CC1021-RTR1 datasheet(39/91 Pages) TI1

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Baud rate

GAUSS _ FILTER

Not Recommended for New Designs NRND

CC1021

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SWRS045D – JANUARY 2006 – REVISED JANUARY 2015

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 Equation 23.

(23)

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 uC to improve the resolution.

NOTE

Each GAUSS_FILTER reading should be synchronized to the MODEM_CLK. As an

example, accumulating four 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. The modulation and the Gaussian filtering are done internally in the chip. The

TX_SHAPING bit in the DEVIATION register enables the GFSK.

Figure 5-19 shows a typical eye diagram for 153.6 kBaud data rate at 868 MHz operation.

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CC1021-RTR1 Datasheet(PDF) 39 Page - Texas Instruments