• revv
In summary, there are various methods for generating AM and FM waves, including square-law modulators, switching modulators, and voltage controlled oscillators. In an AM radio transmitter, the amplitude is modulated by using an audio amplifier to vary the power supply voltage of the RF amplifier. An experiment can be done to demonstrate AM modulation by connecting a lamp to the output of an audio amplifier and observing it flash in sync with the sound.

#### revv

Is the carrier wave modulated or in other words "modified" with the information signal? If so how exactly is this done? I can't find or maybe I am looking at the wrong things but I don't understand how this is done.

I know that AM is modulated with the amplitude and FM is modulated by the frequency but I don't think I really understand how this is done.

revv said:
Is the carrier wave modulated or in other words "modified" with the information signal? If so how exactly is this done? I can't find or maybe I am looking at the wrong things but I don't understand how this is done.

I know that AM is modulated with the amplitude and FM is modulated by the frequency but I don't think I really understand how this is done.

https://en.wikipedia.org/wiki/Modulation#Analog_modulation_methods

Or are you asking about the electrical circuit the does the modulation such as this?

https://en.wikipedia.org/wiki/Heterodyne

davenn and berkeman
With amplitude modulation, you could think of the carrier signal as "sampling" the information signal.

davenn
anorlunda said:

https://en.wikipedia.org/wiki/Modulation#Analog_modulation_methods

Or are you asking about the electrical circuit the does the modulation such as this?

https://en.wikipedia.org/wiki/Heterodyne
There's no limit to the number of sources for methods of realising analogue modulation. One thing I would say about the 'Hetrodyne' reference and that is a simple multiplier circuit will give frequency mixing but not Amplitude Modulation. For AM you need a mean carrier amplitude of half the peak value and that gives an 'envelope' on the carrier (from zero carrier amplitude to peak amplitude ) and not the 'string of beads' that simple mixing gives.

Easy to achieve this with a low power transistor amp and it can be done with an RF amplifier with its Power Supply volts, varied according to the audio signal. It's like having a control knob on the power supply and varying the supply volts by hand - only faster using an audio amp; the negative peaks of the sound signal almost turn off the RF and the positive peaks take it to twice the mean value.

Practical Note:
In a conventional AM broadcast transmitter, the modulation is all done in the high power output stage. You have a powerful RF amplifier, driven by an RF oscillator. and the Power Supply Volts for the amplifier are varied with what is effectively a (very powerful) audio amplifier. It's all scary, steaming components which sit there producing many hundreds of kW of RF and (despite the efficient class C design) some hundreds of kW of HEAT. All this is to avoid having a linear high power amplifier which would require an equal amount of or more wasted Power. (This must be where the expression "Steam Radio" came from, I think)

revv said:
I know that AM is modulated with the amplitude and FM is modulated by the frequency but I don't think I really understand how this is done.

There are several methods for generating AM and FM waves.

For AM, we can use a square-law modulator or a switching modulator.

For NBFM, we can use the components of the integrator, product modulator, and phase shifter to build a NBFM modulator.

For WBFM, we can use voltage controlled oscillator (direct method) or NBFM modulator plus frequency multiplier (indirect method).

Here is an experiment that shows Amplitude Modulation (AM).

• Use an Audio Amplifier or a radio with at least a 1 Watt output.
• Connect a bulb (lamp) from a flashlight across the speaker connections. Typical lamp sizes would be #112, #222, PR2. The #112 is the lowest power, and PR2 is the highest power of the three.
• Turn on the sound.
You should see the lamp flashing in step with the sound. The lamp represents the AM transmitter.

Things that can go wrong with the experiment.
• The amplifier doesn't have enough power to drive both the speaker and lamp.
• Disconnect the speaker and connect only the lamp to the amplifier output.
• You used an LED from a flashlight instead of a lamp. They need a higher voltage.
• Use one of the suggested lamps.
• Use a higher power amplifier.

In this experiment, the lamp will be off when there is no sound. In an AM radio transmitter, there is always some power output, even without any sound to transmit. This is called the Carrier, and is what you tune the radio to when you want to listen to a particular station.

To better represent that in this experiment, a single flashlight battery could be added to keep the lamp on at about half brightness. Details on how to connect a battery depends on the internal circuit of the Audio Amplifier, and since that is an unknown, please don't try it. (unless the amplifier is a throw-away)

Hope this helps.

Cheers,
Tom

berkeman