Analog Switch (2315 only)
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
D
o
pin but when LE is set HIGH the charge pump output also becomes available at BISW A typical circuit is shown below The
second filter stage (LPF-2) is effective only when the switch is closed (in the scanning mode)
TL W 11766 – 11
Typical Crystal Oscillator Circuit
A typical circuit which can be used to implement a crystal
oscillator is shown below
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
TL W 11766 –12
TL W 11766 – 13
12
NSC [ NATIONAL SEMICONDUCTOR ]
