Synthesizer Description And Troubleshooting; 26 Mhz Reference Oscillator (Vcxo); Vco - Nokia RH-6 Series Rf Description & Troubleshooting

Hide thumbs Also See for RH-6 Series:

Table of Contents

RH-6

7 - RF Description & Troubleshooting

Synthesizer Description and Troubleshooting

One PLL synthesizer is generating all the required frequencies for both RX and TX and for

all three bands (EGSM900, GSM1800 and GSM1900). The VCO frequency is divided by 2

or by 4 in Mjoelner depending on which band is active.

The 26 MHz reference oscillator (VCXO, Voltage Controlled Crystal Oscillator) is part of

the Mjoelner RF-ASIC (N601). It uses an external 26 MHz crystal (B601) as external cir-

cuitry.

The reference oscillator has two functions:

• Reference frequency for the PLL synthesizer.

• System clock for BB (RFClk_I = 26 MHz).

For an error free initial synchronization, the 26MHz frequency of the VCXO must be

accurate enough. Therefore, a VCXO-calibration value is written via the serial Bus into

the RefOSCCAL register of Mjoelner and an additional bit in the RefOSCCntl register of

the Mjoelner. That is necessary for the rough calibration of the VCXO

The VCXO is fine tuned by programming the AFC value via the serial bus of Mjoelner. The

necessary AFC value is written into the RefOSCAFC register in Mjoelner.

The VCO is able to generate frequencies in the range from 3420MHz to 3980MHz when

the PLL is working. The frequency of the VCO signal is divided by 2 or by 4 in Mjoelner

RF-ASIC. This way it is possible to generate the frequencies of all channels in EGSM900,

GSM1800 and GSM1900 (both RX and TX).

The output frequency of the VCO is controlled by a DC voltage (Vc) coming from the PLL

loop filter. The valid range of Vc when PLL is in function is 0.7V– 3.8V. The typical tuning

sensitivity of the VCO is 240MHz/V. Even if the PLL is not working (Vc outside the valid

range) there is a frequency at the output of the VCO, which is between 3 and 4 GHz (if

the VCO itself is ok).

Page 28

Company Confidential

CCS Technical Documentation

Issue 1 10/2003

Table of Contents