26 Mhz Reference Oscillator ( Vcxo ); Vco - Nokia NPM-10 Series Troubleshooting Instructions

NPM-10
Troubleshooting Instructions RF
lyzer and high impedance probe it is possible to get an indication if the VCO outputs the
correct frequency. To do this, probe R656, C654 – the frequency should be 3920 MHz
and the power should be around –50 dBm.

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
26 MHz signal is divided by two to achieve 13 MHz inside the UPP IC (D400). The 26 MHz
signal from the VCXO can be measured by probing R420 (must be measured on the UPP
side of R420; i.e., the end not connected to C420). 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 (R420)

VCO

The VCO is generating frequencies in the range of 3296.8 MHz – 3979.6 MHz when the
PLL is running. The output frequency from the VCO is led to the Local oscillator input of
the Mjoelner IC (N600), where the frequency is divided by two or four so that they can
generate all channels in GSM and PCS respectively. Frequency of the VCO is controlled by
a DC-voltage ( Vctrl ) coming from the loop filter. The loop filter consists of the compo-
nents R618, R619 and C639-C641. Range of the Vctrl when the PLL is running (locked) is
0.4V – 2.4V. Even if the PLL is not in locked state (Vctrl out of range) there is some fre-
quency at the output of the VCO (G600) which is between 3 and 4 GHz. This is of course
only true if the VCO is working and if the VCO power supply is present (2.7V).
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Figure 22: VCXO 26 MHz waveform
Nokia Corporation
CCS Technical Documentation
Confidential Issue 1 03/2003