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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

Q1304

Q1305

2ND TX

1ST TX

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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