Low Band, 800 MHz, PassPort & 900 MHz Theory of Operation: 900 MHz Voltage Control Oscillator (VCO) 7-27
In the receive mode, U250 pin 19 is low or grounded. This activates the receive VCO by enabling the
receive oscillator and the receive buffer of U250. The RF signal at U250 pin 8 is run through an
injection amplifier, Q304. The resulting RF signal is the LO RF INJECTION and it is applied to the
mixer at U301.
During the transmit condition, when PTT is depressed, five volts is applied to U250 pin 19. This
activates the transmit VCO by enabling the transmit oscillator and the transmit buffer of U250. The
RF signal at U250 pin 10 is amplified by Q251 and injected into the input of the PA module (U101
pin1). 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 the U201 pin 41.
When a high impedance is applied to U250 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.
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