CIRCUIT DESCRIPTION

800/900 MHz SYNTHESIZER DESCRIPTION

VCO to the center of the operating band when the control voltage is at its midpoint.

The output signal on the collector of Q902 is coupled by C911 to a buffer amplifier formed by Q901 and Q903. This is a shared-bias amplifier which provides amplification and also isolation between the VCO and stages which follow. C918 provides impedance matching on the input, and the resistors in the circuit provide biasing and stabilization (R913 also provides current limiting). C901, C902, and C908 are RF decoupling capacitors, and C909 and C919 provide an AC ground on the emitters of Q901 and Q903.

The output signal on the collector of Q903 is fed through C914 to synthesizer U804. A 50-ohm load is provided by R855, and C841, C842, and C849 provide impedance matching. Likewise, the output signal on the collector of Q901 is fed through C904 to buffer amplifier Q801 (see Section 3.10.5). Resistor R902 lowers the Q of L903 to make is less frequency selective.

VCO Frequency Shifting

In the applicable 800 or 900 MHz band, the VCO must be capable of producing frequencies from the lowest receiver first injection frequency up to the highest talk-around mode transmit frequency. At 800 MHz, the lowest VCO frequency is 798.0625 MHz (52.950 MHz below the lowest receive frequency of 851.0125 MHz) and the highest talk-around frequency is 869.9875 MHz. Therefore, the VCO frequency band spread is approximately 72 MHz. If this large frequency shift was achieved only by varying the VCO control voltage, the VCO gain would be undesirably high. Instead, capacitance is switched in and out of the tank circuit to provide a coarse frequency shift.

This switching is provided by PIN diode CR901 which is controlled by the logic signals from the Q0 and Q1 outputs (pins 4 and 5) of shift register U800. Q803 and Q804 function as inverters and drivers.

When a PIN diode is forward biased, it presents a very low impedance to RF signals. Conversely, when it is reverse biased, it presents a very high impedance.

Forward biasing of PIN diode CR901 adds capacitance to the tank circuit which lowers its resonant frequency. The diode is forward biased in the normal mode and reverse biased in the talk-around mode (both

800 and 900 MHz). The logic levels on pins 3 and 4 of the VCO are as follows:

 

Pin 3

Pin 4

Normal Mode

H (5.5V)

L (0V)

Talk-Around Mode

L (0V)

H (5.5V)

In the normal mode, CR901 is forward biased by current flowing through R903, L905, CR901, and L902. Capacitors C907 and C912 are then effectively AC grounded through CR901 and C905. The control lines are isolated from tank circuit RF by L902/C903, L905/C906, C811, C812, C823, and C824.

Frequency Control and Modulation

Fine VCO frequency control is performed by varying the DC voltage across varactor diodes CR903 and CR904 (coarse control is provided as described in the preceding description). As the DC voltage applied across a reverse-biased varactor diode increases, its capacitance decreases. Therefore, the VCO frequency increases as the control voltage increases and vice versa. The amount of frequency change produced by CR903 and CR904 is set by series capacitor C922. A DC ground is provided by L908, and the control line is isolated from tank circuit RF by L909 and C925.

The VCO is frequency modulated in a similar manner. Another capacitance leg of the tank circuit is formed by C915, C920, and varactor diode CR902. The audio and data modulation signal is applied across CR902, and a fixed bias from a voltage divider formed by R853 and R854 is applied through R851 to pin 5. Isolation and filtering of this DC bias is provided by C838, C839, C840, and R852. Refer to the next section for more information on modulation.

3.10.3 VCO AND TCXO MODULATION

NOTE: If the wideband data input is used, the external device must provide FCC-approved modulation limiting and splatter filter circuitry and a stable 2.5 VDC reference level.

Both the reference oscillator and VCO are modulated in order to provide the required frequency response. If only the VCO was modulated, the phase detector in U804 would sense the frequency difference and change the control voltage to counteract it, espe-

 

February 2001

3-24

Part No. 001-9800-001