Almost everyone has heard of FM, but not everyone has heard of FSK. Frequency Shift Keying is a modulation scheme which utilizes changes in the frequency of the carrier wave to communicate data. For example, you can see the change in frequency in the image to the right. The lower frequency may indicate a “1” and the higher frequency may indicate a “0”, or vice-versa. It really is up to you how you would like the modulation scheme to work.
There only real difference between FM and FSK is in the baseband signal. The baseband signal is the signal applied to the modulator in order to create the actual modulation. In FM it is usually an analog signal, audio, etc. In FSK, it is usually a digital signal.
In the image on the right, there are two distinct changes in frequency, so this is known as 2-FSK.
FSK is commonly created using a varactor diode. This is a diode whose capacitance changes in relation to the voltage applied to it. This makes it very useful in oscillators. Taking a look at my horribly drawn, incomplete, for-illustration-purposes only (it is lunchtime and I want to have time to grab a sandwich!) example schematic to the right, you can see the modulation input arrives at the right, and is injected directly to the anode of the varactor diode. Let us imagine this input is a square wave (a low and high voltage level, or a 1 and a 0 binary) and it results in a change in the varactor diode capacitance. This change in capacitance is used to change the loading on the crystal at the center of the schematic. Loading the crystal with different capacitances changes the crystal’s oscillating frequency, and so changes the oscillator circuit output frequency. So, now we have converted two different voltage levels to two different frequencies. 2-FSK. The output from this circuit can then be amplified and upconverted to the frequency of operation.
4-FSK is an extension of 2-FSK which has four levels of input voltage to the modulator circuit, and so produces four different frequencies dependent on the input voltage.
In this animated example, I’ve fed a 4-FSK modulated signal at approximately 450 MHz into a spectrum analyzer, and labeled the bit values. The sample rate is 5 samples per second (sps), which is very very slow, but allows you to see the discrete frequency levels in the modulation happening in real time. Or, rather, slow enough time for us to see. You can see how 4-FSK is much more efficient data transfer, since for a given deviation, you get four levels (00,01,10,11) with two bits of binary per symbol instead of just two levels (0,1) with one bit of binary per symbol – it has a higher spectral efficiency. In reality, FSK systems modulate the carrier with much higher bit/symbol rates, and it may not be easy (or maybe impossible) to pick out the discrete frequencies. It generally looks like a blur on a spectrum analyzer.