Nokia RH-18 - Synthesizer; 26 Mhz Reference Oscillator ( VCXO ); Vco

RH-18/38 Company confidential
CCS Technical Documentation
Page 46 Nokia Corporation. Issue 1 10/03
Synthesizer
There is only one PLL synthesizer generating Local Oscillator frequencies for both RX and
TX in both bands ( PCN and EGSM). The VCO frequency is divided by 2 for PCN operation
or by 4 for EGSM operation inside the Mjoelner IC.
26 MHz Reference Oscillator ( VCXO )
The 26 MHz oscillator is located in the Mjoelner IC (N600). The coarse frequency for this
oscillator is set by an external crystal (B600). The reference oscillator is used as a refer-
ence frequency for the PLL synthesizer and as the system clock for BaseBand. The
26MHz signal is divided by 2 to achieve 13MHz inside the UPP IC (D400).
The 26 MHz signal from the VCXO can be measured by probing R425 (must be measured
on the UPP side of R425 i.e. the end not connected to C425). The level at this point is
approx. 700mVpp. Frequency of this oscillator is adjusted by changing the AFC-register
inside the Mjoelner IC. This is done via the Mjoelner serial interface.
Example Signal Measured at VCXO output (R425)
Figure 45: VCXO 26 MHz waveform
VCO
The VCO is an ASIC with all the frequency determining parts inside.
In order to reduce the requirements of the tuning voltage and coverage of the VCO, the
VCO core is composed of four VCOs in parallel. This VCO circuit enables a very wide tun-
ing range of 3.4 - 4.0 GHz .