Section 2: Waves
Much of RF design is the study of how energy goes into and out of
a system. It turns out that mechanical waves (sound and vibration) and
RF waves
share a lot in common- these fields use the same
equations to define a propagating (traveling) wave. This section
will not cover propagation in general, but instead will try to
provide some analogies to help visualize what's going on in later
sections. Using a simple plastic megaphone (search your favorite online
retailer for "plastic megaphone"), you can do these experiments at home
or in the class..
The Source and the Load
Consider a talking head. The blue man is our “source”, and
the sound coming from his mouth is the signal. The green man's ear
is the “load”, and we care about how much energy goes to that load.
Notice that audio energy goes all over the place: the energy that
doesn't reach the load (the green man's ear) from the source (the
blue man's mouth) is considered “lost”.
Transmission, Reflection, and Gain
We can introduce a device between the source and the load that can change how much energy is lost, and how much is transmitted. For example, we can use a megaphone. With a megaphone, we can see most of the energy is transmitted to the load, so much less is considered loss. The sound that goes to the load is the “transmitted” energy, the sound that goes back towards the source is the “reflected” energy. The difference between how much energy gets to the load with and without the megaphone is the “gain” of the megaphone.
Insertion Loss and Return Loss
The energy that does not reach the load is considered “lost”.
If the megaphone is turned backwards, more energy is actually reflected back to the source than
is transmitted to the load. The amount of energy that is reflected back, and
does not reach the load, is often referred to as the “return loss”. The energy has been reflected back to the source.
If a large piece of cotton is put in the megaphone, a lot of the sound energy will be absorbed by the cotton- it will be neither reflected nor transmitted. This is the “insertion loss”; generally this is the audio energy that is turned from sound into heat (or RF into heat). So energy may be transmitted, reflected, or lost in the device... energy will always be conserved.
Ports
These examples have had two openings, or “ports”. A port is a place that energy can enter and leave the system. It is possible to consider a megaphone with three ports, such that some of the energy is reflected, some goes towards one port (whose load is the green man's ear), and some is diverted to a third port (whose load is the orange man's ear). This third port is also considered a load, the energy that goes to the third port is not considered loss.
Propagation Delay
Keep in mind that it takes time for sound to travel from the source (the blue man's mouth) to the load (the green man's ear). We normally don't notice the difference in time between when a sound is made and when it is heard, but this effect is very important in RF analysis of circuits. The next section will look into how propagation delay effects the voltage waves bouncing back and forth through a device.
