The Importance of the RF Reference
The RF Reference is the node on a circuit’s schematic that we define as our reference voltage when designing an RF circuit or measuring its performance. For the most cost-effective EMC, all circuits (digital, analogue, switch-mode, etc.) should now be designed using RF techniques.
It is common practice to call the RF Reference ‘earth’ or ‘ground’, although it might instead be called ‘chassis’ or ‘frame’ in some applications, and in circuits it is usually the same structure as the 0V power supply distribution so it is often called 0V. But all these terms are potentially misleading, because what matters in EMC engineering is the impedance of the conductor structure that is being used as the reference for the RF signals or noises, at the frequencies that you wish to control. The RF Reference is very important indeed, for all filters that are more than simple series impedances. For filters to function as desired, the impedance seen by the return currents as they flow in the RF Reference must be much less than the impedance of any filter elements connected to that Reference.
So, if we are using a 10nF capacitor in an RC filter to shunt the RF noise to ‘ground’, and we want the RC filter to operate as close to its theoretical performance as possible up to 100MHz, we should realise that the reactive impedance of the capacitor (assuming a self-inductance of 1nH) at 100MHz is approximately 0.65Ω (almost all of which, incidentally, is due to its self-inductance). To create a ‘ground’ structure that has an impedance of much less than 0.65Ω at 100MHz is quite difficult, because a 10mm length of 1mm diameter wire or 1mm wide PCB trace has an impedance of about 6.3Ω at that frequency. Increasing the diameter of the wire, or the width of the PCB trace, reduces the impedance but not by a great deal – 10mm length of 4mm diameter wire or a 4mm wide trace would still be around 3.2Ω.
A great many earths, grounds, chassis, frames, and 0V systems are made of wire or PCB trace conductors, and designers assume that because they are labelled ‘earth’, ‘ground’ or ‘0V’ they actually are at earth, ground or 0V potential – but in fact they have such high impedances at RF that they have significantly different potentials at various points on their structures, depending on the RF currents flowing in them. Above a few tens of MHz, the only conductive structures that can achieve a low enough impedance to be useful as a reference for circuits and especially for filters, are metal areas or planes – which is why RF References are quite often called RF Reference Planes.
The circuits that use a plane as their RF Reference must be located much closer than one-tenth of a wavelength (λ/10) to it, ideally λ/100 or even less – at the highest frequency to be controlled. This helps prevent the connections to the plane from behaving as resonating antennas with impedances possibly in the hundreds of Ω, instead of the plain old low-impedance conductors that they look like to our eyes. At 1GHz this would mean a maximum spacing of 30mm, and better EMC would be achieved by being much closer than that, ideally 3Nm? or less.
Where a circuit is shielded by placing it in a metal box, it can use one or more walls of the box, and/or the rear, base and top as its RF Reference. Generally, this would still be called an RF Reference Plane, despite that fact that they are different sides of a metal box. An important consideration in the design of the structure of an RF Reference is that surface currents must be able to flow freely where they will, all over the area being used. Surface currents are discussed later in the section on Skin Effect.
Many electronic engineers are familiar with the idea of ‘single point earths/grounds’ – sometimes called ‘star earths’ or ‘star grounds’. In such designs the voltage reference is a single physical point, and everything that needs to use it connects to it by a conductor (a wire or PCB trace). Analysing these conductors in terms of impedance, or in terms of their likelihood of becoming resonating antennas, as discussed in the above paragraphs, quickly shows us that single-point or star conductive structures are no use to us for EMC – their conductors are simply too long.
The continued shrinking of silicon die sizes means that even commonplace digital glue-logic (e.g. HCMOS) now generates significant noise emissions at frequencies up to 1GHz, and modern FPGAs and microprocessors can be very much worse for EMC – both in level and frequency – than such glue logic ICs. To stand any chance of controlling such frequencies requires lengths of wire, PCB traces or via holes, that are no more than a few millimetres long, preferably <1mm. Using flat braid straps instead of round conductors simply raises the useful frequency by a little, but not by enough to control hundreds of MHz. So single-point earthing/grounding techniques are now only of interest to students of the history of technology, regardless of the power or signals involved. All circuits and interconnections now suffer from RF noise that is coupled into them from digital, switch-mode and/or wireless circuits inside the same product, and they also suffer from RF noise coupled from nearby cables and ambient EM fields in their environments.
These coupled noises can cause any circuit or interconnection to be source of RF emissions, and/or a victim of interference, and this is true even for DC instrumentation and low-frequency analogue signals such as audio.
Despite the fact that the design of the RF Reference Plane is crucial to cost-effective EMC design, many engineers (me included) still tend to refer to ‘earth’ ‘ground’ or 0V, thereby often leading to confusion and miscommunication with people who think a length of wire can be part of an ‘earth’ structure as long as it has green/yellow insulation. So it is important to look beyond the terms that are being used to identify the physical structure that will be used as the RF Reference Plane, or to create it if it is not yet there.
