Fractional-N Synthesizer; Figure 7-3. Uhf Frequency Generation Unit Block Diagram - Motorola EP450 Service Manual

403-440 MHz UHF Theory Of Operation: UHF Frequency Generation Circuitry
SPI_CLK, and SYNTH_CS (chip select) lines (U409 pins 100, 1 and 47 respectively). A logic high (3
V) from U201 pin 4 indicates to the microprocessor that the synthesizer is locked.
Voltage
Multiplier
VCP
Vmult1
Synthesizer
Vmult2
U201
16.8 MHz
Ref. Osc.
Modulating
Signal
Transmit modulation from the ASFICcmp (U451 pin 40) is applied to U201 pin 10 (MOD_IN). An
electronic attenuator in the ASFICcmp adjusts overall transmitter deviation by varying the audio level
applied to the synthesizer IC. Internally the audio is digitized by the Fractional-N synthesizer and
applied to the loop divider to provide the low-port modulation. The audio is also routed through an
internal attenuator for the purpose of balancing the low port and high port modulation and reducing
the deviation by 6 dB for 12.5 kHz channels, and is available at U201 pin 41 (VCO_MOD). This audio
signal is routed to the VCO's modulator.
7.4.1

Fractional-N Synthesizer

The Fractional-N synthesizer, shown in
reference frequency for the system. External components C201-C203, R202 and D201 are also part
of the temperature-compensated oscillator circuit. The dc voltage applied to varactor D201 from U201
pin 25 is determined by a temperature-compensation algorithm within U201, and is specific to each
crystal Y201, based on a unique code assigned to the crystal that identifies its temperature
characteristics. Stability is better than 2.5 ppm over temperatures of -30 to 60°C. Software-
programmable electronic frequency adjustment is achieved by an internal DAC which provides a
frequency adjustment voltage from U201 pin 25 to varactor D201.
The synthesizer IC U201 further divides the 16.8 MHz signal to 2.1 MHz, 2.225 MHz, or 2.4 MHz for
use as reference frequencies. It also provides a buffered 16.8 MHz signal at U201 pin 19 for use by
the ASFICcmp.
To achieve fast locking of the synthesizer, an internal adapt charge pump provides higher current at
U201 pin 45 to quickly force the synthesizer within lock range. The required frequency is then locked
by the normal mode charge pump at pin 43. A loop filter (C243-C245 and R243-R245) removes noise
and spurs from the steering voltage applied to the VCO varactors, with additional filtering located in
the VCO circuit.
Both the normal and adapt charge pumps get their supply from the capacitive voltage multiplier made
up of C221-C224 and D220-D221. Two 3 V square waves from U201 pins 14-15 provide the drive
signals for the voltage multiplier, which generates 12.1 V at U201 pin 47. This voltage is filtered by
C225-C228.
HKLN4216A
Aux3
Loop
Filter
MOD Out

Figure 7-3. UHF Frequency Generation Unit Block Diagram

Figure
TRB
Rx Out
Rx VCO
VCOBIC
Circuit
U251
Tx Out
Tx VCO
Circuit
7-4, uses a 16.8 MHz crystal (Y201) to provide the
Buffer
Q280
To Mixer
To PA Driver
November 24, 2003
7-5