Today is Tuesday, 2020-01-21
Some Icons by Dryicons
Today is Tuesday, 2020-01-21
Some Icons by Dryicons
Here we dive a little more deeply into waves and look at three ways that a “pure” radio wave (called the carrier) can be modulated to encode a voice signal (called the baseband signal): AM, SSB, FM.
But first, let's look at the general characteristics of a wave.
Here's a good introductory video for this section:1)
Here are two moving waves. What's different about them? What's the same?
Imagine that the dots moving up and down are creating the waves that are travelling to the right (as we'll see later, this is kind of like how radio waves are created). Here are a few things to notice:
To quantify these observations more precisely, let's look at a snapshot of both waves frozen in time.
Now imagine that the animation is in super slow motion and that the waves are actually travelling at the speed of light, which is roughly 300,000,000 metres per second: How many times does each dot go up and down in one second?
Another way of asking that question is: how many full cycles can you fit in 300,000,000 metres (since radio waves travel 300,000,000 metres each second).
So a quick way to relate the frequency (in MHz) and the wavelength (in metres) is:
Note that the reason we're using just 300, instead of 300,000,000 is that we've cancelled 6 of the zeros so that the frequency is in MHz instead of in Hz.
Now, here's a related question: how long does it take for each wave to complete one cycle?
The time to complete one full cycle is called the period (T) and is the reciprocal of the frequency:
AM stands for Amplitude Modulation. What this means is that the transmitted radio wave is obtained by changing the amplitude of a pure radio waves (the carrier) based on an audio signal (the baseband).
For example, let's transmit a single audio note of 10 kHz at a radio frequency of 200 kHz.3):
So very roughly:
AM Radio Wave = (Audio Signal + 1) × Carrier Wave
The incredible thing about the resulting AM broadcast is that the transmitted radio signal can also be seen as the sum of three pure sine waves:
AM Radio Wave = LSB Wave + Carrier Wave + USB Wave
This is absolutely not obvious but let's see why it's at least plausible. Imagine we start with the following three waves:
Now let's add them together. This is a bit of a mess, but let's look at specific places along the waves:
Note that the carrier as a frequency of 200 kHz just like the original carrier, but the two side bands are 10 kHz lower and higher with half of the amplitude. Notice also how the LSB Wave oscillates slower than the Carrier Wave, while the USB Wave oscillates faster.
An easier way to represent a radio signal is using a spectroscope, which shows the frequency spectrum of a wave. That is, instead of looking at the signal wave itself, the spectroscope shows the strength of each frequency that makes up the sum of the signal.
For example, the spectrum of our 10 kHz note transmitted over a 200 kHz carrier would look like this:
All this is saying is that the radio signal is composed of three pieces: a signal at 190 kHz with an amplitude of 0.5, a signal at 200 kHz with an amplitude of 1, and another at 210 kHz with an amplitude of 0.5.
There are three things to notice here:
One way of saving power (and reduce bandwidth) is to only transmit one of the side bands. In this example, the radio would be tuned to 200 kHz, but…
By itself, neither of these transmissions would carry the information we need (that the baseband signal was a 10 kHz note) since it's the difference between the sideband and the carrier that gives us that information. But if the receiver knows that this signal was generated by a transmitter at a frequency of 200 kHz, then the receiver can re-inject the missing carrier on its side.
This is why an AM signal is not too picky about being slightly off frequency (both the sidebands and the carrier are transmitted). But a SSB signal changes pitch if the receiver is not tuned precisely to the transmitter frequency.
In reality, the voice we transmit contains a whole group of “notes” typically between 300 Hz and 3000 Hz (music could range between 20 Hz and 20,000 Hz). A typical voice signal (baseband) could look something like this:4)
So the AM signal would look like this:
And finally, each individual sideband would look like this:
The two main advantages of using SSB (LSB or USB) are that:
The radio is tuned to 3.880 MHz (where no one is transmitting), but there are two neighbouring conversations going on:
The scope shows the recent history of the radio signal (called waterfall) where the present is at the top and the past at the bottom. Blue represent a weak signal strength and yellow or red represent a strong signal strength. Here are some things to notice:
|LSB (3.875 MHz)||AM (3.885 MHz)|
|Symmetry||The signal is on the left (low side) of where the carrier would be (at 3.875 MHz) and varies with speech.||The signal has a strong, constant carrier in the centre, and two symmetrical sides that vary with speech.|
|Bandwidth||About 2.7 kHz on the low side of 3.875 MHz||About 6 kHz (2.7 kHz on each side of 3.885 MHz, with two gaps near the carrier)|
|Pauses||During pauses, no radio signal is transmitted.||During pauses, the carrier is still transmitted.|
|Relationship||An AM signal can be understood in LSB mode because it contains the lower side band required. But an LSB signal can't be understood in AM mode because both sidebands and the carrier are needed to process the signal.|
FM stands for Frequency Modulation. What this means is that the transmitted radio wave is obtained by changing the frequency of the carrier based on the audio signal.
For example, let's again transmit a single audio note of 10 kHz at a radio frequency of 200 kHz using FM this time instead of AM:
This time, we don't simply multiply the baseband signal to the carrier (as in AM). Instead, we “compress” and “stretch” the carrier (ie, modulate its frequency) based on the baseband signal.
Here, the math is a bit more involved and requires at least 1st year calculus to understand but in a nutshell, if the carrier is and the baseband signal is , then the FM signal will be:
If this looks like Greek to you, don't worry; the math isn't important. The key concept to understand is that the highs and lows of the baseband signal are encoded in the horizontal compression (the frequency) of the radio wave: When the baseband is high, the radio signal is more compressed (its frequency is higher), and when the baseband is low, the radio signal is more stretched out (its frequency is lower).
For those interested in some of the mathematical details, see this optional page.