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4. RF Circuit Technical Brief
4.2 GSM Part
4.2.1 GSM Receiver
[Figure 4.2.1-1] GSM Receiver Part Block Diagram
The constant gain direct conversion receiver contains all active circuits for a complete receiver chain
for GSM/GPRS/EDGE (see [Figure 4.2.1-1] ). The GSM850/900/DCS1800/ PCS1900 LNAs with
balanced inputs are fully integrated. No interstage filtering is needed. The orthogonal LO signals are
generated by a divider-by-four for GSM850/900 band and a divider-by-two for the DCS1800/PCS1900
band.Down conversion to baseband domain is performed by low/high band quadrature direct down
conversion mixers. The baseband chain contains a LNB (low noise buffer), channel filter, output buffer
and DC-offset compensation. The 3rd order lowpass filter is fully integrated and provides sufficient
suppression of blocking signals as well as adjacent channel interferers and avoids anti-aliasing
through the baseband ADC. The receive path is fully differential to suppress on-chip interferences.
Several gain steps are implemented to cope with the dynamic range of the input signals. Depending
on the baseband ADC dynamic range, single- or multiple gain step switching schemes are applicable.
Furthermore an automatic DC-offset compensation can be used (depending on the gain setting) to
reduce the DC-offset at baseband-output. A programmable gain correction can be applied to correct
for front end- and receiver gain tolerances
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