5.9.3 Demodulator, Bit Synchronizer and Data Decision

The block diagram for the demodulator, data slicer and bit synchronizer is shown in Figure 5-10. The built-in bit synchronizer synchronizes the internal clock to the incoming data and performs data decoding. The data decision is done using over-sampling and digital filtering of the incoming signal. This improves the reliability of the data transmission. Using the synchronous modes simplifies the data-decoding task substantially.

The recommended preamble is a ‘010101…’ bit pattern. The same bit pattern should also be used in Manchester mode, giving a ‘011001100110…‘chip’ pattern. This is necessary for the bit synchronizer to synchronize to the coding correctly.

The data slicer does the bit decision. Ideally the two received FSK frequencies are placed symmetrically around the IF frequency. However, if there is some frequency error between the transmitter and the receiver, the bit decision level should be adjusted accordingly. In the CC1021 device, this is done automatically by measuring the two frequencies and use the average value as the decision level.

The digital data slicer in the CC1021 device uses an average value of the minimum and maximum frequency deviation detected as the comparison level. The RXDEV_X[1:0] and RXDEV_M[3:0] in the AFC_CONTROL register are used to set the expected deviation of the incoming signal. Once a shift in the received frequency larger than the expected deviation is detected, a bit transition is recorded and the average value to be used by the data slicer is calculated.

The minimum number of transitions required to calculate a slicing level is 3. That is, a 010 bit pattern (NRZ).

The actual number of bits used for the averaging can be increased for better data decision accuracy. This is controlled by the SETTLING[1:0] bits in the AFC_CONTROL register. If RX data is present in the channel when the RX chain is turned on, then the data slicing estimate will usually give correct results after 3 bit transitions. The data slicing accuracy will increase after this, depending on the SETTLING[1:0] bits. If the start of transmission occurs after the RX chain has turned on, the minimum number of bit transitions (or preamble bits) before correct data slicing will depend on the SETTLING[1:0] bits.

The automatic data slicer average value function can be disabled by setting SETTLING[1:0] = 00. In this case a symmetrical signal around the IF frequency is assumed.

The internally calculated average FSK frequency value gives a measure for the frequency offset of the receiver compared to the transmitter. This information can also be used for an automatic frequency control (AFC) as described in Section 5.9.13.

Figure 5-10 Demodulator Block Diagram