Keywords: RF Microwave PCB

There wasn’t much demand for RF Microwave PCB, a few decades ago. To design into the architectures of the time, they were difficult. They were quite costly projects to be afforded.  But, into a large variety of commercial products, RF circuitry is crammed today. To become portable communications units, most of these are handheld wireless devices for communications, industrial, and medical applications. From desktop models, applications are migrating in a variety of fields. RF, as well as microwave circuitry, is becoming more ubiquitous both capturing ultra-high frequencies and very high frequencies.

Much more than mixed-signal or pure digital technologies are encompassed in Printed circuit boards, and many more challenges are faced by the PCB layout designer when designing sub-assemblies with high-frequency microwave and RF.

From 500 MHz to 2 GHz is the frequency range of The RF typically, and RF designs are the ones above 100 MHz. Anything above 2 GHz is the microwave frequency range. Between analog and typical digital circuits versus microwave and RF circuits, there’s a considerable difference. In essence, very high-frequency analog signals are RF signals. Therefore, between maximum and minimum limits, an RF signal can be at any current and voltage level at any point in time unlike digital.

Between DC and a few hundred megahertz comes the Standard analog signals are assumed. But on a very high-frequency carrier, microwave and RF signals are a band of frequencies or one frequency. Microwave and RF signals operate on a particular frequency.

To pass signals within a certain band, microwave and RF circuits are designed. To transmit signals in a so-called band of interest, they use bandpass filters. The rest of the frequencies of the signal are filtered and the signal passes through this band range within a range of frequency. A single band can be carried upon a very high-frequency carrier wave and is very wide or narrow.

Issues with microwave or RF and PCB design

PCB layouts including microwave or RF circuits are difficult to design for the most part. However, the rule of thumb is to start with the basics and follow the laws of physics regardless of their difficulty. Upfront, microwave signals are highly sensitive to noise that must be known to the PCB designer. The incurring reflections and ringing possibility must be treated with great care.

For example, the PCB designer has to follow certain rules and guidelines when working with very high-speed digital signals in the 10 Gb per second or gigahertz range. The PCB designer must have the same mindset when microwave and RF enter the layout. As RF is far more sensitive as compared to very high-speed digital signals yet multiply that mindset many times simply.

For RF, impedance matching is very critical secondly. Even if they are very high-speed, Digital signals have a certain tolerance. But the smaller the tolerance becomes the higher the frequency for RF Microwave PCB.

Third, the return loss must be reduced. Caused by ringing or signal reflection, this loss occurs. Taken by the return current, the return is the path.

The return signal takes the path of least inductance at very high microwave frequencies. At providing this path, Ground planes underneath the signals are good. Therefore, there should be no discontinuities from the driver to the receiver in the plane underneath the signal. However, the signal will still somehow find its way to the driver if there is a cut in the ground plane or the ground doesn’t exist underneath that trace. Through some other route or a PCB’s multi-layers, it will go through power planes and will find a return path. But such a path is not ideal. Since it will no longer be an impedance-controlled signal, this will cause ringing and reflection.

When designing microwave and RF circuits, the PCB designer must also keep the crosstalk factor in mind. The problem of how to deal with it and cross-talk becomes more crucial as the board densities and system performance increase. Due to shunt capacitance and mutual inductance, Cross talk is the energy transfer between adjacent conductors. On the victim line, the coupled energy from the active line is superimposed.

Forward crosstalk is constituted in the coupled signals flowing toward the receiver. Backward crosstalk is constituted in those traveling toward the source. The sum of the capacitive and inductive coupling is backward crosstalk, whereas the difference between the two is forward crosstalk. In high-frequency designs, Crosstalk is a major issue. This is because the edge rates of the active line, it is directly proportional.

The distance over which the two lines run parallel to each other and the proximity of the two lines are other factors. Hence as far apart as possible, the high-speed signals must be routed. Ideally, four times the trace width for these signals is the distance from center to center.

You must also keep the distance maintained by the lines running parallel to each other minimum. Between its reference plane and the line or introducing a co-planar structure, other solutions include minimizing the dielectric spacing. Between the traces, a ground plane is inserted here. The cross talk can also be decreased by as much as 50 percent by terminating the line on its characteristic impedance.

To wavelength, Frequency is inversely proportional. The frequency is higher if the wavelength is shorter. For example, the critical length is approximately 0.425 inches for 1GHz on a microstrip FR4-based PCB. Therefore, its total length is greater than 425 mils if you are routing a 1GHz signal. This trace must be impedance controlled. On the transmission line technologies and the material of the PCB, the critical length is also dependent.

The designer needs to consider the laminate properties when using RF circuits, such as the dielectric constant value and dissipation factor its variation. As compared to high-frequency laminates, FR4 has a higher dissipation factor. This indicates that when using FR4, insertion losses are much higher. These losses will increase as frequency rises and are also a function of frequency. Secondly, as much as 10%, the Dk value of FR4 can vary. This varies the impedance in turn. More stable frequency properties are present in High-frequency laminates.

Then the Dk value itself is there. The Dk value is tied to the size of the circuit elements when it comes to RF Microwave PCB circuits. So, by choosing a laminate with a higher Dk value, the designer may be able to decrease the size of the circuit.