Nokia 2650 - PLL Synthesizer, Functional Description; Receiver; Agc

RH-53/54
Nokia Customer Care 8-System Module
36 COMPANY CONFIDENTIAL Issue 2 06/2005
Copyright © 2005 Nokia. All Rights Reserved.
The VCO for the 900/1800 (RH-53) bands covers the range of 3420 to 3840 MHz, while the
VCO for 850/1900 (RH-54) (and thereby the quad band) covers 3296 to 3980 MHz.
PLL Synthesizer, Functional Description
The frequency synthesis PLL in conjunction with the VCO and 2/4 dividers generates the LO
signal for both RX and TX paths, locked to the VCXO which again is locked to the base station
through the AFC.
■ Receiver
The Receiver, figure 14, is a dual band direct conversion linear receiver. The received RF signal
is routed from the antenna to the FEM, where the RX/TX switch is located. The RX/TX switch
performs both the switching between receive – transmit routing of the antenna signals as well
as the selection of the band to be used.
The RX signal is routed from the RX/TX switch, in the FEM, to the RX bandpass filter. The filter
input is single ended and the output is balanced. The bandlimited signal is amplified in the in-
ternal LNA and the Pre-gain amplifier before being converted to a BB signal in the passive mix-
er.
Figure 14:Simplified BB, either I or Q channel
The BB signal from the passive mixer is amplified by 24 dB in BBAMP1. BBAMP1 is followed
by LPF1 with a gain of 14 dB and with a pole at 86 kHz. LPF1 is followed by DCN1 (DC com-
pensation amplifier 1) with a minimum gain of 0 dB and a maximum gain of 24. The attenuator
output is filtered in LPF2, a biquad filter, before passing DNC2, (DC compensation amplifier 1).
The total filter combination gives a flat transfer function from DC to 90 kHz. All capacitors for
both filters are located in the RF-ASIC.
The gain characteristic of the BB amplifier is an amplifier with a maximum gain of 80 dB with
an AGC range of 72 dB.
AGC
The AGC keeps the BB level form the receiver within a certain range in order to stay within the
dynamic range for the BB, even during fading.
4 MHz
pole at
input
250
kHz
pole at
output
86 kHz
pole
BBAMP
1
LPF 1 DCN 1 AGC LPF 2 DCN 2
114 kHz
pole pair
Gain ~24 dB Gain ~14 dB
Gain ~ 0 to
24 dB
Gain ~ -48
to 0 dB
Gain ~20 dB Gain ~0 dB
AGC ~ 0 dB AGC ~ 0 dB AGC ~ 24 dB AGC ~ 48 dB AGC ~ 0 dB AGC ~ 0 dB
Step ~ 6 dBStep ~ 6 dB