Motorola HT Series Service Manual page 43

UHF Band 1 Theory of Operation: Voltage-Controlled Oscillator (VCO)
The VCOBIC (U241) in conjunction with the Fractional-N synthesizer (U201) generates RF in both
the receive and the transmit modes of operation. The TRB line (U241 pin 19) determines which
oscillator and buffer will be enabled. A sample of the RF signal from the enabled oscillator is routed
from U241 pin 12, through a low pass filter, to the prescaler input (U201 pin 32). After frequency
comparison in the synthesizer, a resultant CONTROL VOLTAGE is received at the VCO. This
voltage is a dc voltage between 3.5 V and 9.5 V when the PLL is locked on frequency.
The VCOBIC (U241) is operated at 4.54 V (VSF) and Fractional-N synthesizer (U201) at 3.3 V. This
difference in operating voltage requires a level shifter consisting of Q260 and Q261 on the TRB line.
The operation logic is shown in
Desired Mode
Tx
Rx
Battery Saver
In 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
second buffer to improve mixer to receive VCO isolation. The resulting RF signal is LO RF
INJECTION and it is applied to the mixer at T302. (Refer to the UHF Receiver Front-End Schematic
Diagram on page 9-84.)
During the transmit condition, when PTT is depressed, a five-volt current 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 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 4-1.
Table 4-1. Level Shifter Logic
AUX 4 AUX 3
Low High (@3.2V)
High Low
Low Low
TRB
High (@4.8V)
Low
Hi-Z/Float (@2.5V)
4-7