ATA8520D Sensitivity Measurement. Features. Description ATAN0141 APPLICATION NOTE

Transcription

1 ATAN0141 ATA8520D Sensitivity Measurement APPLICATION NOTE Features Sensitivity Measurement for ATA8520D V1.3 SIGFOX Transceiver [1] Uses ATA8520-EK1-E Application kit [2] and test transmitter (-21.5dBm level) Requires a Sensitivity Flash application for testing [3] Requires variable damping unit up to 130dB Description This application note describes the sensitivity measurements for the SIGFOX ATA8520D transceiver device. Due to the requirements from SIGFOX for the downlink direction of -126dBm sensitivity level an additional LNA is required for the ATA8520D device which has a sensitivity level of about -120dBm. The sensitivity measurement is controlled by a Flash application which detects the BER of 10-3 to determine the sensitivity level. Caution: The sensitivity measurement application requires ATA8520D V1.3 device.

2 Table of Contents Features... 1 Description Test Setup RF Test Protocol Bit Error and Sensitivity Application Flow and SPI Commands References

3 1. Test Setup For the test a DUT, i.e. the ATA8520-EK1-E kit programmed with the sensitivity test application is connected via the SMA connector to a test transmitter, i.e. a test transmitter device (-21.5dBm) or a RF generator with arbitrary function generator. In this RF path a variable damping unit up to 130dB is included as shown in Figure 1-1. The UART peripheral of the ATA8520-EK1-E kit is connected with an USB-to-Serial converter to a terminal or a PC with terminal application. The test application is running on the ATmega328P MCU of the ATA8520-EK1-E and is started by pressing the button SW1. Figure 1-1 Sensitivity Test Setup UART Tx Variable Damping Unit 50 to 130dB 50Ω RF ATA8520-EK1-E ATA8520D LNA Rx GND USB to Serial Converter 50Ω RF USB Test Transmitter MHz -20dBm PC with Terminal Application The terminal output is shown in Figure 1-2 giving the results of the sensitivity measurements. Figure 1-2 PC Terminal Window Each line represents the transmission of about 1001bits with the following details: + indicates a received frame which is correct. indicates a bit error 3

4 - indicates a lost frame with 91bits ok indicates a transmission without errors (xxxx/1001) indicates the no. of bit errors within 1001bits RSSI=-yyy shows the ATA8520D signal strength in dbm at the device RF input pin The test is performed by increasing the damping at the damping unit until bit errors occur. The sensitivity level is then determined by Sensitivity [dbm] = -21.5dBm - damping [db] (1) When using a RF generator with sufficient RF output level, i.e. down to -130dBm the damping unit may not be required and the sensitivity level can be directly measured with the RF generator (under consideration of the cable and connector losses). The ATA8520D RSSI level indicates the sensitivity level after the LNA at the RF input pin of the device. The measured sensitivity level includes the gain from the LNA and should be at -126dBm or better. 4

5 2. RF Test Protocol The RF signal for the test has to be setup with the following parameters for the reception at the ATA8520D device: RF frequency: Data rate: MHz 600bit/s, FSK modulation with ±0.8kHz deviation, NRZ coded Data frame: 0xAA 0xAA 0xAA 0xAA 0xAA 0xB2 0x27 0x31 0x32 0x33 0x34 0x35 0x36 0x37 < Preamble > <- Sync -> < Payload > The data reception will start within the preamble when at least 5bits are detected without error. This will result in a data frame length between 74 and 112bits. In the calculation and implementation of the Flash application a mean value of 91bits per frame is assumed which lead to 11 frames with 1001bits. The RF test signal can be generated using a 1. RF test generator at MHz with -20dBm 2. RF generator with arbitrary waveform generator to generate the data pattern When using the RF generator the variable damping unit can be removed if a RF signal level down to -130dBm can be generated. 5

6 3. Bit Error and Sensitivity The bit error rate BER is using 1000 data bits with the following considerations for the ATA8520D implementation: The preamble check uses a sequence of at least 5bits without error The sync word check uses 13bits without error following the preamble check The data payload has 56 bits which are checked within the Flash application In total an amount of 74 bits to 112 bits are checked for each telegram. To measure 1000 bits for the BER about 11 telegrams have to be received without errors. The Flash application uses the following calculations for the BER: 1. The frame transmission is repeated every 500ms 2. Each frame is assumed to have 91bits with a total of 1001 for 11 telegrams 3. The bit errors in the payload are counted 4. A lost frame happens when either the preamble or the sync pattern is missed. This results in a loss of 91bits. 5. If the bit errors are 2bits the frame is considered as failed 6

7 4. Application Flow and SPI Commands For the processing of the RSSI measurement special test modes of the ATA8520D device have to be used. Beside the SPI commands defined in the ATA8520D datasheet a RX test mode has to be activated. The SPI commands for the sensitivity measurement are listed in Table 4-1. Table 4-1 Sensitivity SPI Commands for ATA8520D V1.3 CMD Index Payload TX Data RX System Reset 0x01 None None Atmel Version 0x06 None Major / Minor Enable Test Mode 0x08 TBD None Get Status 0x0A None SSM / Atmel FW / SIGFOX Library Get PAC 0x0F None PAC[0], PAC[1]. PAC[15] Read RX Buffer 0x10 None 8 data Bytes Get ID 0x12 None ID[3] ID[0] RX Test Mode 0x18 0x06/0x09/0xFF/0xFF None To activate the RX test mode the following SPI command sequence is applied: 1. The Enable Test Mode command will enable the internal test modes: 2. The RX Test Mode command will start the RX test mode of the ATA8520D device: When 7 bytes are received the event line PB6 is activated and with the Read RX buffer command the received data can be read by the host. The event line is cleared when reading the status information with the SPI Get Status command. The device should be reset after reception of the data payload or when a timeout occurs to close the RX test mode. The RX data buffer should contain the following 8 byte of payload data: 0x31 0x x37 drssi. The RSSI level at the ATA8520D RF input pin is then calculated with RSSI[dBm] = drssi (2) The flow of the sensitivity measurement application is show in Figure

8 Figure 4-1 Sensitivity Measurement Application Flow Start Init Pins, SPI, UART, ATA8520D Read RX buffer Check ATA8520D version to be V1.3 Check 7 data bytes against reference bytes (bit-wise) and set # of error bits If timeout set # of error bits Start RX Test mode with Spi commands 0x08 and 0x18 Perform system reset and wait until finished Data received? no 1s timeout? yes yes # of errors > 1? yes Print error and # of error bits no Print ok and RSSI value no 8

9 5. References [1] ATA8520D transceiver data sheet [2] ATA8520-EK123-E_Tool_pack_V1.0.zip [3] ATAN0141_SW.zip [4] Application Note: Effect of RF System Parameters on Receiver (ATA5745/ATA5746) Sensitivity 9

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