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content: entries on frequency modulation

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---
tags: [radio, ham-study]
---
# Radio signal modulation
A radio signal comprises two parts:
- the message (information)
- the frequency for transmission of the information (the carrier)
The process of adding information (an audio signal) to a carrier is called
**modulation**.
## Amplitude modulation (AM)
The message is combined with a carrier on a set frequency.
When the message is added to the carrier wave, the amplitude of the wave form
varies depending on the volume of the audio signal.
## Frequency modulation (FM)
Again the audio signal is combined with a carrier on a set frequency however the
amplitude of the resulting signal remains constant. The freequency changes
relative to the volume of the audio signal. When the volume of the audio signal
is louder, the waves are closer together. In other words, the frequency of the
waves per second increases.
## Continuous wave modulation (CW - Morse code)
A continous wave has a constant amplitude and frequency. To send information you
simply switch this wave on and of. When it is on, the radio wave is transmitted.
When it is off, nothing is transmitted.
This is the basis for communication using Morse code over radio waves.

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---
tags: [radio, ham-study]
---
# Single Sideband Modulation
With [AM-modulated transmission](./Frequency_modulation.md), "sidebands" are
created just below and just above the carrier frequency known as the upper and
lower sidebands (USB and LSB, respectively).
AM radio stations typically transmit on both sidebands. That is to say, they use
the full available bandwidth. However the two sidebands are copies of each
other.
This is not technically necessary, however. It is possible to transmit by just
using a single sideband. This is more efficient and means you are focusing your
power to transmit a single frequency.
The formula for the generation of the full audio signal is:
$$
\text{USB: } f_c + f_m \\
\text{LSB: } f_c - f_m \\
\text{Carrier: } f_c
$$
![Standard AM transmission](../img/standard-am-transmission.png)
The standard process of transmitting AM:
- Audio signal multiplies with carrier in accordance with formulae
- Two sidebands are created (USB, LSB)
- All three components are transmitted
![Single sideband transmission](../img/single-sideband-transmission.png)
With SSB:
- Audio signal multiplies with carrier in accordance with formulae
- Two sidebands are created (USB, LSB)
- A filter removes the carrier and one sideband
- Only one sideband (USB or LSB) is transmitted
> Crucially, this is possible because the carrier wave frequency is not needed
> for transmission and receiving. It is only needed in order to get the audio
> signal (_f_m_) up to the transmitter frequency ready to transmit. But it it is
> kept in during normal AM transmission.