This low power fm transmitter has good spectral purity. Harmonics and spurious emissions are better than -40dbc due to the use of RF filtering in the final stage. It uses some “old fashioned” components but it has been tested and proved to be excellent. Do not look to find any tuning elements in this circuit. As a broadband fm V.C.O. it does not require any special tuning in any amplifier stage. Just connect to the power supply and adjust the control voltage in order to achieve the desired frequency. With 2N4427 or MRF237 in the final stage, the output power is about 200mW or 800mW respectively.
This V.C.O was a nice match for the LMX2306 P.L.L IC which is now obsolete. The low pass filter on tuning voltage input has been especially designed for the LMX2306. However, this circuit will work with any other P.L.L. Consider this circuit as a classic design. If you have some experience in high frequency circuits, you will not face any difficulties to build it. With the use of a spectrum analyzer you will be able to achieve perfection with some minor adjustments on RF coils.
Posted by: A.G
Broadband FM Exciter electronic schematic