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5.9.14 Digital FM
It is possible to read back the instantaneous IF from the FM demodulator as a frequency offset from the
nominal IF frequency. This digital value can be used to perform a pseudo analog FM demodulation.
The frequency offset can be read from the GAUSS_FILTER register and is a signed 8-bit value coded as
2-complement.
The instantaneous deviation is given by
GAUSS _ FILTER Baud rate
F
=
The digital value should be read from the register and sent to a DAC and filtered in order to get an analog
audio signal. The internal register value is updated at the MODEM_CLK rate. MODEM_CLK is available at
the LOCK pin when LOCK_SELECT[3:0] = 1101 in the LOCK register, and can be used to synchronize
the reading.
For audio (300 to 4000 Hz) the sampling rate should be higher than or equal to 8 kHz (Nyquist) and is
determined by the MODEM_CLK. The MODEM_CLK, which is the sampling rate, equals 8 times the baud
rate. That is, the minimum baud rate, which can be programmed, is 1 kBaud. However, the incoming data
will be filtered in the digital domain and the 3-dB cut-off frequency is 0.6 times the programmed Baud rate.
Thus, for audio the minimum programmed Baud rate should be approximately 7.2 kBaud.
The GAUSS_FILTER resolution decreases with increasing baud rate. A accumulate and dump filter can
be implemented in the µC to improve the resolution. Note that each GAUSS_FILTER reading should be
synchronized to the MODEM_CLK. As an example, accumulating 4 readings and dividing the total by 4
will improve the resolution by 2 bits.
Furthermore, to fully utilize the GAUSS_FILTER dynamic range the frequency deviation must be 16 times
the programmed baud rate.
5.10 Transmitter
5.10.1 FSK Modulation Formats
The data modulator can modulate FSK, which is a two level FSK (Frequency Shift Keying), or GFSK,
which is a Gaussian filtered FSK with BT = 0.5. The purpose of the GFSK is to make a more bandwidth
efficient system as shown in
the chip. The TX_SHAPING bit in the DEVIATION register enables the GFSK. GFSK is recommended for
narrowband operation.
Figure 5-18
and
Figure 5-19
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Equation
´
8
Figure
5-17. The modulation and the Gaussian filtering are done internally in
show typical eye diagrams for 434 MHz and 868 MHz operation, respectively.
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SWRS046H – NOVEMBER 2006 – REVISED MARCH 2015
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