Low-power radio transmitters

Low-power radio transmitters

INTRODUCTION

Low-power transmitters has always been a fascinating one for hobbyists and experimenters.

They enable you to operate your own radio station, broadcast music, control

devices remotely without wires, do surveillance work, and other things such as remote sensing. Most of recent designs are based on FM, due to inevitable problems of AM transmission. These devices broadcast in the 88–108 MHz band and can have a range of several hundred feet when a sensitive receiver is used. FCC and NCC regulations limit range and field strength at a certain distance.

The modern low-power transmitters may employ varactor modulators, microprocessor-controlled frequency synthesis using a phase locked loop (PLL), audio preemphasis, audio mixing and control facilities, and stereo modulator circuitry that generates a “real” baseband multiplex signal. Integrated circuit (IC) devices make these tasks easily implemented with reasonable amounts of circuitry.

The following factors should be considered when designing transmitter:

1. Cost: Cost benefit analysis should be a determinant in choosing a specific solution.

2. Physical size: This may be an important factor, as in hidden surveillance camera in an aircraft, or even attached to a shirt pocket will be required to be small in size.

Obviously, such transmitters and their accessory components (e.g., battery,

microphone or other sensor, and antenna) have to be kept small, requiring surface-

mount construction techniques. Other applications may have few size

restrictions.

3. Power considerations: Available power might be limited, such as in hearing aid battery, or continuously available but limited, such as from a telephone

line, or unlimited for all practical purposes, as from a 120- or 240-volt

AC power line. The low-power transmitter will have to derive all of its operating

voltages from a specific source.

4. Type of emission (FM, AM, video, pulse, etc.): The transmitter design is determined

by these factors. Simple audio modulation or tone signals are relatively

easy, but video and other complex waveforms may require more complicated

circuitry with a higher component count.

5. Quality of transmission desired: Attributes such as audio or video quality, RF

output power, frequency stability, and range dictate the need for certain circuit

features. It may be possible or necessary to omit certain circuit elements and

features to obtain a certain goal, such as size, cost, or power consumption.

Certain factors, such as range of transmission or frequency stability, may mandate

other requirements. Often, compromises must be made.

6. Legal: FCC and NCC (Nigeria) rules dictate certain power, emission type, range, antenna, RF field strength, and frequency restrictions; however, in government,

police, or military work, these restrictions may not apply or are often ignored.

ARCHITECTURE

Some of the basic low-power transmitter architecture are as follows:

1. Basic Oscillator

      2. Master Oscillator-Power amplifier.

      3. Heterodyne mixer-Power amplifier