Therefore asymmetric corner frequencies can be observed. The use of AFC results in
more symmetry.
A multi-stage bandpass limiter at a center frequency of 274 kHz completes the receiver
chain. The -3dB corner frequencies of the bandpass limiter are typically at 75 kHz and
at 520 kHz.
An RSSI generator delivers a DC signal proportional to the applied input power and is
also used as an ASK demodulator. Via a programmable anti-aliasing filter this signal is
converted to the digital domain by means of a 10-bit ADC.
The limiter output signal is connected to a digital FSK demodulator.
The immunity against strong interference frequencies (so called blockers) is determined
by the available filter bandwidth, the filter order and the 3rd order intercept point of the
front end stages. For Single-Channel applications with moderate requirements to the
selectivity the performance of the on-chip 3rd order bandpass polyphase filter might be
sufficient. In this case no external filters are necessary and a single down conversion
architecture can be used, which converts the input signal frequency directly to the 2nd IF
frequency of 274 kHz.

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