Introduction to RF Design

CHAPTER-1

Introduction to RF Design

Radio-frequency (RF) electronics differ from other electronics because the higher frequency circuit operation is hard to understand due to

1.   Stray capacitance is the capacitance that exists between conductors of the circuit, between conductors or components and ground, or between components. (VHF TV Tuners)

2.   Stray inductance is the normal inductance of the conductors that connect components, as well as internal component inductances.

3.   Skin effect refers to the fact that ac flows only on the outside portion of the conductor, while dc flows through the entire conductor. As frequency increases, skin effect produces a smaller zone of conduction and a correspondingly higher value of ac resistance compared with dc resistance.

4.   The signals at Radio Frequencies easily radiate both from the circuit and within the circuit.

5.   Coupling effects between elements of the circuit, between the circuit and its environment, and from the environment to the circuit become a lot more critical at RF. Interference and other strange effects are found at RF that are missing in dc circuits and are negligible in most low frequency ac circuits.

The electromagnetic spectrum

When an RF electrical signal radiates, it becomes an electromagnetic wave that includes not only radio signals, but also infrared, visible light, ultraviolet light, X-rays, gamma rays, and others.

The electromagnetic spectrum is broken into bands for the sake of convenience and identification. The extremely low frequency (ELF) range includes ac power-line frequencies as well as other low frequencies in the 25- to 100-hertz (Hz) region. The U.S. Navy uses these frequencies for submarine communications.

Fig.1: The Electromagnetic Spectrum from VHF to X-ray. The RF region covers from less than 100KHz to 300GHz.

The amplitude-modulated (AM) broadcast band (540 to 1630 kHz) spans portions of the LF and MF bands.

The high-frequency (HF) region, also called the shortwave bands (SW), runs from 3 to 30 MHz. The VHF band starts at 30 MHz and runs to 300 MHz. This region includes the frequency-modulated (FM) broadcast band, public utilities, some television stations, aviation, and amateur radio bands.

The ultrahigh frequencies (UHF) run from 300 to 900 MHz and include many of the same services as VHF.

The microwave region begins above the UHF region, at 900 or 1000 MHz, depending on source authority.

Microwaves almost become a separate topic in the study of RF circuits because at these frequencies the wavelength approximates the physical size of ordinary electronic components. Thus, components behave differently at microwave frequencies than they do at lower frequencies.

At microwave frequencies, a 0.5-W metal film resistor, for example, looks like a complex RLC network with distributed L and C values  and a surprisingly different R value.

Units in RF Design

In accordance with standard engineering and scientific practice, all units in this book will be in either the CGS (centimeter-gram-second) or MKS (meterkilogram- second) system unless otherwise specified.

 

Wavelength and frequency

For all wave forms, the velocity, wavelength, and frequency are related so that the product of frequency and wavelength is equal to the velocity. For radiowaves, this relationship can be expressed in the following form:

eq1

Where λ – Wavelength in m, f – Frequency in Hz, ε – Dielectric Constant of the Propagation Medium, c – Velocity of Light.

The dielectric constant (ε ) is a property of the medium in which the wave propagates.

The value of is defined as 1.000 for a perfect vacuum and very nearly 1.0 for dry air (typically 1.006). In most practical applications, the value of ε in dry air is taken to be 1.000. For media other than air or vacuum, however, the velocity of propagation is slower and the value of ε relative to a vacuum is higher. Eg.: Teflon can be made with values from about 2 to 11. eq2

 

Microwave letter bands

During World War II, the U.S. military began using microwaves in radar and other applications. For security reasons, alphabetic letter designations were adopted for each band in the microwave region. T1 T2 T3 

          

Skin effect

Reasons for ordinary lumped constant electronic components to not work well at microwave frequencies are

1.   Component size and lead lengths approximate microwave wavelengths.

2.   The distributed values of inductance and capacitance become significant at these frequencies.

3.   The phenomenon of skin effect. While dc current flows in the entire cross section of the conductor, ac flows in a narrow band near the surface. Current density falls off exponentially from the surface of the conductor toward the center. At the critical depth (δ, also called the depth of penetration), the current density is 1/e = 1/2.718= 0.368 of the surface current density. 

Fig.2: In ac Circuits, the current flows only in the outer region of the    conductor. This effect is frequency-sensitive and it becomes a serious consideration at higher RF frequencies.
eq3

Where δ – Critical Depth, f – Frequency in Hz, μ – Permeability in H/m, σ – Conductivity in mhos/m.

INTRODUCTION TO RF AND WIRELESS TECHNOLOGY

Phone sets with increasingly higher performance require reducing power consumption and cost by nearly 30% every year. An “omnipotent” wireless terminal can handle voice, data and video as well as computing power – Personal Communication Services (PCS).

COMPLEXITY COMPARISON

Fig.3: FM Transmitter                           Fig.4: FM Receiver

In FM Transmitter, Q1 operates as both an Oscillator and a frequency modulator.i.e., the audio signal produced by the microphone varies the bias voltage across the varactor diode D1, thereby modulating the frequency of oscillation. In FM Receiver, Q1 operates as both an Oscillator and a demodulator.

Fig.5: RF Section of a Cell Phone

DESIGN BOTTLENECK

Phone contains small fraction operating in RF Range and the rest performing Low-Frequency “Baseband” analog and digital signal Processing.

 

Fig.6: RF and Baseband processing       Fig.7: Disciplines required in RF

          in a Transceiver.                                   Design

RF DESIGN HEXAGON

RF Circuits must process analog signals with a wide dynamic range at high frequencies. RF Circuits cannot be manufactured using IC Technology as it requires external components such as inductors that are difficult to fabricate.

Fig.8: RF Design Hexagon

DESIGN TOOLS

Computer Aided Analysis and Synthesis Tools are still in developing Stage due to the following issues.

1.   As Nonlinearity, Time Variance and Noise in RF Circuits usually require studying the spectrum of signals, but the standard ac analysis in SPICE uses only Linear, Time Invariant models. Hence circuits are simulated in time domain and transformed to frequency domain to obtain spectrum, but time domain simulation has to run for a long period to resolve closely spaced frequency components. Also spectral averaging techniques may be necessary if random noise is used in time domain analysis.

2.   The external components like surface acoustic wave (SAW) filters, used in both transmit and receive paths exhibit input and output impedances that can be characterized by only S-Parameters (essentially a table of numbers), cannot be modeled by typical devices in SPICE. Hence modeling such circuits with RLC networks provides a first order approximation and it may not predict effects such as instability and impedance mismatch.

APPLICATIONS

1.   Pagers

2.   Cell Phones

3.   WLANs – use frequency bands around 900MHz and 2.4GHz. WLAN Tranceivers can provide mobile connectivity in offices, hospitals, factories, etc instead of using cumbersome wired networks. Prominent features are Portability and reconfigurability.

4.   GPS -   To obtain one’s location and direction. Operates in 1.5GHz for automobile industry as low-cost hand held products.

5.   RFIDs – RF Identification systems are small low cost tags attached to objects or persons to track their position. Luggabe bags in airports to troops in military operations. Advantages – Low power consumption, long tag’s lifetime, single small battery. 900MHZ to 2.4GHz.

6.   Home Satellite Network – operate in 10 GHz, require additional dish antenna and a receiver to a television set.

ANALOG AND DIGITAL SYSTEMS

 

Fig.9: Block Diagram of a generic analog RF system Transmitter and Receiver.

In analog architecture, in the transmit path, the signal generated by the microphone modulates a HF carrier and the resultant signal is amplified and buffered to drive the antenna. In the receive path, the signal is amplified by a LNA, the spectrum is translated to a LF by a downconverter (mixer) to facilitate subsequent demodulation, and the demodulated output is amplified to drive the speaker.

Fig.10: Block Diagram of a generic digital RF system with transmitter and receiver.

In digital architecture, Voice Compression is used to reduce bit rate and Bandwidth. Coding and Interleaving to detect and minimize errors. Pulse Shaping is used as rectangular pulses are usually not optimum for modulation.

The performance of transceivers can be quantified in terms of maximum distance across which they operate while providing satisfactory reception which is determined by both the power delivered to the antenna and the sensitivity of the receiver due to noise of LNA.

Practically, many different transceivers operate simultaneously, often in close proximity, thereby interfering with each other. Also the communication medium is quite complex: reflections from buildings and other obstacles can result in destructive interference at a given point, suppressing the received signal strength to undetectable levels. Hence in such an environment, signal processing in the digital transmitter achieves higher performance than that of the analog system.

 In RF domain, the spectrum is not centered around zero frequency. The Modulator, Power Amplifier, Low Noise Amplifier and the Downconverter operate in RF range.

CHOICE OF TECHNOLOGY

Performance, Cost and Time to Market are three critical factors that influence the choice of technology in competitive RF industry.

Issues such as level of integration, form factor and prior experience play an important role in the decisions made by the designers.

Technologies used in RF:

1.   GaAs

2.   Silicon Bipolar

3.   BiCMOS Technology

GaAs Field effect and heterojunction devices are low yield, high power and high cost option used Power Amplifies and front-end switches.

GaAs processes offer useful features like higher (breakdown voltage)(cutoff Frequency) product, semi-insulating substrate, and high-quality inductors and capacitors.

Silicon (Bipolar) devices in a VLSI Technology can potentially provid both higher levels of integration and lower overall cost, as demonstrated in complex circuits such as frequency synthesizers.  

CMOS Technology devices have high transit frequencies, e.g., tens of gigahertz in the 0.35μm generation. CMOS technology resolves a number of practical issues like substrate coupling of signals that differ in amplitude by 100dB, parameter variation with temperature and process, and device modeling for RF operation.