content: entries on frequency modulation

zk/Frequency_modulation.md

@@ -0,0 +1,36 @@
+---
+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.

zk/Single_sideband_modulation.md

@@ -0,0 +1,47 @@
+---
+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.