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Setbacks and Solutions in RF PCB Design

Lots of uncertainties are available on RF (radio frequency) PCB (printed circuit board) design that is therefore described as "black art". Generally speaking, when it comes to circuits at frequency below microwave (including low frequency and low-frequency digital circuit), careful layout is the guarantee of first-time success in circuit design with all the design principles mastered. When it comes to frequency at above microwave and high-frequency PC-level digital circuits, however, two to three versions of PCB are capable of ensuring circuit quality. As far as RF circuits are concerned at a frequency above microwave, however, more versions of PCB design are needed for constant improvement. Therefore, many difficulties are certain to be met during RF circuit design.

The Most Commonly-Seen Problems in RF Circuit Design

• Interference between Digital Circuit Module and Analog Circuit Module

When analog circuit (RF circuit) and digital circuit work independently, it's most possible that they can work perfectly. But as soon as they are mixed together on the same circuit board with the same power supply depended on, the whole system will possibly become instable because digital signals frequently swing between ground and positive power supply (>3V) and period will be very short at a scale of nanosecond. Due to larger amplitude and shorter switching time, all the digital signals will contain high-frequency elements that are independent to switching frequency. In analog sections, the voltage is usually less than 1μV from radio tuning loop to radio equipment receiver. Therefore, difference between radio tuning loop and RF signals can reach 120dB. Evidently, if digital signals and RF signals fail to be neatly separated, weak RF signals will possibly suffer from damage. As a result, radio equipment workability will become deteriorated or it can't even work.

• Noise Interference of Power Supply

RF circuit is quite sensitive to noise, which is especially true for glitch voltage and other high-frequency harmonic wave. Microcontroller will suddenly absorb majority of current within each internal clock period, which is because all modern microcontrollers are manufactured with the application of CMOS technique. Therefore, suppose that a microcontroller runs at an internal clock frequency of 1MHz and it will then extract current from power supply at such a frequency. If suitable power decoupling isn't applied, voltage glitch will be aroused on power lines. When voltage glitches arrive at power pins of RF circuit, failure will possibly be caused if serious.

• Unreasonable GND

If GND is unreasonably set for RF circuit, some strange results could be generated. When it comes to digital circuit design, even if GND isn't available, most digital circuit functions are capable of being excellently implemented. As far as RF is concerned, however, even a short ground line will play an equivalent role as inductor. It is known that inductance with 1nH is compatible with a length of 1mm, based on which it can be roughly figured out that inductive reactance of a PCB with length of 10mm should be approximately 27Ω. If GND isn't applied, most ground lines will be so long that circuit fails to feature characteristics based on design.

• Radiated Interference by Antenna on Other Analog Circuits

In PCB layout design, other analog circuits are also available on the board. For example, lots of circuits contain analog-digital converter (ADC) or digital-analog converter (DAC). The high-frequency signals transmitted by RF transmitter perhaps arrive at analog input terminal of ADC because any circuit line will transmit or receive RF signals like antenna does. If input terminal of ADC is inappropriately processed, RF signals will possibly become self-excited within ESD diode of ADC input, which then arouses ADC deviation.

RF Circuit Design Principles and Scheme

• Definition of RF Layout

As RF layout is designed, the following general principles should be conformed to first:
① High power amplifiers (HPAs) and low noise amplifiers (LNAs) should be separated as much as possible. Briefly speaking, high-frequency RF transmission circuits are placed far away from low-frequency RF receiving circuits.
② At least a complete ground should be available on high-frequency area on PCB board and it's best that no through holes take place on that. The larger copper foil area is, the better.
③ It's equivalently significant for circuits and power to go through decoupling.
④ RF output should be far away from RF input.
⑤ Sensitive analog signals should be as far away high-speed digital signals and RF signals as possible.

• Design Principles of Physical Partitioning and Electrical Partitioning

Partitioning can be classified into physical partitioning and electrical partitioning. The former is mainly concerned with component layout, orientations and shielding while the latter can be further classified into power distribution, RF routing, sensitive circuit, signals and ground partitioning.

a. Physical Partitioning Principle

Component layout principle. Component layout plays an essential role in contributing to a well-performed RF design. The most effective technology is to first fix components that are placed along RF path and get their orientations modified so that RF path can be minimized with input far away from output and high-power circuits and low-power circuits separated as much as possible.

PCB lamination design principle. The most efficient circuit lamination method is to arrange main ground plane at the second layer below the first plane and to arrange RF traces at the first plane. The size of through holes on RF path should be reduced to the minimum, which can reduce path inductance and decrease the number of cold solder joints on the main ground. Furthermore, less RF energy will be leakaged to other areas within lamination.

RF components and RF tracing principle. Within physical space, linear circuits like multi-stage amplifiers are capable of separating all RF areas but duplexer, mixer and mid-frequency amplifier/mixer often lead to mutual interference between multiple RF/IF signals. Therefore, this type of influence should be carefully minimized. RF/IF traces should be crossed and a ground should be left between them. Correct RF path is quite important to PCB performance, which is why component layout accounts for majority of time in cell phone PCB design.

b. Electrical Partitioning Principle

Power transmission principle. DC in most circuits of cell phones is usually quite low so trace width needn't be carefully considered. However, a trace with large current whose width is as wide as possible must be independently designed for the power supply of high-power amplifiers so as to reduce transmitted voltage to the minimum. To avoid too much current loss, multiple through holes should be applied to transmit current from one plane to another.

Power decoupling of high-power devices. If complete coupling fails to be achieved at power pins of high-power amplifier, high-power noise will be radiated to the whole board with many issues generated. High-power amplifier grounding is very essential and a metal shield cover is usually needed for its design.

RF input/output segregation principle. For most situations, it's equivalently essential to guarantee that RF output be far from RF input, which also works for amplifier, bumper and filter. In the worse situations, if input of amplifier and bumper is returned to their input terminal at an agreeable phase and amplitude, self-excited vibration may be caused. In the best situations, they will be able to stably work at any temperature and voltage. As a matter of fact, they may become unstable and add noise and intermodulation signals to RF signals.

All in all, RF circuit features skin effect and coupling effect due to its distributed parameter circuit, which makes it different from low-frequency circuit and DC. As a result, the issues discussed above should be specially emphasized during RF circuit PCB design so that circuit design can be effective and accurate.

Helpful Resources
Guidelines for RF and Microwave PCB Design
PCB Design for Radio Frequency Circuit and Electromagnetic Compatibility
PCBCart offers Radio Frequency PCB fabrication service

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