Motorola PR860, Detailed Service Manual

Page: 35 / 197

2-18 Theory of Operation : UHF2 Voltage Controlled Oscillator (VCO)
The operation logic is shown in Table 2-6 .
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 RF signal is the LO RF INJECTION and it is applied to the mixer at
T302 (refer to UHF (450–512 MHz) Receiver Front End Schematic Diagram on page 4-49).
During the transmit condition, when PTT is depressed, 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 the 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 2-6. 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)