Basic CW AM FM Modulation Tutorial

This video is very basic demonstration using a Function Generator to show CW, AM, and FM Modulation in the Frequency Domain using a Spectrum Analyzer and an Oscilloscope to show these Modulations in the Time Domain. The video also shows how Modulation affects signal Bandwidth.

Take aways include:

1. CW (using Morse Code) has an approximate Bandwidth of 200 Hz thus is the narrowest mode normally used in radio communications.

This mode typically uses On/Off keying to generate short pulses ("dots") and long pulses ("dashes") to transmit using Morse Code... though any other sequence of pulses could be used as long as the sender and receiver both agree on what those pulses mean.

2. AM bandwidth is approximately equal to the highest modulating frequency applied times two (Modulation Frequency[max] x 2 ). This is about 6 kHz for normal HF (Short Wave) communications when the modulation frequencies are all below 3 kHz. Distortion may increase this to about 9 kHz in a typical transmitter.

3. FM is somewhat wider than AM because it swings the carrier frequency higher and lower with modulation amplitude and often has a modulation index of more than 1. Thus FM will occupy most of the bandwidth of a 20 kHz "channel" when adjusted for a typical 5 kHz deviation.

Another tutorial about CW and AM modulation can be found here: youtube.com/watch?v=3I_e7gIyfQg

(Note: For purists who look at the video carefully you will note that the "200 Hz" shown in the video is actually more like 600 Hz wide since I had the Spectrum Analyzer set for a 300 Hz receive bandwidth. This, of course, adds to the apparent bandwidth of the signal as the filter sweeps across the signal.)