The VCO frequencies are locked by PLLs into a reference oscillator, VCTCXO.
The PLLs are located in N7500 and N7501 respectively and controlled via RFBus. PLL charge pump charges or
discharges the integrator capacitor in the loop filter depending on the phase of the measured frequency
compared to the phase of the reference frequency. The integrator output voltage is connected to the control
input of the VCO.
The VCOs operate at the channel frequency multiplied by two in the upper bands (for example, GSM1800/1900/
WCDMA) and by four in EGSM900 (and GSM850, if applicable). The required frequency dividers required for
modulators are integrated in N7501 and those for demodulators in N7500. The dividers are controlled via RFBus.
Figure 128 Phase locked loop in N7500 and N7501 (PLL)
Reference oscillators
As a reference oscillator for the frequency synthesizers a 38.4MHz VCTCXO (voltage controlled temperature
compensated crystal oscillator) is used.
The output signal of the VCTCXO is directly connected to both N7500 and N7501 where it’s used as synthesizer
reference. N7500 also contains a balanced buffered output for supplying the clock signal to the digital BB ASIC
and a single ended buffer for Bluetooth.
The frequency of the reference oscillator is locked into the frequency of the base station with the help of an AFC
voltage, which is generated in BB by DSP and converted by dedicated DAC.
Regulators
N7500 and N7501 contain integrated regulators to supply regulated voltages for their internal circuitry and
other RF parts. Rx VCO supply is got via a switch from N7500 VR1 regulator. VCO can be switched on and off by
controlling the switch via RFBus.
Supply voltage for the VCTCXO is provided by a BB mixed mode ASIC. The same supply is used for reference clock
input buffers (in N7500 and N7501), output buffers (from N7500 to BB) and for the digital control blocks of both
RF ASICs. When the VCTCXO regulator is set active, the control blocks of the RF ASICs also wake up. After that the
integrated regulators can be controlled via RFBus.
Other supplies, like 4.7V supply for PLL charge pumps and bias reference (VREFRF01) are also provided by the
BB mixed mode ASIC.
RM-42
System Module
Nokia Customer Care
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Company Confidential
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