User's Manual

synthesizer. In most synthesizer designs, the divider must be an integer, which forces the reference
frequency to be equal to the synthesizer step size. This synthesizer, however, uses a fractional-N technique
to generate smaller step sizes. The reference frequency is derived by digitally dividing the frequency of the
14.4 MHz master oscillator. When locked, the VCO attains the same relative frequency stability as that of
the master oscillator.
The output of the VCO is amplified to a level of about 0 dBm by Q401 and Q402. Q603 with R602 and
C604 act as a very low noise power supply filter for the VCO.
8.2.2 Transmitter
PA Driver Stages
The output of the VCO buffer drives Q203 and Q204 to provide a drive level to the RF PA device of about
+13 dBm. The supply voltage to these two stages is switched on in transmit by Q201 and Q202.
PA Module, Lowpass Filter, and T/R Switch
When driven by +13 dBm, the PA module, U201, is capable of producing full rated power at the antenna
connector. Pin 2 of the module is used for power control. The output power level can be varied from less
than 0.5 watt to full power by changing the voltage at this pin.
To reduce carrier frequency harmonics of the PA module output to acceptable levels, lowpass filters are
inserted between the module and the antenna connector. To isolate the PA module from the receiver, an
electronic T/R switch is used. The switch is formed around PIN diodes CR101 and CR201 which are
turned on in transmit and are off in receive.