Introduction to RF Design
Today there are many wireless technologies that utilize RF design ranging from mobile phones to satellite TV to wireless Internet connections and Bluetooth devices.
This article is intended to provide insight into how these technologies work and considerations during the design, development and verification process.
A Simplified Communication System
Let’s start with the basic blocks of a typical communications system (Figure 1). Basically any system will include an information source, information processing, the transmitter, transmission medium, a receiver, and then again information processing, and information destination. In the early days the signal was primarily voice, but of course today data is a major part of the communicated information.
The information processing is taking the source information and putting into a format which can be transmitted over whatever medium is required. In a wireless or RF system, we’re typically communicating through free space or air, as opposed to through a wire or a fiber network.
In the early days of RF design it was more common to have a one-way path for the RF signals. Radio and television are good examples. These signals were transmitted via large antennas to many radios and televisions. Today’s mobile phone is a great example of a more common device which is able to both transmit and receive signals. This paper focuses on transmit and receive portion of an RF communication system, discusses the key components, and test equipment to ensure performance verification.
Figure 1. Simplified Communications System Block Diagram
In the early days of wireless communications signals were basically sine waves. Remembering back to our school days that a sine wave can be represented with a frequency, an amplitude, and a phase.
Figure 2 shows two signals in the time domain. In terms of our communications the intent is to sendinformation from a source to a destination by modifying these sine waves. Today of course it is more common to have modern digital signals that are transmitting higher data rates with a signal that is much more complex.
Figure 2. Basic sine waves are used to carry information by varying their frequency, amplitude, and/or phase.
Typically, in terms of RF and microwave signals, we tend to look more in the frequency domain than in the time domain. Figure 3 shows a basic signal on a spectrum analyzer display. As the transmitted signals become more complex modulated signals or signals with more information put on them, the spectrum analyzer displays are excellent for understanding the multiple frequencies and modulation techniques.
Figure 3. Spectrum analyzers are excellent tools for evaluating transmitted RF and microwave signals.
Frequency is a critical parameter in RF design. Early applications were focused on audio frequencies, now referred to as analog systems. While there still are plenty of audio devices in use, there has been a broad increase of RF applications such as mobile phones, Bluetooth devices and Wi-Fi. Today, there are many commercial applications that use microwave and even millimeter wave frequencies.
Table 1 highlights the fact that as the frequency increases, the wavelength decreases. The effects of wavelength on a design can have implications on design complexity and end product costs. First let’s look at how to determine the wavelength:
Table 1. Examples of Wireless Applications and Their Wavelengths
So how does wavelength affect the RF design? When you consider the device size relative to wavelength the physical geometry may become an important consideration. From the signals in Figure 5 we see that the sine wave starts at zero, it goes up to a maximum, comes back down to zero, goes to a minimum, and come back up to zero across a single wavelength. From Table 1, we can see that at audio frequencies this happens across a distance of meters to hundreds of meters. The phase effects of moving through a typical analog device are therefore minimal. However, as you move into RF frequencies or higher, the effect of these phase variations becomes a design consideration. Certain circuit design techniques take advantage of /4 and /2 effects to optimize or cancel signals, which is really a way to minimize the effects of the topic in our next section.
Reflections and Interference
As the sine wave propagates down a transmission line or a cable, what happens when it hits a discontinuity or some change in impedance? This typically occurs at a connector or a solder point or even changes in the widths of a transmission line. Figure 5 shows a signal that is propagating down a transmission line and hitting a discontinuity represented by a green box. While some percentage of that signal will pass through the green box, some of the signal is reflected back.
This reflected signal will add in and out of phase depending on the phase of the signals. The red sine wave that’s reflecting back could be completely out of phase with the incoming signal and could actually cancel out the signal. These effects need to be analyzed and mitigated when designing your transmission lines and circuit boards at higher frequencies.
Figure 5. Discontinuities in transmission lines often cause reflections which create new signals that may interfere with and distort the desired signals.