Voltage Controlled Oscillator (Vco) - Motorola LB3 (42.0 - 50.0MHz) Service Information

Low Band Frequency Synthesis
4.2

Voltage Controlled Oscillator (VCO)

Separate VCO and buffer circuits are used for receiver injection and transmitter carrier frequency
generation. Since the receiver uses high-side injection, the receiver VCO frequency range is 10.7
MHz above the transmit VCO range. The VCO/buffers are bandsplit into three ranges depending on
radio model, covering radio operating frequencies of 29.7 to 36.0 MHz, 36.0 to 42.0 MHz, or 42.0 to
50.0 MHz. The corresponding three frequency ranges for the receive VCO are 40.4 to 46.7 MHz,
46.7 to 52.7 MHz, and 52.7 to 60.7 MHz.
The VCOs, together with Fractional-N synthesizer U1201, generate the required frequencies for
transmit and receive mode. The TRB line (U1201 pin 2) determines which VCO/buffer circuit is to be
enabled. A high level on TRB will turn on the transistors in U1378 to turn on via R1376, applying the
8.3 volt VSF source to the receiver VCO and first buffer. The second buffer in each string operates
from the 9V3 source and become active when RF is applied to their inputs.
The RF signal at the bases of the second buffers are combined and fed back to the Fractional-N
synthesizer via PRE_IN where it is compared to the reference frequency as described below in
"Synthesizer Operation". The Fractional-N IC provides a DC steering voltage VCTRL to adjust and
maintain the VCO at the correct frequency.
With a steering voltage from 2.5V to 11V at the appropriate varactor diode (CR1302 for the RX VCO,
or CR1310 for the TX VCO), the full VCO tuning range is obtained. Each VCO uses and AGC circuit
to maintain a constant VCO output level across the frequency band. A diode (CR1306 in the receive
VCO, or CR1314 in the transmit VCO) is configured as a voltage doubler which rectifies the RF level
sampled at the VCO drain and applies a proportional negative DC voltage to the VCO gate.
Increased RF level reduces the VCO gain to compensate.
STEERING
LINE
(VCTRL)
RXVCO
Q1303
~
AGC
TXVCO
Q1306
~
AGC
SFOUT
(U1201 PIN28)
(U1201 PIN2)
Figure 2-4 LowBand VCO/Buffer Block Diagram
1ST RX
2ND RX
BUFFER BUFFER
Q1304 Q1305
2ND TX
1ST TX
BUFFER
BUFFER
Q1307 Q1308
U1377-8
DC SWITCH
RXINJ
(TO 1ST MIXER)
TO Q1202
PRESCALER BUFFER
TXINJ
(TO U1401 PIN16)
RX (TO Q1303-5)
TX (TO Q1306-8)
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