Today, RF circuitry is crammed into a large variety of commercial products. Most of these are handheld wireless devices for medical, industrial and communications applications. NCAB’s Harry Kennedy explains what to keep an eye on when designing the circuit boards.
The RF frequency range is usually from 300 kHz to 300 GHz, with microwave being anything above 300 MHz. There’s a considerable difference between RF and microwave circuitry versus typical digital and analog circuits. In essence, RF signals are very high-frequency analog signals. Therefore, unlike digital, at any point in time, an RF signal can be at any voltage and current level between the minimum and maximum limits. Moreover, a single band of signal can be very narrow or very wide and carried upon a very high-frequency carrier wave.
RF PCB design is also very much different and difficult, compared to high-speed digital-signal board design. When handling RF boards, there are many new challenges for PCB designers.
First, RF is far more sensitive to noise, incurring ringing and reflections, which must be treated with great care. The noise can be dealt with by properly terminating the signal, thereby solving the ringing and reflection issues. Another method is to optimize the return path with proper ground.
Second, impedance matching is extremely critical for RF. The higher the frequency, the smaller the tolerance. Practically, if the total length of the trace from the driver to the receiver is greater than 1/16th of the wavelength of the signal, impedance control of that trace is required – 1/16th of the wavelength is called the signal’s critical length. If you’re routing a 1 GHz signal and its total length is greater than 425 mils, that trace needs to be impedance controlled. For example, 50 ohms out from the driver, 50 ohms during transmission and 50 ohms into the receiver.
Third, the return loss must be minimized. At very high microwave frequencies, the return signal takes the path of least inductance. As a result, without good PCB design, it will go through power planes, through the PCB’s multi-layers or take some other route, and it will no longer be impedance controlled. Ground planes underneath the signals are good at providing an impedance-controlled path. Therefore, there should be no discontinuities in the plane underneath the signal all the way from the driver to the receiver. Ground planes help minimize not only ground loop currents but also RF leakage into circuit elements.
Fourth, crosstalk is a major issue in high-frequency designs. This is because crosstalk is directly proportional to the edge rates of the active line. In this case, the coupled energy from the active line will be superimposed on the victim line. When the board densities increase, the problem of crosstalk becomes more critical.
As a solution, always leave adequate space around the signal trace for smooth bends and isolation of the RF signal. Keep all the traces coming out of or going into the transmitter and receiver modules as small as possible. The high-speed signals should be routed as far apart as possible. The length that lines run parallel to each other should also be kept to a minimum. All these measures will reduce crosstalk. Other solutions include reducing the dielectric spacing between the line and its reference plane and introducing a co-planar structure, where a ground plane is inserted between the traces. Terminating the line on its characteristic impedance can also reduce the crosstalk by as much as 50 percent.
There are other signal losses as well. There’s the skin effect loss, more specifically the skin effect loss on the trace of a signal. Dielectric loss is a companion since both can be created at extremely high frequencies, when electrons flowing through a conductor bounce back and forth with the electrons on the FR4 PCB substrate, for example. During this interaction, some of the energy from the electrons flowing through the conductor is transferred to the electrons on the FR4. Consequently, that energy is converted to heat and subsequently lost.
In such instances, for extremely high-frequency microwave circuits, it’s best to use polytetrafluoroethylene Teflon (PTFE). These laminates have a dissipation factor of around 0.001 – compared to 0.02 for FR4. Using gold body on RF circuits can also greatly reduce skin losses.
When using RF circuits, PCB designers need to consider the laminate properties, such as the dissipation factor and dielectric constant value and its variation. FR4 has a higher dissipation factor than high-frequency laminates like Rogers and Nelco. This means that insertion losses are much higher when using FR4. These losses are also a function of frequency and will increase as the frequency rises.
The dielectric constant value of FR4 can vary as much as 10 percent. This in turn causes the impedance to vary. High-frequency laminates have more stable frequency properties. Then there’s the dielectric constant value itself. When it comes to microwave circuits, this value is tied to the size of the circuit elements, so designers may be able to decrease the size of the circuit by choosing a laminate with a higher dielectric constant value.
To create better designs and improve the anti-interference for high-frequency PCBs, there are a couple of tips engineers can take to heart. First, use inner layers as power ground layers. This will have the effect of shielding and even decreasing spurious inductance while shortening the length of the signal wire reduces cross-interference between signals.
Turning the circuit layout 45 degrees will help reduce high-frequency signal emission and coupling. The shorter the layout length, the better, and the less the better for through holes. The layout between layers should be in a vertical direction, on the top layer in a horizontal direction and on the bottom layer in a vertical direction. This will help reduce signal interference. Increasing copper on the ground layer is helpful, too.
Packaging important signal traces can obviously improve the signal’s anti-interference ability. Of course, we can also package interference sources to avoid interference on other signals. For the layout of signal traces, avoid loops and use a chrysanthemum link instead.
In the power section of integrated circuits, it’s important to bridge the decoupling capacitor. Make the RF signal 50 ohms and always lay out the RF first. Last but definitely not least, isolation is key.