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FM Transmitter [INTRO]

The above picture shows the basic stages of the commercial transmitter, its blocks/stages and how they are connected with each other. I would like to describe each of these block independently so it could help the beginners see the big picture and thus better understand how transmitters are designed. So lets start!


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"Audio Source

 

Also referred to as Audio Frequency (AF) which usually is around 20Hz-20KHz. This can be either CD player, computer, tape, microphone or just about any other audio device. The audio signal should have as good characteristics and quality as possible. Connectors from Audio Source to the Audio Limiter should also be of a better quality to make sure that there isn't any noise coming to the audio stages of a transmitter (yellow blocks).

Audio Limiter



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.

Stereo Encoder





Also known as encoder, coder or multiplexer (MPX). Stereo encoder is a circuit that takes both channels of audio (L and R) and mixes it into one so called MPX channel. Stereo encoder uses a process called multiplexing which allows to transmit more than one signal over a single link, route, or a channel. The circuit is driven by an oscillator that switches between the two channels of audio with a frequency of 38KHz per second to merge these channels into one. Additionally, 38KHz frequency is divided in half to produce a 19KHz PILOT tone that receiver will need to use to decode MPX signal back into two audio channels. Once the two audio channels are mixed and 19KHz frequency is generated they are then merged together to form a MPX signal that FM transmitter will transmit.

Modulator



This block places audio frequency on top of a carrier frequency (frequency in which one wants to transmit, in our case 88-108MHz). We may also refer to this block as a mixer because it mixes two different frequencies.

Oscillator



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.

Separator



This is usually a single transistor that separates a low oscillator's signals from an antenna or the rest of the blocks. When separator is in place it usually brings a greater oscillator's stability especially in low power transmitters where there is no amplifier at all. When an amplifier is used this part may be omitted because an amplifier acts as a separator too.

RF Amplifier



RF amplifier is a circuit that takes small incoming RF signal and increases its strength multiple times. Most amplifiers use several transistor stages; driver and output stages that amplify RF signal gradually. For instance if you connect a 50mW signal to a 10W transistor you cannot expect a 10W output signal. This is because such transistor might need at least 1W of incoming signal to produce an output at its maximum power peak.

You should always take precautions when dealing with amplifiers that produce 1W or more output power. Never connect them to power without a proper antenna or a dummy load. By doing so you are running a risk of destroying your output stage transistor(s).

Power Meter



This device connects to an output of an amplifier to see how many watts are being transmitted. You may also connect it to the end of the antenna cable if it's a long one to see how much power has been lost through that cable. In lower power transmitters you may use a single transistor and a LED as a power indication, but to have a precise measurement you will definitely need a power meter.

Antenna



Antenna is an equally important element of every transmitter because it is used to dispatch or radiate the signal of the transmitter. You may have a powerful amplifier, but if you have a poor antenna only a fraction of that signal's strength will transmitted to the air. Transmitter's amplifier should always be matched with the antenna by using variable capacitors to achieve maximum signal performance. Avoid running a long antenna cables form your amplifier to an antenna to minimize the power lose, and if you have no choice use better quality antenna cables.

Glossary


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"

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