RF Circuits PLL Synthesizer

12.8 MHz TCXO

The TCXO contains the 3-stage thermistor network compen- sation and crystal oscillator and modulation ports. Compen- sation is +/-5 PPM or less from -30c to +60c.

PLL IC Dual Modules Prescaler

Input frequency of 12.8 MHzto IC2 MC14519 pin 20 is divided to 6.25 kHz or 5 kHz by the reference counter, and then supplied to comparator. RF signal input from VCO is divided to 1/64 at prescaler in IC2, Divided by A and N counter in IC2 to determine frequency steps, and then sup- plied to the comparator. PLL comparison frequency is 6.25/5 kHz so that minimum programmable frequency step is 5/

6.25kHz. A and N counter is programmed to obtain the desired frequency by serial data in CPU. In comparator, the phase difference between reference and VCO signals is com- pared. When the phase of reference frequency is leading, Fr is output, but when VCO frequency is leading, Fv is the out- put. When Fv=Fr, phase detector out is very small 0v pulse. 64/65 modulus prescaler is comprised in IC2, and has two output ports:

Port A pin 16: tx enable 2

Port B pin 15: prescaler power save control in PLL IC pin 13 labeled test2 allows the technician to see the output of the dual modules pre-scaler for trouble shooting purposes, no connection should be made to this pin.

Level Shifter & Charge Pump

The charge pump is used for changing output signals Fr, Fv at PLL IC from 0-5v to 0-12v necessary for controlling vco.

Reference Frequency LPF

The Loop Filter contains R12, C21 and C22. LPF settling time is 12 mS with 1 kHz frequency. This also reduces the residual side-band noise for the best signal-to-noise ratio.

DC to DC Converter

The DC to DC converter converts the 5v to 14-16v to supply the necessary voltage for wide range of frequencies in the VCO.

VCO

The VCO consist of an RX VCO and TX VCO. Is switched TX/RX by power source. It is configured as a colpits oscilla- tor and connected to buffer as cascade bias in order to save power.

The varicap diode D201/D301 are low-resistance elements and produce a change in frequency with a change in reverse bias voltage (2-11v). L203/L303 are resonant coil, which changes the control voltage by the tuning core. D202 modu- lation diode, modulates the audio signal. C204 compensate

MAXON

SP-300 HAND HELD

for the non-linearity of the vco due to modulation diode, and maintain a constant modulation regardless of frequency.

RECEIVER

Front End

The receive signal is routed backward through the low pass filter, then onward to Pin 1 of the Hybrid Receiver Front End Module to a bandpass filter consisting of (VHF C622 through C608, L607 through L604) and (UHF C601 through C610, L601 through L603) is coupled to the base of Q601 which serves as an RF amplifier. Diode D601 serves as pro- tection from static RF overload from nearby transmitters. The output of Q601 is then coupled to a second bandpass fil- ter consisting of (VHF C607 through C601, L603 through L601) and (UHF C612 through C623 and L604 through

L607). The output of Pin 6 is then coupled to the doubly bal- anced mixer D9. The receiver front end module is factory pre-tuned and requires no adjustment. Repair is effected by replacement of the entire module of the proper banded mod- ule. These are VHF 148 MHz to 174 MHz and UHF 440 MHz to 470 MHz. The receiver front end module signal pins are as follows:

1.RF Input

2.Input Ground

3.N/A

4.Receive +5V

5.Ground

6.Output

First Mixer

D9, T2 and T3 are double balanced mixers which provide the 45.1 MHz intermediate frequency output. The filtered frequency from the front end module is coupled to T2. The

45.1MHz IF output is matched to the input of the 2-pole monolithic filter by L14, L31, C69 and C97. The crystal fil- ter provides a bandwidth of +/-7.2 kHz from the operating frequency providing a high degree of spurious and intermod- ulation protection. Additionally, a 90 MHz trap (XF1) is also placed at the filter output to provide additional attenuation of the second order IMD. The output of the filter is impedance matched by C97 and C69 to the base of the post of filter IF amplifier Q25.

Second Oscillator Mixer Limiter And FM Detector

The output of the post filter amplifier, Q25, is coupled, via C98 to the input of IC5 (MC3371). IC5 is a monolithic sin- gle conversion FM transceiver, containing a mixer, the sec- ond local oscillator, limiter and quadrature detector. Crystal X1 44.645 MHz is used to provide resultant 455 kHz signal from the output of the second mixer. The mixer output is then routed to CF1 (455F) or CF2 (455HT). These ceramic

Page -11- May 2001