The following is excerpted from Chapter 8 from a new edition of the book, RF Circuit Design, 2e by Christopher Bowick. (If you order a copy of this book before March 30, 2008 you can receive additional 20% off. Visit www.newnespress.com or call 1-800-545-2522 and use code 91603. )
Click here for another excerpt: "What's in an RF Front End?"
Click here for Radio Architectures, Pt 2: Receivers, LOs, and Mixers.
Click here for " Radio Architectures, Pt 3: Intermodulation and Intercept Points"
Click here for "Radio Architectures, Pt 4: Sensitivity, Noise, Front End Amps"
Click here for "Radio Architectures, Pt 5: ADCs and Receivers"
The fundamental operation of an RF front end is fairly straightforward: it detects and processes radio waves that have been transmitted with a specific known frequency or range of frequencies and known modulation format. The modulation carries the information of interest, be it voice, audio, data, or video.
The receiver must be tuned to resonate with the transmitted frequency or frequencies in order to detect them. Those received signals are then filtered from all surrounding signals and noise and amplified prior to a process known as demodulation, which removes the desired information from the radio waves that carried it.
These three stepsfiltering, amplification and demodulationdetail the overall process. But actual implementation of this process (i.e., designing the physical RF receiver printed-circuit board (PCB)) depends upon the type, complexity, and quantity of the data being transmitted. For example, designing an RF front end to handle a simple amplitude-modulated (AM) signal requires far less effort and hardware (and even software) than building an RF front end for the latest third-generation (3G) mobile telecommunications handset.
Because of the enhanced performance of analog components due to IC process improvements and decreasing costs of more powerful digital-signal-processing (DSP) hardware and software functions, the ways that different RF front-end architectures are realized has changed over the years. Still, the basic requirements for an RF front end, such as the frequency range and type of carrier to be received, the RF link budget, and the power, performance, and size restrictions of the front-end design, remain relatively the same in spite of the differences in radio architectures.
Let's start by looking at the simplest of radio architectures or implementations.
AM Detector Receivers
One of the basic RF receiver architectures for detecting a modulated signal is the amplitude modulation (AM) detector receiver (see Fig. 8-2). The name comes from the fact that information like speech and music could be converted into amplitude (voltage)
modulated signals riding on a carrier wave. Such an RF signal could be de-modulated at the receiving end by means of a simplediode detector. All that is needed for a basic AM receiverlike a simple crystal radiois an antenna, RF filter, detector, and (optional) amplifier to boost the recovered information to a level suitable for a listening device, such as a speaker or headphones.
The antenna, which is capacitive at the low frequencies used for AM broadcasting, is series matched with an inductor to maximize current through both, thus maximizing the voltage across the secondary coil. A variable capacitance filter may be used to select the designed frequency band (or channel) and to block any unwanted signals, such as noise. The filtered signal is then converted to demodulate the AM signal and recover the information. Fig. 8-3 represents a schematic version of the block diagram shown in Fig. 8-2.
The heart of the AM architecture is the detector demodulator. In early crystal radios, the detector was simply a fine metal wire that contacted a crystal of galena (lead sulfide), thus creating a point contact rectifier or "crystal detector." In these early designs, the fine metal contact was often referred to as a "catwhisker." Although point-contact diodes are still in use today in communication receivers and radar, most have been replaced by pn-junction diodes, which are more reliable and easier to manufacture.
For a simpleAM receiver, the detector diode acts as a half-wave rectifier to convert or rectify a received AC signal to a DC signal by blocking the negative or positive portion of thewaveform (see Fig. 8-4).A half-wave rectifier clips the input signal by allowing
either the positive or negative half of theAC wave to pass easily through the rectifier, depending upon the polarity of the rectifier.
A shunt inductor is typically placed in front of the detector to serve as an RF choke. The inductor maintains the input to the detector diode at DC ground while preserving a high impedance in parallel with the diode, thus maintaining the RF performance.
In a simple detector receiver, the AM carrier wave excites a resonance in the inductor/tuned capacitor (LC) tank subcircuit. The tank acts like a local oscillator (LO) to the current through the diode is proportional to the amplitude of the resonance and this gives the baseband signal (typically analog audio).