CIRCUIT DESCRIPTION
800/900 MHz SYNTHESIZER DESCRIPTION
cially at the lower audio frequencies. Conversely, if only the reference oscillator was modulated, the VCO frequency would not change fast enough, especially at the higher audio frequencies. By modulating both, a relatively flat response is provided for all modulation frequencies.
Separate audio, data, and wideband data modulation signals are applied to the synthesizer on J201, pins 18, 17, and 16, respectively. The data signal includes LTR and Call Guard signaling, and the wideband data signal (if used) comes directly from an external device such as a modem and does not pass through the limiting and filtering circuitry.
The levels of the audio and data signals are set by digitally controlled variable resistors in U802. These resistors are adjusted in 256 steps by serial data from the microcontroller (see Section 3.3.1). The transmit audio signal is applied to pins 2 and 4 which are the wiper and one end of a potentiometer. It is fed out on pin 3 which is the other end of the potentiometer (see Figure
The output signals on pin 23 and 3 are then combined with the wideband data signal and applied to summing amplifier U803. The output signal from U803 is then applied to the reference oscillator on pin 1 and also across the potentiometer connected to pin 19 of U802. The output on pin 18 is applied to the VCO on pin 5. This modulates both the reference oscillator and VCO. The resistor across pins 19 and 18 of U802 adjusts the balance of these signals.
3.10.4 ACTIVE FILTER (Q800)
Q800 functions as a capacitance multiplier to provide a filtered
of C814. Therefore, base current does not change and the voltage on the emitter remains constant.
3.10.5BUFFER AMPLIFIER (Q801), TX/TX SWITCH (CR801/CR802)
The output signal on pin 2 of the VCO is applied to buffer amplifier Q801. Impedance matching on the input is provided by C822, a section of microstrip, and C826. Microstrip is a form of transmission line with distributed series inductance and shunt capacitance. The characteristic impedance is determined by the width of the microstrip and the PC board material and thickness (distance from ground plane). This stage provides isolation and also amplifies the signal to produce an output level of approximately 0 dBm.
The bias current of Q801 is fixed at a constant level by Q802. The collector current of Q801 flows through R830. The voltage drop across that resistor (and therefore the current) is set by R836 and R837. For example, if current through R830 attempts to increase, the emitter voltage of Q802 decreases. Q802 then conducts less and turns Q801 off slightly to maintain a constant bias current. This provides a stable bias over changes in temperature.
Capacitors C813, C817, C827, and C844 decouple RF signals, and a section of microstrip and C818 provide impedance matching with the transmit/ receive switch. R832 lowers the Q of the microstrip to make it less frequency selective.
The transmit/receive switch formed by CR801, CR802, and several other components switches the VCO signal to the receiver in the receive mode and the transmitter in the transmit mode. CR801 and CR802 are PIN diodes similar to CR901 described in Section
3.10.2.Therefore, they present a very low impedance when forward biased and a very high impedance when reverse biased.
These diodes are controlled by the Q2 signal from shift register U800. This signal is high in the transmit mode and low in the receive mode. Therefore, when the transmitter is keyed, both Q805 and Q806 turn on and CR801 and CR802 are forward biased by the current flowing through Q806, R856, L800, CR801, CR802, R857, and Q805.
| February 2001 |
Part No. |