Description

This new FM transmitter is very easy to make and doesn't need any RF tuning.

First of all ,we have used an integrated VCO: The POS150 from Mini-circuits.
This excellent RF circuit covers all the FM Band in a voltage range of 4V to 8V.
The Kvco factor is very stable all over the FM band, consequently, we have applied the AF signal directly on the control voltage line coming from the PLL.
The VCO drives directly a new RF stage amplifier in order to get 23dBm at the output.
To reach this target, we have used a BFG135 transistor with few components to adapt the input and output impedances to 50 ohms.
All the components have been determined by using Genesys software tool.

Finally,a low pass filter ensures a good harmonics rejection.(H3<-60dBc).

This new design conducts to new RF performances like:

_Phase/noise equals to -103dBc/Hz at 10KHz from the carrier.
_+24dBm output power level (250mW).

 

Parameter	Value
Power Output	250mW
Frequency Output	87.5MHz to 108 MHz
Frequency Step	10KHz
Phase Noise	-103dBc/Hz
Frequency Stability	+/- 400Hz
Harmonic Filtering (Except H2)	<-60dBc
Spurious Emissions	<-70dBc
Modulation	Direct Frequency Modulation
MPX Input Response	+/-0.5dB 30Hz to 80KHz
MPX Input Level	300mV RMS Min
Power Supply	14 to18V

 

Conclusion:

This new FM transmitter is definitely more simple to assemble but more expensive than the previous versions.

 

VCO/PLL Schematic PDF File