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MAXON
SP-300 HAND HELD
Page -11-
May 2001
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/
6.25 kHz. 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
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
45.1 MHz 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