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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.



Join Microwave Journal and Rogers Corp. for a free one-hour webinar on March 26, 2015:

Measurement of PIM Distortion in Microstrip Transmission Lines: Effect of Laminate Properties and Measurement Repeatability 

Date: March 26, 2015, 8am PT / 11am ET / 3pm UTC

Presenter: Allen F. Horn III, Ph.D., Rogers Corp.

Screen shot 2015-03-17 at 5.13.04 PMUnderstanding the factors contributing to the generation of passive intermodulation (PIM) distortion in multi-frequency communication systems is a subject of interest to antenna designers. As telecommunications antennas are becoming more complicated, microstrip circuits made on copper clad laminates are replacing bent metal designs. PIM distortion is a circuit or system property, not a basic material property, and depends on the overall design, connectors, and local power densities, among many other factors. However, there are basic laminate properties that can contribute to PIM. In this seminar, we will discuss the measurement of PIM in microstrip transmission lines and the pertinent laminate properties, most notably conductor profile. We also discuss experiments on the effects of circuit processing, laminate thickness, and power level. Understanding small contributions to PIM is complicated by the very low power levels involved. We discuss the special attention that must be paid to the statistics of the measurement method.



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