CIRCUIT DESCRIPTION
800/900 MHz RECEIVER DESCRIPTIONFor example, if current through R207 attempts to increase, the emitter voltage of Q200 decreases. Q200 then conducts less and turns Q201 off slightly to maintain a constant bias current. This provides a stable bias over changes in temperature.
The output signal of Q201 is fed to another bandpass filter similar to the one on the input described in the preceding section. Impedance matching with the filter is provided by a section of microstrip on the collector and C227. Resistor R209 lowers the Q of the microstrip to make it less frequency selective. C222- C226 decouple various unwanted AC signals from the circuit.
3.11.3FIRST MIXER (Q202), CRYSTAL FILTER (Z204)
The signal from Z201 is then applied to mixer Q202. A
Q202 is biased by constant current source Q203 similar to Q200 described in the preceding section. The injection signal is applied to the emitter of Q202 and is at a level of approximately 10 dBm. With 800 MHz models, the injection frequency is 52.950 MHz below the receive frequency, and with 900 MHz models, it is 45 MHz below the receive frequency. Filtering of the injection signal is provided by two- pole bandpass filter Z203. With 800 MHz models, it has a center frequency of 807 MHz and a bandwidth of 18 MHz; and with 900 MHz models, it has a center frequency of 893 MHz and a bandwidth of 6 MHz.
The 52.950 or 45.000 MHz output signal of mixer Q202 is then applied to crystal filter Z204. Impedance matching between Q202 and
Z204 is a
from intermodulation, and other undesired frequen- cies. Impedance matching between this filter and U201 is provided by C271, C272, L215, and R228.
3.11.4 SECOND MIXER/DETECTOR (U201)
Introduction
U201 contains second mixer, IF amplifier, detector, RSSI, and audio amplifier stages as shown in Figure
Second Mixer
From the IF amplifier the signal is internally fed to the mixer which combines it with the 52.500 MHz (800 MHz models) or 44.550 MHz (900 MHz models) second injection frequency to produce a second IF of 450 kHz.
The injection frequency on pin 4 is produced by tripling the frequency of reference oscillator U806. To do this, part of the reference oscillator signal is applied to tripler Q207. This stage is an amplifier with the output tuned for the third harmonic of the reference oscillator frequency. This output tuning is provided by a
Ceramic Filters (Z202/Z206, Z205)
The 450 kHz output of the internal mixer is fed out of U201 on pin 20 and routed to ceramic filter Z202 for all 900 MHz and 800 MHz
Routing of the IF signal to the appropriate filter is provided by Q205 and Q206, PIN diodes CR207- CR210, and several resistors and capacitors. It is controlled by the microcontroller through the Q4 output of shift register U800. This output is low for
| February 2001 |
Part No. |