وبلاگ آموزش کامپیوتر،الکترونیک

سرتیتر صفحه جدید

RF Components and Circuits

What are the ‘radio frequencies’?

The radio frequencies (RF) are, roughly speaking, those which are above human hearing,

and extend into the microwave spectrum to the edge of the infrared region. That means

the RF frequencies are roughly 20 000 Hz to many, many gigahertz. In this book, we will

assume the radio frequencies are up to about 30 GHz for practical purposes.

There are radio frequencies below 20 kHz, however. In fact, there are radio navigation

transmitters operating in the 10 to 14 kHz region. The difference is that the waves

generated by those stations are electromagnetic waves, not acoustical waves, so humans

cannot hear them.

Why are radio frequencies different?

Why are radio frequencies different from lower frequencies? The difference is largely due

to the fact that capacitive and inductive stray reactances tend to be more significant at

those frequencies than they are at lower frequencies. At the lower frequencies, those stray

or distributed reactances exist, but they can usually be ignored. Their values do not

approach the amount required to establish resonance, or frequency responses such as high

pass, low pass or bandpass. At RF frequencies, the stray or distributed reactances tend to

be important. As the frequency drops into the audio range (1–20 kHz), and the ultrasonic

range (20–100 kHz) the importance of stray reactances tends to diminish slightly.

What this book covers

We will look at a number of different things regarding RF circuits. But first, we will take

a look at signals and noise. This sets the scene for a general look at radio receivers. Most

radio frequency systems have one or more receivers, so they are an important type of

4 RF Components and Circuits

circuit. They also include examples of many of the individual RF circuits we will be

looking at. So Part 1 is an introduction.

It is hard to say very much about RF circuits without talking about the components, but

the special RF components don’t make much sense unless you already know something

about the circuits. So which should come first? (Think of chickens and eggs!) The way

round this is to put circuits in Part 2 and components in Part 3, but then think of them as

running in parallel rather than one after the other. So you can swap between them, but

because this is a paper-based book we have to print Part 3 after Part 2.

Part 2 looks at the various types of RF circuits in roughly the order a radio signal sees

them as it goes through a normal superhet receiver. Many of these circuits are also used

in transmitters, test equipment and other RF stuff but there isn’t enough space to go into

all that in this book.

Part 3 mainly deals with the sort of components you won’t see in lower frequency or

digital circuits. Radio frequency is a bit unusual because some components, mainly

various types of inductors, can’t always be bought ‘off the shelf’ from catalogues but

instead have to be made from parts such as bits of ferrite with holes in them and lengths

of wire. So I will give you design information for this.

We finish up by looking at some RF measurements and techniques in Part 4.

One big important RF topic has been left out of this book – antennas. This is because

doing this properly would make the book much too long, but a quick look would not tell

you enough information to be useful. You can learn about antennas from my book

Antenna Toolkit, second edition (published by Newnes/RSGB, Oxford 2001). This includes

a CD-ROM to help you with antenna calculations.

Now, let’s get started . . .

2 Signals and noise

Types of signals

The nature of signals, and their relationship to noise and interfering signals, determines

appropriate design all the way from the system level down to the component selection

level. In this chapter we will take a look at signals and noise, and how each affects the

design of amplification and other RF circuits.

Signals can be categorized several ways, but one of the most fundamental is according

to time domain behaviour (the other major category is frequency domain). We will therefore

consider signals of the form v = f(t) or i = f(t). The time domain classes of signals include:

static, quasistatic, periodic, repetitive, transient, random, and chaotic. Each of these categories

has certain properties that can profoundly influence appropriate design decisions.

Static and quasistatic signals

A static signal (Fig. 2.1A) is, by definition, unchanging over a very long period of time

(Tlong in Fig. 2.1A). Such a signal is essentially a DC level, so must be processed in low

drift DC amplifier circuits. This type of signal does not occur at radio frequencies because

it is DC, but some RF circuits may produce a DC level, e.g. a continuous wave, constant

amplitude RF signal applied to an envelope detector.

The term quasistatic means ‘nearly unchanging’, so a quasistatic signal (Fig. 2.1B) refers

to a signal that changes so slowly over long times that it possesses characteristics more

like static signals than dynamic (i.e. rapidly changing) signals.

Periodic signals

A periodic signal (Fig. 2.1C) is one that exactly repeats itself on a regular basis. Examples

of periodic signals include sine waves, square waves, sawtooth waves, triangle waves,

and so forth. The nature of the periodic waveform is such that each waveform is identical

at like points along the time line. In other words, if you advance along the time line by

exactly one period (T), then the voltage, polarity and direction of change of the waveform

will be repeated. That is, for a voltage waveform, V(t) = V(t + T).

Repetitive signals

A repetitive signal (Fig. 2.1D) is quasiperiodic in nature, so bears some similarity to the

periodic waveform. The principal difference between repetitive and periodic signals is

seen by comparing the signal at f(t) and f(t + T), where T is the period of the signal. Unlike

periodic signals, in repetitive signals these points might not be identical although they

will usually be similar. The general waveshape is nearly the same. The repetitive signal

might contain either transient or stable features that vary from period to period.

Transient signals and pulse signals

A transient signal (Fig. 2.1E) is either a one-time event, or a periodic event in which the

event duration is very short compared with the period of the waveform (Fig. 2.1F). In

terms of Fig. 2.1F, the latter definition means that t1 <<< t2. These signals can be treated

as if they are transients. In RF circuits these signals might be intentionally generated as

pulses (radar pulses resemble Fig. 2.1F), or a noise transient (Fig. 2.1E).

Fourier series

All continuous periodic signals can be represented by a fundamental frequency sine wave,

and a collection of sine or cosine harmonics of that fundamental sine wave, that are

 

summed together linearly. These frequencies comprise the Fourier series of the waveform.

The elementary sine wave (Fig. 2.2) is described by:

v = Vm sin(_t) (