Audio frequency-shift keying (AFSK) is a modulation technique by which digital data is represented by changes in the frequency (pitch) of an audio tone, yielding an encoded signal suitable for transmission via radio or telephone. Normally, the transmitted audio alternates between two tones: one, the “mark”, represents a binary one; the other, the “space”, represents a binary zero.
The need for a compact telemetry system represents a challenge design for a small, lightweight, low component count.
Connection to serial data is of the microprocessor also difficult because most Low-cost RF transmitters do Do not accept dc levels at the entrance.
Commercial FSK (frequency-Shift-keying) modulators are bulky and need many Passive components. the circuit uses a single NOT gate (inverter), On Semiconductor NL27WZ14 into a surface mount Package, to generate continuous FSK data from the TTL level signals. the Spending of this range are compatible
with the available channels. If the TTL input is low, the circuit is a continuous execution of the oscillator output about 2400 Hz (adjustable with R1). If the input assumes a high level of The oscillator frequency reduces by half with the
Introduction of a capacitor in the circuit over time Q1. converter IC provides space for surgical Frequency of approximately 80 kHz. You can easily Operation of the FSK modulator at higher frequencies eg, 4800 and 9600 Hz, by reducing the values ??of the timing capacitors C1 and C2.
source : http://ednmag.com
Here is Decoder and Encoder for FM transmitter and FM radio Receiver, This circuit is a solution for transmit data in long distance.
The RF Solutions RF600E, & RF600D are easy to use encoder and decoder I/C’s. They have been designed to achieve the maximum possible range from any radio / Infra Red transmitter receiver set.
Unlike other encoder /decoder devices the RF600E/600D provide an unprecedented level of security which prevents copying or grabbing whist also obtaining optimum range from the transmitter and receiver.
The devices are very easy to use and can be inserted directly into a circuit. The RF600D has an easy learn feature enabling it to learn up to 7 unique RF600E encoder devices in standalone mode or up to 48 encoder devices when used in conjunction with an external EEPROM. These devices enable a simple but secure remote telemetry application whilst obtaining the maximum range from the radio set.
The RF600D has a serial data output. This outputs the serial number, button and battery status of the transmitter encoder (RF600E) it has successfully received. It also gives an indication of which inputs of the encoder have been activated and the status of the encoder battery. This data may be fed directly to a microcontroller or RS232 type driver circuit which may then be fed directly to a PC serial port. (An RS232 serial data output is provided on SA123 & RFEVAL2).
Serial data is output continually whilst data is being received from the RF600E transmitter. i.e. this output is valid regardless of whether the RF600E encoder has been learnt to the RF600D decoder or not. If the user requires the serial number of pre-learnt RF600E encoders only, then the serial output should be configured only to output data when one of the digital outputs from the RF600D is asserted. (Digital outputs from the RF600E will only activate when a RF600E encoder has been pre-learnt.)
The original scheme of this project was published in May of 2005 of the electronic magazine for you. The diagram down is practically the same. A module of infrared receiver is used TSOP1738 in the side of the entrance to receive the 38 KHz of frequency of the pulses TO GO at a distance of the control. In normal conditions, the pin of exit of the module TO GO is in the discharge logic, which means that transistor T1 (BC557 PNP) is of cuts and its terminal of collector is logical low. The collector of T1 units of the line of clock of the accountant of decades CD4017. Now we are going to see what happens when somebody faces a remote TV or DVD towards the TSOP1738 and presses a key in him. The module receives TSOP the 1738 train of 38 pulses of KHz TO GO of the remote control, that causes that its exit oscillates too much. These pulses are reversed in the collector of T1, which finally goes to the entrance of clock of the accountant of decades. The impulses that arrive could increase accountant CD4017 in the same proportion (38 KHz), but due to the presence of the circuit of filter RC (R1 = 100K, C1 = 10 uF) between the collector and the Earth, the train of pulses appears like a single pulse for the accountant. Therefore, in each pulsation of key, the CD4017 advances against only by a single position. When the usuary one loosen the key, the unloadings condensing C1 through the R1 resistance, and the line of clock is to zero. So whenever the user presses and loose a key of the remote control, accountant CD4017 receives a single pulse in his entrance of clock.