Service manual

2-23
5V signal goes to pins 13 and 30 while the 3.3V signal goes to pins 5, 20, 34 and 36 of U201. The
synthesizer in turn generates a superfiltered 4.3V which powers the VCOs and buffers.
In addition to the VCO, the synthesizer also interfaces with the logic and ASFIC circuitry.
Programming for the synthesizer is accomplished through the data, clock and chip select lines (pins
7, 8 and 9) from the microprocessor, U409. A 3.3V dc signal from pin 4 indicates to the
microprocessor that the synthesizer is locked.
Transmit modulation from the ASFIC is supplied to pin10 of U205. Internally the audio is digitized by
the Fractional-N and applied to the loop divider to provide the low-port modulation. The audio runs
through an internal attenuator for modulation balancing purposes before going out at pin 41 to the
VCO.
Figure 2-17: Low Band Frequency Generation Unit Block Diagram
2.15 Synthesizer
The Fractional-N synthesizer, shown in Figure 2-18, uses a 17.0 MHz crystal (Y201) to provide a
reference for the system. Along with being used in the LVFracN, the 17.0 MHz signal is provided at
pin19ofU205forusebytheASFICandLVZIF.
The LVFractN IC (U205) further divides this by 8 internally to give 2.125 MHz to be used as the
reference frequency in the frequency synthesis. While UHF and VHF can use other references,
(divide by 7 or divide by 7/8), only the divide by 8 function is valid for lowband.
The internal oscillator device in the LVFracN together with C236, C237, C242, R219, CR211and Y201
comprise the reference oscillator. This oscillator is temperature compensated is capable of 2.5 ppm
stability over temperatures of -30 to 85°C. There is temperature compensation information that is
unique to each crystal contained on Y201 that is programmed into the radio when built.
The loop filter consists of components C256, C257, C259, R224, R225 and R228. This circuit
provides the necessary dc steering voltage for the VCO and determines the amount of noise and spur
passing through.
To achieve fast locking for the synthesizer, an internal adapt charge pump provides higher current at
pin 45 of U205 to put the synthesizer within lock range. The required frequency is then locked by
normal mode charge pump at pin 43.
Both the normal and adapt charge pumps get their supply from the capacitive voltage multiplier made
up of C247, C283, C284, C285, C286, and triple diodes D210 and D211. This circuit provides 13.3V
at U205, pin 47.
Synthesizer
U205
Voltage
Multiplier
Switching
Network
Loop
Filter
To Mixer
To PA Driver
VCP
VSF
Vmult2
Vmult1
Aux2
Prescaler Input
Modulating
Signal
Rx VCO
Circuit
Tx VCO
Circuit
17.0 MHz
Ref. Osc.
Buffer
Amplifier
Aux3
MOD Out