2.7.5.2 TX Filtering

Gaussian Shaping Filter

The ATA8510/15 embeds a digital Gaussian shaping filter to improve the spectral efficiency of the FSK transmission signal. The filter enables a smooth transition between the two FSK frequencies.

Four different bit-time-bandwidth product (BT) settings can be selected as a parameter (BT = 2, BT = 1.5, BT = 1 or BT = 0.5). A smaller BT value reduces the occupied bandwidth of the spectrum but also generates intersymbol interference.

The module works with an oversampling ratio of 12 compared to the symbol rate. To achieve this value, a clock divider has to be configured appropriately. The correct value is calculated automatically by the configuration tool based on the configured TX data rate. As a result, the data rate configuration in the configuration tool must also be correct in TXMode(transparent) if the Gaussian shaping filter is enabled.

Due to the discrete oversampling, the activation of the Gaussian shaping filter introduces a jitter of approximately 8.5% on the symbol time. If both filters, pre-emphasis and Gaussian shaping are enabled, the combined jitter is limited to 8.8%.

The Gaussian shaping filter can be enabled in both buffered and transparent TX mode, but is not useful for ASK transmission. The Gaussian shaping filter introduces a system delay of approximately one symbol period.

The filter can be enabled independently for path A and path B of each service in the eepServices.txSetPathx[0].GAUS variables.

The BT value can be configured independently for path A and path B of each service in the eepServices.FSFCRx.BTSEL[1:0] variables.

The clock divider can be configured independently for path A and path B of each service in the eepServices.GACDIVx[1:0].GACDIVH[4:0] and GACDIVL[7:0] variables.

Pre-Emphasis Filter

The ATA8510/15 embeds a digital pre-emphasis filter to compensate for the PLL loop bandwidth of 50 kHz that limits the FSK transmission data rate. The activation of the filter is mandatory for symbol rates above 40 Ksym/s (20 Kbit/s Manchester or 40 Kbit/s NRZ). There are no further settings associated with this filter.

If the filter is enabled, the data stream is re-sampled at approximately 2 MHz, which introduces a jitter on the symbol time of about 2% at 20 Kbit/s Manchester (40 Kbit/s NRZ), 4% at 40 Kbit/s Manchester (80 Kbit/s NRZ), 6% at 60 Kbit/s Manchester (120 Kbit/s NRZ) and 8% at 80 Kbit/s Manchester (160 Kbit/s).

The pre-emphasis filter is required in both buffered and transparent TX mode. It is not useful for ASK transmission.

The filter can be enabled independently for path A and path B of each service in the eepServices.txSetPathx[0].PREE variables.

ASK Shaping Filter

The ATA8510/15 embeds an ASK shaping filter to improve the spectral efficiency of the ASK transmission signal. The filter introduces a smooth transition in seven equally spaced steps between the off-state of the power amplifier and the on-state with the configured transmit power.

The rise/fall time of the shaping signal can be configured by a clock divider setting. The configuration tool provides the correct timing which goes with the configured divider value. A reasonable setting is to use 20% of the symbol time, tsym, for the rise/fall (trise/tfall) time, as shown in the following figure.

The transmitted power is approximately 27 dB below the active value for the standard 433 MHz application. In High-Band applications, this value can go down to approximately 23 dB.

The ASK shaping filter can be enabled in both buffered and transparent TX modes and can also be used for ASK and FSK transmissions. When enabled for FSK transmission, the shaping is performed at the beginning and end of the transmission when the carrier is switched on and off.

The filter can be enabled independently for path A and path B of each service in the eepServices.txSetPathx[1].ASKshapingenable variables.

The clock divider can be configured independently for path A and path B of each service in the eepServices.FSFCRx.ASDIV[3:0] variables.