LGE Internal Use Only
Copyright © 2008 LG Electronics. Inc. All right reserved.
Only for training and service purposes
The digital transmitter architecture is based on a fractional-N sigma-delta synthesizer for constant
envelope GMSK modulation. This configuration allows a very low power design with a reduced
external component count.
The modulation is transferred between baseband- and RF-part of the PMB7880 via a digital interface
signal into the digital modulator. The following Gaussian filter shapes the digital data stream for the
GMSK modulation. Additionally a pre-distortion filter compensates the attenuation of the PLL transfer
function resulting in a very low distortion at the transmit output.
The filtered digital data stream is scaled appropriately and added to the channel word.
This sum is fed into the MASH modulator. The output of the MASH modulator is a sequence of integer
divider values representing the high resolution fractional input signal. This sequence controls the MMD
(multi modulus divider) at a sample rate of 26MHz. Thus a tightly controlled frequency modulation of
the VCO is achieved.
3. TECHNICAL BRIEF
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