SOURCES
Block Diagram
Figure 3-19. Block diagram of an oscillator.
The last of the major building blocks is called a source or an oscillator (see Figure 3-19). An oscillator which provides one of the inputs to a mixer is referred to as a local oscillator or LO. (I guess this is to distinguish it from a remote oscillator, which is located on Venus.) Now you know what the LO port in a mixer does: it gets connected to an oscillator.
How Oscillators Work
While the actual workings of an oscillator is somewhat involved, it is very simple to understand conceptually. A power supply is connected and, in an ideal world, out comes a perfect sine wave signal at a predetermined frequency. (That is why there is a little sine wave inside the circle.) Needless to say, all oscillators are active devices.
Oscillators are where the RF comes from in the first place. They are the "source" of the RF. Visually, it is simply a sine wave signal coming out the top of the oscillator, as shown in Figure 3-19. Several-surface mount oscillators used in today's wireless systems are shown in Figure 3-20. Notice how small they are.
Did You Know?
Almost every solid object that exists has what's known as a self-resonant frequency. What that means is if you can excite the material with electrical energy (or by tickling it), the material will actually produce a sine wave! No kidding.
Figure 3-20. Surface-mount oscillators. Courtesy of VarI-L Company, Inc.
Different Kinds of Oscillators
There are many different oscillator types which make up an alphabet soup of acronyms, a selection of which is detailed in Table 3-3. Every one of these oscillators has the same objective: to provide the most perfect sine wave under the given conditions (temperature, frequency, etc.). It is imperfections in the sine wave that cause problems and require the use of additional filters.
| Acronym | Oscillator | Use |
|---|---|---|
| DRO | Dielectric Resonator Oscillator | Accurate, high frequency |
| DTO | Dielectrically Tuned Oscillator | Variable DRO |
| OCXO | Oven-Controlled XO | Accurate, with a built-in oven |
| SAW | Standing Acoustic Wave | Low frequency |
| TCXO | Temperature-Compensated XO | Accurate over temperature |
| VCO | Voltage-Controlled Oscillator | Variable frequency |
| VCXO | Voltage-Controlled XO | Very accurate and variable |
| XO | Crystal Oscillator | Very accurate |
| YIG | Yttrium-Iron-Garnet | Accurate, very high frequency |
The Reason for the Different Oscillators
The reason for all the different oscillators comes down to how "perfect" the sine wave must be. Obviously, the more perfect the sine wave, the more the oscillator is going to cost. The actual sine wave requirement is dictated by the system's requirements. For instance, digital wireless communication systems, like the newer cellular phones, require a more perfect sine wave from their LO than the communication systems of FM radio stations. Imperfections in the LO's sine wave show up as noise in the system and degrade the system's performance.
What Distinguishes Them
Inside the oscillator, the thing which determines the actual frequency and "perfectness" of the sine wave is just some small piece of material like ceramic or crystal. The exact frequency is determined by the composition and size of the material, and you should know by now that the higher the required frequency, the smaller the given piece of material will be.
Did You Know?
There is actually a type of oscillator called a YIG (rhymes with pig) oscillator. YIG stands for Yttrium-Iron-Garnet which is a metallurgical compound that just happens to produce an incredibly perfect sine wave. How some RF engineer stumbled onto this little tidbit of information is beyond me.
How the Frequency is Determined
What frequency should the oscillator be? To answer this question, you have to go back to the operation of a mixer. A mixer adds and subtracts frequencies, which in the case of a transmitter is used to raise the frequency of the RF signal before shooting it out the antenna. In this case, the mixer is used to add two frequencies together (rather than subtract them). If the signal going into the mixer (IF) is 400 MHz and the signal we want out (RF) is 900 MHz, what must we make the frequency of the LO? I will wait for you to get your calculator. The answer is 500 MHz. Of course, the mixer gives two frequencies and the unwanted one (100 MHz) is eliminated with a filter, but you already knew that.
A Special Oscillator-The VCO
A special subset of oscillators is known collectively as voltage-controlled oscillators or VCOs. A VCO is an oscillator which can vary the frequency of its output sine wave in response to a change in input voltage. A block diagram of a VCO is shown in Figure 3-21.
Figure 3-21. Block diagram of a VCO.
How They Work
As the voltage applied to the VCO goes up, the output frequency of the VCO goes up and vice versa. Unlike oscillators with a single fixed frequency, VCOs have a range of frequencies over which they operate. All the same rules apply to VCOs: the wider the frequency range, the more they cost, and the poorer their performance. In this case, poor performance means imperfections in the sine wave. When RF engineers design a system which requires a VCO, they choose one with a wide enough frequency range to accommodate their requirements, but no wider.
Where are VCOs used in RF systems? As you will learn later in the section on modulation, there is a type of modulation called frequency modulation, or FM, which is the modulation used in FM radio signals (duh). Frequency modulation works by taking the information to be transmitted wirelessly and "imprinting" it onto the RF carrier by varying the RF carrier's frequency. What device do you suppose the RF system uses to vary the RF carrier's frequency? Hint: look at Figure 3-21.
Synthesizers
The Use of Feedback
There is another type of device related to the oscillator known as a synthesizer. In some circumstances the sine wave coming out of an normal oscillator is just not perfect enough. Eventually, RF engineers discovered that if an oscillator is combined with a bunch of other electronic circuitry, and it utilizes something known as feedback, it can make the sine wave even more perfect, which is what a synthesizer is: an oscillator plus some other circuitry which employs feedback to make a more perfect sine wave.
Phase-Locked Loops
Feedback, in any electrical system, is nothing more than sensing the output of some component and, if it is not exactly the way it should be, changing the input to make it so. That is basically how synthesizers workâby sensing the output of an oscillator and, if the sine wave isn't perfect enough, making a slight modification to something inside the oscillator to improve it. When synthesizers perform this feedback activity they are sometimes referred to as phase-locked loops or PLLs. (Apparently the word synthesizer is just too simple.)
Real life synthesizers can become pretty sophisticated and quite costly. They also perform many more functions than just simple feedback. One of the functions a synthesizer can perform is frequency programmability. A programmable synthesizer is like a box with several different oscillators inside, all at different frequencies. The system can then select which frequency the synthesizer puts out, and in this way, the synthesizer can instantaneously switch between different frequencies. The ability to instantaneously switch between different frequencies is useful in secure military communications, as well as the newest digital wireless communication technologies.