Synthesizer Operation; Control Head (Com 1550•Ls+); Power Supplies - Motorola CDM1550 LS+ Detailed Service Manual

Theory of Operation 2-31
2.9.4 Synthesizer Operation
The complete synthesizer subsystem comprises mainly of a low voltage FRAC-N (LVFRACN) IC,
Reference Oscillator (crystal oscillator with temperature compensation) , charge pump circuits, loop
filter circuits, and de supply. The output signal (PRESC_OUT) of the VCOBIC (U3301, pin12) is fed
to of U3201, pin 32 (PRE IN) via a low pass filter (L3205 and C3246) which attenuates harmonics
and provides correct level to close the synthesizer loop.
The pre-scaler in the synthesizer (U3201) is basically a dual modulus pre-scaler with selectable
divider ratios . The divider ratio of the pre-scaler is controlled by the loop divider, which in turn
receives its inputs via the SRL. The output of the pre-scaler is applied to the loop divider. The output
of the loop divider is connected to the phase detector, which compares the loop divider's output
signal with the reference signal. The reference signal is generated by dividing down the signal of the
reference oscillator (Y3201).
The output signal of the phase detector is a pulsed de signal which is routed to the charge pump.
The charge pump outputs a current at pin 43 of U3201 (lOUT). The loop filter (which consists of
R3214-15 , R3218, C3235-7, C3245 and L3204) transforms this current into a voltage that is applied
to the varactor diodes CR3303 for transmit, CR3301-2 for receive and alters the output frequency of
the VCO.The current can be set to a value fixed in the LVFRAC-N IC or to a value determined by the
currents flowing into BIAS 1 (U3201-40) or BIAS 2 (U3201-39). The currents are set by the value of
R3211 or R3213 respectively. The selection of the three different bias sources is done by software
programming .
LOCK (U3201-4) signal is routed to one of the J,JP's ADCs input U101-56. From the voltage the J,JP
determines whether LOCK is active.
In order to modulate the PLL the two spot modulation method is utilized Via pin 10 (MODIN) on
U3201. The audio signal is applied to both the AID converter (low frequency path) as well as the
balanced attenuator (high frequency path) . The AID converter converts the low frequency analog
modulating signal into a digital code which is applied to the loop divider, thereby causing the carrier
to deviate. The balance attenuator is used to adjust the VCO 's deviation sensitivity to high frequency
modulating signals. The output of the balance attenuator is present at the MODOUT port (U3201-
41) and connected to the steering line via R3219-20 and C3241.
2.10 Control Head (CDM1550•LS+)
The control head contains the internal speaker, the on/off/volume knob, the microphone connector,
several buttons to operate the radio, several indicator light emitting diodes (LEOs) to inform the user
about the radio status, and a 14 character liquid crystal display (LCD) for alpha - numerical
information, e.g . channel number or call address name. To control the LED's and the LCD, and to
communicate with the host radio the control head uses the Motorola 68HC11 E9 J,JP.
2.10.1 Power Supplies
The power supply to the control head is taken from the host radio's FLT A+ voltage via connector
J0801 pin 3 and the regulated +5V via connector J0801 pin 7. The voltage FLT A+ is at battery level
and is used for the LED's, the back light and to power up the radio via on
I
off
I
volume knob. The
stabilized +5 volt is used for the
J.JP,
display, display driver, and keypad buttons. The voltage USW 5V
derived from the FLT A+ voltage and stabilized by the series combination of R0822 , VR0822 is used
to buffer the internal RAM of the J,JP (U0831 ). C0822 allows the battery voltage to be disconnected
for a couple of seconds without losing RAM parameters. Dual diode 00822 prevents radio circuits
from discharging this capacitor. When the supply voltage is applied to the radio, C0822 is charged
via R0822 and 00822. To avoid that the J.JP enters the wrong mode when the radio is switched on
while the voltage across C0822 is still too low, the regulated 5V charge C0822 via diode 00822 .