RADIO FREQUENCY CIRCUIT

SWR Protection Circuit

This scheme is an SWR meter, which is quite simple and easily built. Once you make a directional coupler, the results will be displayed with the measure led meters as a ratio SWR.

IC used in the scheme SWR meter is an LM3914, which are available on the market. Use non-inductive resistor (carbon-film or metal-oxide).

Complete set of SWR protection circuit schematic, see example image below to prototype soldering its components and the layout..

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FM Telephone Transmitter

The FM telephone circuit is built on a PC board that is so small it can easily be fitted inside the housing of a telephone making it an instant pseudo-speak earphone.

Here's the FM telephone transmitter circuit :

This FM circuit connects in series with telephone line, steals power from it, and transmit at both sides of the conversation to an FM radio tuned between 90 and 95 MHz.

FM Telephone Transmitter in Detail.

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Synthesised WideBand FM Transmitter

This project is a complete crystal-controlled Wide Band Frequency Modulated (WBFM) transmitter delivering a power output in the order of 10 milli-watts (+10dBm) using simple components. The transmitter is based upon the Phase-Locked Loop (PLL) principle, but due to the circuit's simplicity a true "phase lock" can never be achieved.

The transmitter has both 1v peak-to-peak 'LINE' input and 10mV 'MIC' audio inputs. These will accept audio input sources from external equipment, such as hi-fi, CD and computer equipment. The microphone input also has an in-built power source to energise an 'Electret' type condenser
microphone. The Radio Frequency (RF) output circuitry includes a three-pole filter for reduction of harmonics and other spurious signals. The spurious output signal level is better than -40dBc (0.0001 times the power of the wanted signal level), which makes the project suitable for driving an external power amplifier.

The transmitter is powered from a 12v supply, but it will operate from 9 Volts to 16 Volts. The DC power input is equipped with a diode (D1), which protects the transmitter in the event the supply voltage is inadvertently connected the wrong way round.

Download Kit Intro Datasheet

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