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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Figure 3-9-3. PAM(Power Amplifier Module) interface
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