Frequency Generation Circuit 2-11
In the receive mode, U241 pin 19 is low or grounded. This activates the receive VCO by enabling
the receive oscillator and the receive buffer of U241. The RF signal at U241 pin 8 is run through a
matching network. The resulting LO RF INJECTION signal is applied to the mixer at T302.
During the transmit condition, when PTT is pressed, five volts is applied to U241 pin 19. This
activates the transmit VCO by enabling the transmit oscillator and the transmit buffer of U241. The
RF signal at U241 pin 10 is injected into the input of the PA module (U101 pin16). This RF signal is
the TX RF INJECTION. Also in transmit mode, the audio signal to be frequency modulated onto the
carrier is received through U201, pin 41.
When a high impedance is applied to U241 pin19, the VCO is operating in BATTERY SAVER mode.
In this case, both the receive and transmit oscillators as well as the receive transmit and prescaler
buffer are turned off.
Table 2-1 Level Shifter Logic
Desired
Mode
AUX 4 AUX 3 TRB
Tx Low High (@3.2V) High (@4.8V)
Rx High Low Low
Battery Saver Low Low Hi-Z/Float (@2.5V)
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