As we all know, PLl means a phase-locked loop or phase lock loop which is a control
system that tries to generate an output signal whose phase is related to the phase of the input "reference" signal.
Phase-locked loops are widely used for synchronization purposes; in space communications for coherent demodulation and threshold[disambiguation needed] extension, bit synchronization, and symbol synchronization. Phase-locked loops can also be used to demodulate frequency-modulated signals. In radio transmitters, a PLL is used to synthesize new frequencies which are a multiple of a reference frequency, with the same stability as the reference frequency.
In digital wireless communication systems, PLLs are used to provide the local oscillator (LO) for up-conversion during transmission and down-conversion during reception. In most cellular handsets this function has been largely integrated into a single integrated circuit to reduce the cost and size of the handset. However, due to the high performance required of base station terminals, the transmission and reception circuits are built with discrete components to achieve the levels of performance required.
Some like USB FM Transmitter is a USB device intended to transmit audio from your desktop computer or notebook to nearby FM radios all over your home or office without cables or wires. The USB transmitter is a truly plug-and-play device and is simply connected to your computer via the supplied USB cable. No drivers are needed for supported operating systems and the transmitter installs itself within seconds as soon as you plug it into your computer.
The transmitter offers a choice of 7 frequencies selectable though an easy to change DIP switch. The frequency is PLL (Phase-Locked-Loop) synthesized for high-quality stereo reception. It also offers a stereo audio output jack so that you continue to listen to the audio on your existing speakers at the same time as the transmitter is connected to the computer.