Motorola HT Series Service Manual page 63

VHF Theory of Operation: Voltage-Controlled Oscillator (VCO)
The RF section of the VCOBIC(U3801) is operated at 4.54 V (VSF), while the control section of the
VCOBIC and Fractional-N synthesizer (U3701) is operated at 3.3 V. The operation logic is shown in
Table 6-1.
Desired Mode
Tx
Rx
Battery Saver
In receive mode, U3801 pin 19 is low or grounded. This activates the receive VCO by enabling the
receive oscillator and the receive buffer of U3801. The RF signal at U3801 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
T3302.
During the transmit condition, when PTT is depressed, 3.2 volts is applied to U3801 pin 19. This
activates the transmit VCO by enabling the transmit oscillator and the transmit buffer of U3801. The
RF signal at U3801 pin 10 is injected into the input of the PA module (U3501 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 U3701 pin 41.
When a high impedance is applied to U3801 pin 19, 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 6-1. VCO Control Logic
AUX 4 AUX 3
n.u. High (@3.2V)
n.u. Low
n.u. Hi-Z/Float
(@1.6V)
TRB
High (@3.2V)
Low
Hi-Z/Float (@1.6V)
6-11