The Fractional-N synthesizer, shown in Figure 2-4, uses a 16.8 MHz crystal (FL201) to provide a
reference for the system. The LVFractN IC (U201) further divides this to 2.1MHz, 2.225MHz, and
2.4MHz to be used as reference frequencies. Together with C206, C207, C208, R204 and CR203,
they build up the reference oscillator which is capable of 2.5ppm stability over temperatures of -30 to
85
C. It also provides 16.8 MHz at pin 19 of U201 for use by the ASFIC and LVZIF.
The loop Þlter consists of components C231, C232, C233, R231, R232 and R233. This circuit
provides the necessary dc steering voltage for the VCO and determines the amount of noise and
spur passing through.
To achieve fast locking for the synthesizer, an internal adapt charge pump provides higher current at
pin 45 of U201 to put the synthesizer within lock range. The required frequency is then locked by
normal mode charge pump at pin 43.
Both the normal and adapt charge pumps get their supply from the capacitive voltage multiplier
made up of C258, C259, C228, triple diode CR201, and level shifters U210 and U211. Two 3.3V
square waves, 180 degrees out of phase, are Þrst shifted to 5V, then along with regulated 5V, put
through arrays of diodes and capacitors to build up 13.3V at pin 47 of U201.
To Next Page
Loading...
