CIRCUIT DESCRIPTION
800/900 MHz TRANSMITTER DESCRIPTIONIf a narrowband channel is selected, a low signal is applied to the base of Q205. That transistor is then turned off and inverter Q206 turned on. CR209/CR210 are then forward biased and CR207/CR208 reverse biased. This routes the 450 kHz IF signal through
Z202 and blocks it from Z206. If a wideband channel is selected, the opposite occurs. For more information on the operation of PIN diodes, refer to Section 3.11.1.
The filtered 450 MHz IF signal is then applied to pin 18, amplified by an internal amplifier, and fed back out on pin 16 and applied to ceramic filter Z205. This filter is identical to Z206 and provides additional attenuation of wideband noise. The loss introduced by each ceramic filter is approximately 12 dB.
Limiter/Detector
The signal from Z205 is applied to an internal limiter connected to pin 14. The limiter amplifies the 450 kHz signal and then limits it to a specific value to remove amplitude variations. From the limiter, the signal is fed internally to the quadrature detector. An external phase shift network connected to pin 10 shifts the phase of one of the detector input signals 90° at 450 kHz (the other input is unshifted in phase). When modulation occurs, the frequency of the IF signal changes at an audio rate as does the phase of the shifted signal. The detector, which has no output with a 90° phase shift, converts this phase shift into an audio signal. L218 is tuned to provide maximum undistorted output from the detector.
The audio signal is then fed internally to an audio amplifier. The gain of this stage is set at about three by R255 and R256. The audio signal is then fed out on pin 8 and routed to the audio/logic board.
Also in U201 is an RSSI detector which provides a temperature compensated RSSI (Receive Signal Strength Indicator) signal on pin 5. This is a low impedance (2k ohm) output with a dynamic range of 70 dB. The DC voltage of this output changes in proportion to IF signal strength. This signal is routed to an A/D input of the microcontroller (pin 59) and used along with the squelch signal to determine receive signal strength. R259/C304 and R258/C303 provide low pass filtering of the audio and RSSI signals, and C305 and C306 decouple RF.
NOTE: The transmitter block diagram is located in Figure
3.12.1 PREDRIVER (Q506), DRIVER (Q507)
The input signal to the exciter is the transmit frequency from buffer amplifier Q801 in the synthe- sizer. It is at a level of approximately 0 dBm and is applied to predriver Q506. Impedance matching on the input of Q506 is provided by C529, C515, two sections of microstrip (see description in Section 3.10.5), R518, and C516. Biasing is provided by R519, R520, R522, and R523. Various AC signals are decoupled from the DC supply by C520, C525, C526, C528, and C531.
The
This transmit 8V supply is not delayed which allows Q505 and the transmitter frequency to stabilize before power is produced. The delayed PTT signal is applied to the RF board on J201, pin 2. This signal controls the power control circuit described in Section
3.12.5.The emitters of Q503 and Q505 are grounded through Q509. That transistor is turned off when the logic is in an undetermined state such as during Flash programming. This ensures that the transmitter is turned off during these times.
From Q506 the transmit RF output signal is then applied to driver Q507. Impedance matching between Q506 and Q507 is provided by several capacitors and sections of microstrip and L501. Resistor R521 lowers the Q of the parallel microstrip which makes it less frequency selective. Q507 is biased for class C operation by L504 and ferrite bead EP501.
Supply voltage to Q507 is from the power control circuit described in Section 3.12.5. This circuit varies the supply voltage of Q507 which changes its power output to maintain constant transmitter power output.
| February 2001 |
Part No. |