We have laid out hundreds of RF boards, and while each one is different there are several good guidelines we like to follow when laying out RF boards. Here are some tips we wanted to share with you:
General Overview:
- Lay out the important stuff first
- Keep traces short
- Ground planes
- Decoupling caps
- guard rings
- shielding traces
More Specific:
- If signal traces are constant width and height above the ground plane, and are properly terminated, then their characteristic impedance is more well-behaved and may be calculated.
- Inputs and outputs should be far apart, so that RF energy will not leak back from output to input. stages should line up, rather than snake around.
- Decouple the RF/Analog parts of the circuit from the DC parts of the circuit.
- When possible DC ground should be separated from the RF ground and only connected at 1 point. Be careful not to have ground loops.
- Analog ground is a required.
- The analog power should come from its own source, if possible. This is often done by low drop out linear regulators and one or more switching regulator for the entire system.
- The analog ground should be connected to digital at a single point, preferably with a ferrite bead and filter caps to reduce analog to digital ground crosstalk
- For high order QAM or analog video runs on the PCB, shielded point to point connections should be used for electrically long runs >1 wavelength.
- Whitepapers exist for good reason, if available analyze the whitepaper design. Often these papers provide insight that data sheets do not.
- For signals leaving the PCB on cabling, use backmatch resistors to reduce standing wave reflections analog systems need these, and other components to ensure matching impedance.
- Understand what circuits and systems need to be matched to a specific characteristic impedance, does the system need to be run as a balanced system or unbalanced.
- Via stitching and guard rings are used in RF designs to create a via barrier. This helps to keep random electromagnetic energy from effecting other systems on and off the board.
- Know when to use balanced vs unbalanced RF feed lines, and what the proper impedance matching is for the circuit you are using.
- If the electrical length of a trace is longer then 1/10 wavelength, you need to treat it as a transmission line. At a minimum, this means you must terminate with a resistor matched to the impedance of the circuit. The resistor will keep the system from ringing, this resistor may have to vary depending on the final system layout and design.
- Be careful when increasing the drive strength of a signal, this can cause ringing to appear as the new circuit will have more coupled energy. i.e. A higher drive strength can turn a line that does not ring into one that does.
- If a line is meant to be open depending on the circuit might ring. To stop this a system can be terminated with a few pF and the characteristic impedance of the circuit (50 ohms). This avoids the problem of a terminator directly across a logic line.
- Small valued resistors in small tracks can help to prevent a circuit from self-oscillating. This is often useful for wide band designs.
- Long traces can radiate, to prevent this a trace should be run as a balanced line. This will help as the far field is zero due to the EM fields cancelling one another out. Coax and balanced stripline work well for this type of design.
- Substantial radiation is not often caused by microstrip traces radiating, often the cause of radiation problems is due to when the signal leaves a low impedance realm near the reference plane. Areas of concern are often connectors, cables, and transformers.
- An RF circuit will always radiate. Picture the signal being guided by the trace, not existing inside the trace. The signal on one trace can jump onto another trace if they are close enough. This is called coupling and how some of our best coupling designs work. To minimize coupling, separate traces by at least 2*(distance to the reference plane). A via walls should / can be used to ensure two traces are more isolated from each other.
- Make sure all traces are terminated into something.
- Avoid any and all discontinuities. As a professor once put it: You can think of a trace as a train track. If a train is going down the track and it hits a hard turn (i.e. 70° – 90°), the train is not going to follow the track, the train is the high frequency signals and it is important that those go where you want them to go.
- Metal enclosures are important! All circuits radiate, they will radiate either in the near-field or in both near and far-field. If a signal does radiate away from a circuit, it should be contained with a metal enclosure or absorber. Boards without a solid top metal layer are usually covered with a metal enclosure.
- Lumped elemement device must be less than 1/10th of a wavelength at the maximum frequency.
- A microstrip board height should never exceed 1/10 of a wavelength at the maximum frequency of it usage.
- For microstrip and stripline curved lines, use a minimum radius of three line widths. At frequencies above 6GHz it is best to use five line widths for minimum radius. However it is better to use a optimized miter bend.
- Keep the ground poor greater than three line widths away from a microstrip line to insure minimal loss and any impact to line impedance.
- A circuit can be matched with transmission lines. A wider line effectively looks shorter and more capacitive, a narrow line looks inductive. These can be used to cancel out the inductance or capacitance of a device.
- For common PCB designs skin depth should not be of concern for frequencies above 200MHz and half-ounce copper or thicker.
