V1.00 Page 12 of 49 ICM MP CCQ GRM
S55/S57 Company Confidential © Copyright Siemens AG 12/02
VCO
4.2.3 Synthesizer: LO2
The second local oscillator is required for transmitter operations only. It consists of a
PLL and a VCO which are integrated inside the Bright 4, and an external second order
loopfilter (R831; C830; C832). Before the VCO generated 640 or 656MHz signal
arrives at the modulator, it is divided by 8. So the resulting frequency after the IQ
modulator is 80/82MHz (depending on channel and band). Programming of the LO2
PLL is done in the same way as described at the LO1. The tree-wire-bus (EGOLD+
signals RFDATA; RFCLK; RFSTR.
(RF Control J15, J16, J17) is used. To ensure the
frequency stability, the 640MHz VCO signal is compared by the phase detector of the
2
nd
PLL with the 26MHz reference signal. The resulting control signal passes the
external loop filter and is used to control the 640/656MHz VCO.
The required voltage VCC_SYN is provided by the ASIC D361
Circuit diagram
Loop-filter LO2
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