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.
In 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 constant 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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