This is a demo of the simple FM transmitter using PSoC5 (KIT-059). The project is very simple and uses no software code. AC input from the audio source is amplified (0-4V) and applied to the varactor diode, which controls frequency of the ceramic resonator (4.19 MHz). PSoC5 PLL is set to operate at 75.4MHz, which is 18x harmonic of the XTAL. It appears that PSoC5 internal PLL lock time is quite short (<250us), so that PLL can track the audio frequency (up to ~10kHz), though it less efficient at high range. Antenna is made from a 2" piece of wire, which is connected directly to the BUS_CLOCK output. There is no tank circuit, so transmitter emits on many harmonics of the main (1x, 3x, 5x...). Fortunately, 75MHz and 226MHz regions are empty, and emitted power is rather low (<100nW) so there is no issues with FCC.
Any other ceramic resonator will work, the 4.19MHz is what i had at hands. PLL will lock to any XTAL with frequency >=4MHz. The varactor is also not specific, look for any which adjusts in 10-100pf in 0-4V range (hyperabrupt type). Since PLL feedback decreases with modulation frequency, adding simple CR high-pass filter at audio input with Fcutoff~1-4kHz improves FM output.
RTL-SDR receiver dongle was used to receive the signal.
Project archive is attached.
Project uses the Annotation Library: Annotation library for CY8CKIT-059 Prototyping Kit
YouTube video of transmitter action: PSoC5 FM audio transmitter (PSoC, PSoC5LP, PSoC 5) - YouTube