Motorola Radius CM200, Руководство по обслуживанию

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2-4 THEORY OF OPERATION
3.6 Harmonic Filter
Inductors L111, L112, L124 and L113 along with capacitors C1132, C1022, C1020, C1137, C1018,
and C1017 form a low-pass filter to attenuate harmonic energy coming from the transmitter.
Resistor R150 drains any electrostatic charges that might otherwise build up on the antenna. The
harmonic filter also prevents high level RF signals above the receiver passband from reaching the
receiver circuits to improve spurious response rejection.
3.7 Power Control
The output power is regulated by using a forward power detection control loop. The directional
coupler samples a portion of the forward and reflected RF power. The forward sampled RF is
rectified by diode D105, and the resulting DC voltage is routed to the operational amplifier U100.
The error output current is then routed to an integrator, and converted into the control voltage. This
voltage controls the bias of the pre-driver (U101) stage. The output power level is set by way of a
DAC, PWR_SET, in the audio processing IC (U504), which acts at the forward power control loop
reference.
The sampled reflected power is rectified by diode D107,The resulting DC voltage is amplified by an
operational amplifier U100 and routed to the summing junction. This detector protects the final stage
Q100 from reflected power by increasing the error current. The temperature sensor protects the
final stage Q100 from overheating by increasing the error current. A thermistor RT100 measures the
final stage Q100 temperature. The voltage divider output is routed to an operational amplifier U103
and then goes to the summing junction. The Zener Diode VR101 keeps the loop control voltage
below 5.6 V and eliminates the DC current from the 9.3 regulator U501.
A local loop for the Pre Driver (U101) is used in order to stabilize the current for this stage.
In Rx mode, the two transistors Q101 and Q102 go to saturation and shut down the transmitter by
applying ground to the Pre Driver U101.
4.0 VHF (146–174 MHz) Frequency Synthesis
The synthesizer consists of a reference oscillator (Y201), low voltage Fractional-N (LVFRAC-N)
synthesizer (U200), and a voltage controlled oscillator (VCO) (U201).
4.1 Reference Oscillator
The reference oscillator is a crystal (Y201) controlled Colpitts oscillator and has a frequency of
16.8 MHz. The oscillator transistor and start-up circuit are located in the LVFRAC-N (U200) while
the oscillator feedback capacitors, crystal, and tuning varactors are external. An analog-to-digital
(A/D) converter internal to the LVFRAC-N (U200) and controlled by the microprocessor via SPI sets
the voltage at the warp output of U200 pin 25. This sets the frequency of the oscillator.
Consequently, the output of the crystal Y201 is applied to U200 pin 23.
The method of temperature compensation is to apply an inverse Bechmann voltage curve, which
matches the crystal’s Bechmann curve to a varactor that constantly shifts the oscillator back on
frequency. The crystal vendor characterizes the crystal over a specified temperature range and
codes this information into a bar code that is printed on the crystal package. In production, this
crystal code is read via a 2-dimensional bar code reader and the parameters are saved.