APPLICATION NOTE ATA583x/ATA578x - RF Options ATAN0026 Features ● RF configuration options ● SAW filters ● Low noise amplifiers (LNA) ● Implementation tradeoffs Description Atmel®'s ATA583X/578X family of devices features an RF pin out scheme that accommodates multiple RF configurations. These configurations include operation in multiple frequency bands, minimized component count or maximized sensitivity. This application note will evaluate several chip and external device configurations and the tradeoffs involved. Additionally, the use of external RF components to tailor system performance to increasingly stringent automotive requirements is becoming the standard. Commonly used external components that will be reviewed in this article are the surface acoustic wave (SAW) filters used to limit RF frequency exposure and low noise amplifiers (LNA) used to enhance system sensitivity. As wireless devices become more common and the RF spectrum denser SAW filters are needed to achieve specified performance levels in the presence RF interferers. LNAs are used to preamplify RF signals thereby enhancing sensitivity and range performance. 9309B-RKE-04/15 1. Atmel ATA583X/ATA578X Device Overview3 The Atmel® ATA583X/ATA578X is a highly integrated, low power UHF ASK/FSK RF transceiver/receiver family utilizing an integrated AVR® Microcontroller. The part is capable of operating in all three ISM bands, 310-318MHz, 418-477MHz and 836-956MHz using a single crystal. The device family is characterized in Table 1-1. Table 1-1. Atmel ATA583X/ATA578X Family ATA5831 ATA5832 ATA5833 ATA5781 ATA5782 ATA5783 ROM (KB) 24 24 24 24 24 24 FLASH (KB) 20 - - - 20 - User ROM (KB) - 20 - - - 20 RX +TX RX only - - - Services (profiles) 5 5 5 5 5 5 The Atmel ATA583X/ATA578X block diagram in Figure 1-1 on page 3 illustrates the two RF input paths; one RF output path and the integrated RX/TX switch all of which require external connections. These RF ports and their associated functions will be the primary focus of this application note; ● RFIN_LB (pin 1) - Low band (310-318MHz and 418-477 MHz) receiver input pin ● ● ● ● RFIN_HB (pin 2) - High band (868-956MHz) receiver input pin SPDT_RX (pin 3) - RX/TX switch output to receive paths SPDT_ANT (pin 4) - RX/TX switch common path connected to the antenna SPDT_TX (pin 6) - RX/TX switch output to transmit path (not available in Atmel ATA578X) The internal single pole double throw (SPDT) RF switch is provided to isolate transmit and receive paths however its usage is optional. If used it must be interconnected externally to the appropriate receive and transmit ports on the chip. This switch also performs an optional function called “damping” that helps the chip operate in high signal level conditions. When the RF input port senses a very strong input signal at the tuned frequency, the RX/TX switch adds 15dB of attenuation to the RF signal path. This prevents the input port from being overloaded and the part is able to continue receiving in this case. However, the use of this switch also introduces a fixed signal loss of ~0.75dB that does subtract from the parts sensitivity and transmit power. Using the switch is optional and if chosen to be bypassed this fixed signal loss can be avoided. If bypassed it is up to the user to isolate transmit and receive signal paths and to manage operation in high signal level conditions. 2 ATAN0026 [APPLICATION NOTE] 9309B–RKE–04/15 Figure 1-1. Atmel ATA583X/ATA578X Block Diagram AVCC Sequencer State Machine SRC, FRC Oscillators Watchdog Timer VS DVCC Supplies and Reset Voltage Monitor Clock Management Debug Wire Front-end Registers RFIN_LB LNA, Mixer IF AMP 16 Bit Sync Timer A Rx DSP RFIN_HB D Temp (ϑ) SPDT_RX SPDT SPDT_ANT Damping ANT_TUNE Antenna Tuning SPDT_TX RFOUT VS_PA Power Amplifier Support FIFO 8 Bit Async Timers 2x AVR CPU Data FIFO Tx Modulator NVM Controller 16 Bit Async Timers 2x IRQ Fractional N-PLL ROM 24kB CRC Flash 20kB EEPROM 1024B SRAM 1kByte Tx DSP DATA BUS XTO XTAL1 XTAL2 Port B (8) PB[7..0] SPI Port C (6) PC[5..0] = Not Present in ATA578X Block Diagram ATAN0026 [APPLICATION NOTE] 9309B–RKE–04/15 3 2. Typical RF Performance Requirements Some of the key radio parameters to be considered in this application note are detailed in this section. In many applications these parameters are traded off, e.g., sensitivity versus dynamic range and so the system is optimized for one or two parameters. 2.1 RF Transmit Power Transmit power is the power level of the output RF signal. The Atmel® ATA583X device has an output power specified at pin 7 (RF_OUT) of the IC package. The addition of components to this signal path reduces the power available for transmitting signals. These components typically include the internal TX/RX (SPDT) switch and SAW filters. The range of the RF link is dependent on the RF output power so the less power lost to external components the greater the range. The device has a programmable output power level with a range from –12dBm to +14.5dBm. Note that placement of SAW filters in the transmit path requires a careful approach to the design particularly when used with high output power levels. If the filter output is not matched perfectly then phase-shifted reflected signals (due to the group delay introduced by SAW filter) are present that will interfere with the desired signal and cause instabilities. 2.2 RF Receive Sensitivity Receive sensitivity is the lowest RF power level of a signal that can be successfully be received. The Atmel ATA583X device has it's receive sensitivity specified at the RF input ports pin 1 (RFIN_LB) and pin 2 (RFIN_HB) of the IC package. The addition of any components to this signal path reduces the minimum sensitivity level. These components typically include the internal TX/RX (SPDT) switch and SAW filters. The device has a sensitivity level range of –108dBm to –123dBm depending on which IF bandwidth, data rate and modulation types are selected. An active external low noise amplifier (LNA) can optionally be added to the RF receive path that will improve the sensitivity. 2.3 RF Blocking Performance Blocking is defined as the degradation of receiver sensitivity in the presence of stronger (blocking) signals at frequencies outside the tuned frequency of the receiver. This parameter is critical to receiver performance in the presence of other RF radiating devices such as cell phones, garage door openers and pagers. To successfully operate in dense RF environments a SAW filter is typically used. 2.4 RF Dynamic Range - In Band Signal RF dynamic range is the range of power levels, at the tuned frequency, over which the receiver can operate. The dynamic range of the receiver is important because it must be able to successfully operate with strong signals well as weak ones. This is critical as a receiver must be able to perform when very strong signals are present such as when the transmitter is physically close to the receiver. To successfully operate in high signal level conditions the device has an internal damping function (part of the SPDT switch) that adds 15dB of attenuation to the RF front end in strong signal conditions. 4 ATAN0026 [APPLICATION NOTE] 9309B–RKE–04/15 3. Atmel ATA583X/ATA578X RF Configuration Options The Atmel® ATA583X/578X part can be configured in numerous ways. This section will highlight several of these those possible options highlighting the pros and cons of each. No one configuration is right for all requirements so each case must be evaluated on an individual basis. The configuration flexibility of this family of parts is one of its key advantages allowing the end user to optimize performance according to the requirements for the design. Atmel ATA583X Transceiver Typical Application with SAW Filter In this example a single band transceiver is illustrated. A SAW filter is used at the antenna input to limit out of band frequencies enabling operation in dense RF environments. This is perhaps the most common use case and represents a good compromise for all aspects of transmit and receive RF performance. See comment on use of SAW filters in the transmit path in Section 2.1 “RF Transmit Power” on page 4. Use of SAW filters in this configuration requires optimum matching between SAW filter and antenna and is not recommended for power settings greater than 5dBm. Figure 3-1. Atmel ATA583X Transceiver Application with SAW Filter IRQ NSS VS RFIN_LB 26 MISO 25 PB3 27 PB4 28 PB5 ATEST ATEST _IO1 _IO2 29 PB6 30 PB7 1 31 AGND 32 24 PB2 2 23 RFIN_HB PB1 3 22 SPDT_RX PB0 Atmel ATA5831 ATA5832 ATA5833 4 SPDT_ANT 5 ANT_TUNE 6 20 DVCC PC5 PC4 18 VS_PA PC3 17 9 10 11 12 13 14 CLK_IN 15 Microcontroller PC2 PC1 PC0 VS AVCC TEST _EN XTAL2 7 8 SCK DGND RF_OUT SPDT_TX MOSI 21 19 XTAL1 3.1 16 VS = 5V VDD PROS ● ● ● ● ● Minimized component count Good blocking performance with SAW filter Enhanced dynamic range utilizing SPDT damping function Common TX/RX antenna Transmitted harmonic limited by SAW filter CONS ● ● ● SAW filter loss (~2dB) effects both transmit and receive paths Transmit power can exceed SAW max limits (typically +10dBm) RF transmit mismatch to the SAW filter can result in reflected RF voltages exceeding chip limits when transmitting at high power levels. Per the concerns of using a SAW filter in the transmit path outlined in Section 2.1 “RF Transmit Power” on page 4 care needs to be taken to isolate phase shifted reflected signals from the desired signal. This includes board-level coupling and IC-level coupling with the latter being subject to degradation if RF voltage levels exceed IC limits. ATAN0026 [APPLICATION NOTE] 9309B–RKE–04/15 5 3.2 Atmel ATA583X Transceiver Optimized for Transmit Power In this example a single frequency transceiver is illustrated. The SAW filter is relocated to the receive side of the TX/RX switch. This eliminates the SAW signal loss for the transmitter increasing output power however the transmitted output spectral content is no longer filtered by the SAW. Note that the TX/RX switch can be saturated by wide band interference (at any frequency) and that tight EMC or blocking specifications may not be met with this circuit configuration. The isolation provided by the SPDT switch function is limited to ~60dB. Figure 3-2. Atmel ATA583X Transceiver Optimized for Output Power IRQ NSS VS 26 MISO 25 PB3 27 PB4 28 PB5 ATEST ATEST _IO1 _IO2 RFIN_LB 29 PB6 30 PB7 1 31 AGND 32 24 PB2 2 23 RFIN_HB PB1 3 22 SPDT_RX PB0 Atmel ATA5831 ATA5832 ATA5833 4 SPDT_ANT 5 ANT_TUNE 6 20 DVCC PC5 RF_OUT PC4 18 VS_PA PC3 17 9 10 11 12 13 14 15 16 VS = 5V PROS ● ● ● Transmit power increased with no SAW filter loss Good receiver blocking at lower signal levels with SAW filter in receive path Common TX/RX antenna CONS ● ● ● 6 SAW filter loss (~2dB) effects receive path Susceptible to out of band receiver jamming (SPDT saturation) Transmitter harmonic content no longer limited by SAW ATAN0026 [APPLICATION NOTE] 9309B–RKE–04/15 Microcontroller PC2 PC1 PC0 VS AVCC XTAL2 XTAL1 TEST _EN CLK_IN DGND 7 8 SCK 21 19 SPDT_TX MOSI VDD Atmel ATA583X Transceiver Typical Dual Band (One TRX and One RX) Application with SAW Filters In this example a dual band application is illustrated. One band functions as a transceiver while the other band is receive only. Figure 3-3. Atmel ATA583X Transceiver Dual Band Application IRQ NSS VS RFIN_LB 26 MISO 25 PB3 27 PB4 28 PB5 ATEST ATEST _IO1 _IO2 29 PB6 30 PB7 1 31 AGND 32 24 23 22 Atmel ATA5831 ATA5832 ATA5833 SPDT_ANT ANT_TUNE 6 21 DGND 20 PC5 19 RF_OUT PC4 18 VS_PA PC3 17 SPDT_TX 9 10 12 13 14 15 PC2 PC1 PC0 VS AVCC XTAL2 11 Microcontroller DVCC 7 TEST _EN CLK_IN PB0 SPDT_RX 4 8 SCK PB1 RFIN_HB 3 5 MOSI PB2 2 XTAL1 3.3 16 VS = 5V VDD PROS ● ● ● ● ● ● ● ● Two band operation TX/RX RX on separate band Minimized component count Good blocking performance - both RX paths with SAW filters Enhanced dynamic range utilizing SPDT damping function transceiver band only Common TX/RX antenna Transmitted harmonic limited by SAW filter CONS ● ● ● SAW filter loss (~2dB) effects transmit and both receive paths ● Two antennas required and optionally an external SPDT to effectively isolate both receive paths Transmit power can exceed SAW max limits (typically +10dBm) RF mismatch to the SAW filter can result in reflected RF voltages exceeding chip limits when transmitting at high power levels ATAN0026 [APPLICATION NOTE] 9309B–RKE–04/15 7 3.4 Atmel ATA578X Typical Receiver Application with SAW In this example a single frequency receiver is illustrated. A SAW filter is used at the antenna input to limit out of band frequencies enabling operation in dense RF environments. This is perhaps the most common use case and represents a good compromise for all aspects of receive RF performance. Figure 3-4. Atmel ATA578X Typical Receiver Application IRQ NSS VS RFIN_LB 26 MISO 25 PB3 27 PB4 28 PB5 ATEST ATEST _IO1 _IO2 29 PB6 30 PB7 1 31 AGND 32 24 PB2 2 23 RFIN_HB PB1 3 22 SPDT_RX PB0 Atmel ATA5781 ATA5782 ATA5783 4 SPDT_ANT 5 ANT_TUNE 6 20 DVCC PC5 RF_OUT PC4 18 VS_PA PC3 17 9 10 11 12 13 14 15 16 VS = 5V PROS ● ● ● Minimized component count Saw filter improves RX out of band blocking performance Enhanced dynamic range utilizing SPDT damping function CONS ● 8 SAW filter loss (~2dB) ATAN0026 [APPLICATION NOTE] 9309B–RKE–04/15 Microcontroller PC2 PC1 PC0 VS AVCC XTAL2 XTAL1 TEST _EN CLK_IN DGND 7 8 SCK 21 19 SPDT_TX MOSI VDD Atmel ATA578X Receiver Optimized for Receiver Sensitivity In this example a single frequency receiver is illustrated. The RF input is routed directly to the input port on the chip bypassing the SPDT switch. This avoids the loss in the switch but sacrifices the damping function provided by the switch. A SAW filter is used at the antenna input to limit out of band frequencies enabling operation in dense RF environments. Figure 3-5. Atmel ATA578X Receiver Optimized for Sensitivity IRQ NSS VS LNA RFIN_LB 26 MISO 25 PB3 27 PB4 28 PB5 ATEST ATEST _IO1 _IO2 29 PB6 30 PB7 1 31 AGND 32 24 PB2 2 23 PB1 RFIN_HB 3 22 PB0 SPDT_RX Atmel ATA5781 ATA5782 ATA5783 4 SPDT_ANT 5 ANT_TUNE 6 20 DVCC RF_OUT PC4 18 VS_PA PC3 17 9 10 11 12 13 14 CLK_IN 15 Microcontroller PC2 PC1 PC0 VS AVCC TEST _EN XTAL2 7 8 SCK DGND PC5 SPDT_TX MOSI 21 19 XTAL1 3.5 16 VS = 5V VDD PROS ● ● Good receiver blocking performance with SAW filter SPDT switch bypassed for best sensitivity CONS ● ● SAW filter loss (~2dB) Compromised dynamic range with SPDT damping function not used ATAN0026 [APPLICATION NOTE] 9309B–RKE–04/15 9 3.6 Atmel ATA578X Dual Band Receiver Optimized for Sensitivity In this example a dual band application is illustrated. One band functions as a transceiver while the other band is receive only. The RF inputs are routed directly to the input ports on the chip bypassing the SPDT switch. This avoids the loss in the switch but sacrifices the damping function provided by the switch. A SAW filter is used at both antenna inputs to limit out of band frequencies enabling operation in dense RF environments. Figure 3-6. Atmel ATA578X Dual Band Receiver SPDT Bypassed IRQ NSS VS RFIN_LB 26 MISO 25 PB3 27 PB4 28 PB5 ATEST ATEST _IO1 _IO2 1 29 PB6 30 PB7 31 AGND 32 24 PB2 2 23 RFIN_HB PB1 3 22 SPDT_RX PB0 Atmel ATA5781 ATA5782 ATA5783 4 SPDT_ANT 5 ANT_TUNE 6 20 DVCC PC5 RF_OUT PC4 18 VS_PA PC3 17 9 10 11 12 13 14 15 16 VS = 5V PROS ● ● ● Saw filter improves RX out of band blocking performance SPDT switch bypassed for best sensitivity Dual band receiver CONS ● ● ● 10 Degraded high level desired signal operation (SPDT switch damping function bypassed) SAW filter loss (~2dB) Requires two antennas ATAN0026 [APPLICATION NOTE] 9309B–RKE–04/15 Microcontroller PC2 PC1 PC0 VS AVCC XTAL2 XTAL1 TEST _EN CLK_IN DGND 7 8 SCK 21 19 SPDT_TX MOSI VDD 4. External Component Details This section provides an overview of components commonly applied externally to a RF receiver and discussed in the preceding paragraphs. 4.1 Surface Acoustic Wave (SAW) Filters SAW filters are used in wireless car access receivers to function as RF pre-selection filters. Usually placed immediately after the antenna in the RF circuit path the goal is to preselect or filter out all frequencies not used by the system. This serves to “block” those undesired frequency components that might interfere with operation of the radio receiver. It is particularly important to block signals from intentional RF radiators such cell phones (RF output power up to +30dBm) and garage door openers operating near the receiver. If there is no filtering of these incoming RF signals the internal LNA or damping switch can be saturated by these out of band transmitters and rendered incapable of operation. Automotive SAW filters are available with a pass bandwidth from 200KHz to 2MHz and with an insertion loss of 2-4dB. While the insertion loss reduces the overall system sensitivity the ability to operate in a dense RF environment is usually more desirable than maximum radio sensitivity. 4.2 Low Noise Amplifier (LNA) A LNA is an electronic amplifier placed before the RF input port on a radio receiver. It is used to enhance the sensitivity of the receive enabling a longer reception range. Note that the sensitivity of a receiver is determined primarily by the system noise figure which establishes the noise floor of the system. While an external LNA provides additional gain for the incoming signal it more significantly establishes a good noise figure for the incoming signal. The desired characteristics of an LNA are first a low noise figure and then some amplification or gain. It is a common misconception that the sensitivity of a receiver is increased by the gain of the LNA, i.e., LNA gain = 12dB so sensitivity of the system improves by 12dB. An LNA typically does improve sensitivity but it is primarily due to noise figure improvement. A well executed external LNA device typically improves the sensitivity of an Atmel® ATA583X/578X system by 3-5dB. A good LNA has a low noise figure of ~1.5dB and a gain of ~5-20dB. Be aware that adding an LNA the device also establishes several other critical receiver specifications such as intermodulation and compression points. These specifications are critical to operation in high signal level environments and need to be traded off versus the resultant increase in sensitivity. Special care should be exercised when adding an LNA since it is typically operates over a wide frequency range and so is susceptible to interference over the whole range of operation. ATAN0026 [APPLICATION NOTE] 9309B–RKE–04/15 11 5. Revision History Please note that the following page numbers referred to in this section refer to the specific revision mentioned, not to this document. 12 Revision No. History 9309B-RKE-04/15 Put document in the latest template ATAN0026 [APPLICATION NOTE] 9309B–RKE–04/15 XXXXXX Atmel Corporation 1600 Technology Drive, San Jose, CA 95110 USA T: (+1)(408) 441.0311 F: (+1)(408) 436.4200 | www.atmel.com © 2015 Atmel Corporation. / Rev.: 9309B–RKE–04/15 Atmel®, Atmel logo and combinations thereof, Enabling Unlimited Possibilities®, and others are registered trademarks or trademarks of Atmel Corporation in U.S. and other countries. Other terms and product names may be trademarks of others. DISCLAIMER: The information in this document is provided in connection with Atmel products. No license, express or implied, by estoppel or otherwise, to any intellectual property right is granted by this document or in connection with the sale of Atmel products. EXCEPT AS SET FORTH IN THE ATMEL TERMS AND CONDITIONS OF SALES LOCATED ON THE ATMEL WEBSITE, ATMEL ASSUMES NO LIABILITY WHATSOEVER AND DISCLAIMS ANY EXPRESS, IMPLIED OR STATUTORY WARRANTY RELATING TO ITS PRODUCTS INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTY OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, OR NON-INFRINGEMENT. IN NO EVENT SHALL ATMEL BE LIABLE FOR ANY DIRECT, INDIRECT, CONSEQUENTIAL, PUNITIVE, SPECIAL OR INCIDENTAL DAMAGES (INCLUDING, WITHOUT LIMITATION, DAMAGES FOR LOSS AND PROFITS, BUSINESS INTERRUPTION, OR LOSS OF INFORMATION) ARISING OUT OF THE USE OR INABILITY TO USE THIS DOCUMENT, EVEN IF ATMEL HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. Atmel makes no representations or warranties with respect to the accuracy or completeness of the contents of this document and reserves the right to make changes to specifications and products descriptions at any time without notice. Atmel does not make any commitment to update the information contained herein. Unless specifically provided otherwise, Atmel products are not suitable for, and shall not be used in, automotive applications. Atmel products are not intended, authorized, or warranted for use as components in applications intended to support or sustain life. SAFETY-CRITICAL, MILITARY, AND AUTOMOTIVE APPLICATIONS DISCLAIMER: Atmel products are not designed for and will not be used in connection with any applications where the failure of such products would reasonably be expected to result in significant personal injury or death (“Safety-Critical Applications”) without an Atmel officer's specific written consent. Safety-Critical Applications include, without limitation, life support devices and systems, equipment or systems for the operation of nuclear facilities and weapons systems. Atmel products are not designed nor intended for use in military or aerospace applications or environments unless specifically designated by Atmel as military-grade. Atmel products are not designed nor intended for use in automotive applications unless specifically designated by Atmel as automotive-grade.