Motorola PM400 Service Manual download pdf (Page 163)

2-4 THEORY OF OPERATION

3.6 Harmonic Filter

Inductors L111, L113 and L128 along with capacitors C1011, C1023, C1020, C1016, C1025 and

C1026 form a low-pass filter to attenuate harmonic energy coming from the transmitter. Resistor

R150 along with L130 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. A 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 PWR_SET at

ASFIC (U504 pin 6) which acts as the reference for forward power control loop.

The reflected coupled 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.

One local loop for the Pre Driver (U101) is used in order to stabilize the current for each 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 UHF (438-470 MHz) Frequency Synthesizer

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.

This oscillator is temperature compensated to an accuracy of +/-2.5 PPM from -30 to 60 degrees C.

The temperature compensation scheme is implemented by an algorithm that uses five crystal

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Motorola PM400 Service Manual Page 163