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ATA5724 Datasheet(PDF) 8 Page - ATMEL Corporation |
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ATA5724 Datasheet(HTML) 8 Page - ATMEL Corporation |
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8 / 46 page  8 9106E–RKE–07/08 ATA5723/ATA5724/ATA5728 4.3 FSK/ASK Demodulator and Data Filter The signal coming from the RSSI amplifier is converted into the raw data signal by the ASK/FSK demodulator. The operating mode of the demodulator is set using the bit ASK/_FSK in the OPMODE register. Logic L sets the demodulator to FSK, applying H to ASK mode. In ASK mode an automatic threshold control circuit (ATC) is employed to set the detection refer- ence voltage to a value where a good signal to noise ratio is achieved. This circuit also implements the effective suppression of any kind of in-band noise signals or competing transmit- ters. If the S/N (ratio to suppress in-band noise signals) exceeds about 10 dB the data signal can be detected properly. However, better values are found for many modulation schemes of the competing transmitter. The FSK demodulator is intended to be used for an FSK deviation of 10 kHz ≤Δf ≤100 kHz. The data signal in FSK mode can be detected if the S/N (ratio to suppress in-band noise signals) exceeds about 2 dB. This value is valid for all modulation schemes of a disturber signal. The output signal of the demodulator is filtered by the data filter before it is fed into the digital signal processing circuit. The data filter improves the S/N ratio as its pass-band can be adopted to the characteristics of the data signal. The data filter consists of a 1 st order high-pass and a 2nd order low-pass filter. The high-pass filter cut-off frequency is defined by an external capacitor connected to pin CDEM. The cut-off frequency of the high-pass filter is defined by the following formula: In self-polling mode the data filter must settle very rapidly to achieve a low current consumption. Therefore, CDEM cannot be increased to very high values if self-polling is used. On the other hand, CDEM must be large enough to meet the data filter requirements according to the data signal. Recommended values for CDEM are given in the electrical characteristics. The cut-off frequency of the low-pass filter is defined by the selected baud-rate range (BR_Range). The BR_Range is defined in the OPMODE register (refer to Section 11. “Configur- ing the Receiver” on page 25). The BR_Range must be set in accordance to the baud-rate used. The ATA5723/ATA5724/ATA5728 is designed to operate with data coding where the DC level of the data signal is 50%. This is valid for Manchester and Bi-phase coding. If other modulation schemes are used, the DC level should always remain within the range of V DC_min = 33% and V DC_max = 66%. The sensitivity may be reduced by up to 2 dB in that condition. Each BR_Range is also defined by a minimum and a maximum edge-to-edge time (t ee_sig). These limits are defined in the electrical characteristics. They should not be exceeded to main- tain full sensitivity of the receiver. fcu_DF 1 2 π × 30 k Ω × CDEM × ------------------------------------------------------------- = |
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Similar Description - ATA5724 |
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