Analog Switch

The analog switch is useful for radio systems that utilize a frequency scanning mode and a narrow band mode The purpose of

the analog switch is to decrease the loop filter time constant allowing the VCO to adjust to its new frequency in a shorter

amount of time This is achieved by adding another filter stage in parallel The output of the charge pump is normally through the

second filter stage (LPF-2) is effective only when the switch is closed (in the scanning mode)

TLW12339 – 24

Typical Crystal Oscillator Circuit

A typical circuit which can be used to implement a crystal

oscillator is shown below

TLW12339 – 52

Typical Lock Detect Circuit

A lock detect circuit is needed in order to provide a steady

LOW signal when the PLL is in the locked state A typical

circuit is shown below

TLW12339 – 26

http

www nationalcom

13