Also known as Compressor or Automatic Level Control. This circuit is usually built using operational amplifiers in conjunction with other controlling IC's. And these are the following task of this device:

1. Provides 100% of allowable modulation. In other words the level of incoming sound is a maximum that modulator can handle (perfectly matched with the sensitivity of the modulator). That's why the sound on the receiver is very high but at the same time very clear.

2. Prevents over modulation. The best thing is to keep modulation to the maximum (100%) but if that allowable line is crossed then there would be over modulation of audio cycle when the RF carrier is removed completely from the air thus producing distortion in the transmission. How does that happen? In most of the transmitters the same block or even a transistor is responsible for both modulation and generation of the carrier frequency (88-108MHz). If over modulation takes place transistor (Q1) becomes so unstable that it cannot generate a clear carrier frequency and in result we hear the distortion.

3. Keeps an audio on the same level. This is especially crucial when using microphone as a source because its dynamics (audio levels) are never steady. When playing music, some of the tracks might be recorded at different audio levels and when a user receives a given radio station he or she doesn't want to set an audio level every few minutes or so, you want the sound loudness to always be the same.

If you just want to build a simple transmitter so you can transmit your music around the house you can live without automatic level control but then you'll need to make sure that audio signal of your source is set to minimum level and that it is matched with your transmitter.

Just as the name implies oscillator oscillates or generates a carrier frequency (88-108MHz). Oscillator can generate various types of frequencies and may be used for many different purposes. It can be found in most of the electronic devices and in our case it is found in all FM transmitters and receivers as well. A simple one transistor FM transmitter is in fact nothing else but an oscillator and a modulator. An output power of one transistor oscillators found in these transmitters is often very small, 50mW or below. If such transmitter does not have at least a separator or an amplifier then in that case this oscillator is very prone to frequency drifts. A single touch to its antenna may cause a slight frequency change.

Oscillator Types:

1. VFO (Variable Frequency Oscillator) - An oscillator whose output frequency can be changed by adjusting a variable inductor or variable capacitor.

2. VCO (Voltage Controlled Oscillator) - An oscillator whose output frequency is controlled or changed by an application of external voltage. VCO uses varicap diode that changes the capacitance as different levels of voltage are applied.

3. PLL (Phase Locked Loop) - A circuit that synchronizes a frequency of VCO with a frequency of a reference oscillator by using a comparison of phase between the two signals. PLL takes a frequency of VCO, divides it into a lower frequency which can be compared with a stable reference oscillator. Then amplifiers are used to send an appropriate voltage back to the VCO to keep the desired frequency stable.

4. Crystal Oscillator - Oscillator that uses a crystal to generate a frequency.

FM - Frequency Modulation
VHF - Very High Frequency (30MHz to 300MHz)
UHF - Ultra High Frequency (300MHz to 3GHz)
VFO - Variable Frequency Oscillator
VCO - Voltage Controlled Oscillator
PLL - Phase Locked Loop
Oscillator - device that generates a frequency