This project is a complete crystal-controlled Wide Band Frequency Modulated (WBFM) transmitter delivering a power output in the order of 10 mW (+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 energize 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.
This figure is schematic the transmitter;
Construction should begin with the wire link WLK, all horizontally mounted resistors, then the power diode D1. WLK is formed from one of the resistor lead off-cuts. All vertically mounted resistors and ceramic capacitors are next, followed by the electrolytic capacitors and the four transistors. Finally, fir the vericap diode, D2, and the integrated circuits.Although internally protected, the ICs should be fitted on a static protected workbench. In the absence of a suitable workbench, then aluminum foil can be taped to a work board and connected to the soldering iron Earth (Ground) via a 1M0 resistor. The ICs should be left in the packaging supplied until required. Before handling them, the worker should also be electrically connected to the workbench anti-static mat (foil) via a 1M0 resistor.
Note that C25 and C29 must be miniature capacitor types, due to space restrictions on the PCB. These two components should ideally be fitted after all the resistors and other components have been fitted, and the component bodies positioned about 3mm away from the PCB. These two capacitors should be mounted vertically, as shown to the right. Long component leads will have no effect on the operation of the project. The distance between TR2 / TR3 body and the PCB must be at least 4mm minimum and no greater than 6mm. 4mm is required for clearance between TR3 and L3.
Failure to observe this may cause the collector of TR3 to touch L3 causing a collector-emitter short circuit. A good tip is to fit TR2 and TR3 after L3 has been fitted. L3 should be fitted touching the PCB. C30 has been added to the circuit to restrict the upper operating frequency of the transmitter. 4p7 restricts this to approximately 125MHz. Without C30 fitted the transmitter is capable of operation over 150MHz. It is possible that certain manufacturers of IC1 produce a slightly less sensitive version. If this is used then C7 may be increased in value to 120pf.
