The unlicensed ISM bands below 1GHz are widely used for various consumer and industrial applications where long range, system cost, and long battery life concerns are critical.
Applications in these market segments are diverse and can include home automation, security, industrial control, remote sensing, automatic meter reading, toys, weather stations, among many other consumer applications.
Due to excellent propagation characteristics at sub-GHz frequencies, greatly extended ranges can be obtained at much lower current consumptions than from to the 2.4GHz band solutions. In addition, these sub-GHz bands are free from microwave, WiFi and Bluetooth interference making links substantially more robust than their 2.4GHz counterpart solutions.
The main technical challenge in designing any transceiver for the consumer and industrial markets is managing the conflicting goals of high performance with low cost and low power.
Range and link budget
The EZRadioPRO (Fig. 1) integrates a Class E power amplifier (PA) designed to deliver up to +20dBm and an embedded antenna diversity algorithm. The +20dBm PA was designed to be highly efficient so that transmit current consumption did not significantly effect battery life. The PA is used to simultaneously achieve the conflicting goals of low power consumption and high output power. Several circuit design issues were solved, and when combined with an optimised PCB layout, a highly efficient output power is achieved using a standard CMOS process.
In addition to the highly efficient PA options, the radio has a sensitivity of -118dBm combined, giving a large link budget. Despite the advantages from the sub-GHz bands and a large link budget, range can still be affected by multi-path fading and interference.
Multi-path fading is caused by receiving the transmitted signal from multiple directions, generally caused by signal reflections from other objects such as people, walls, trees and vehicles, which can result in cancellation and attenuation of the signal at the receive antenna. A standard method to counter the effects of multi-path fading is through the use of antenna diversity which according to leading scientific papers and experimentation can increase recover link budget in a typical multi-path fading environment by 8-10dB.
In most cases, however, antenna diversity is often not used as it can be very MCU intensive; leading to greater MCU on-times, an increased MCU specification and a greater overall system current consumption. But the combination of a PA delivering up to +20dBm, a low noise and highly linear receiver front-end providing -118dBm of sensitivity, a sensitive high performance digital GFSK/FSK/OOK modem, and a fully integrated antenna diversity algorithm that does not require MCU overhead can be used to recover link budget from the effects of multi-path fading.
Most applications in these wireless market segments are battery powered, and current consumption forms a critical parameter in the selection of a radio device. A deep sleep mode with minimal current consumption is required to extend the battery lifetime through several years.
Receive current consumption is the second most critical parameter in a transceiver as the radio spends a significant amount of time searching and polling for an available and valid signal most applications. In many applications, a low frequency clock with minimal current consumption is used to maintain timing between the various nodes of the application.
Low power modes including a 32kHz RC oscillator, a 32kHz crystal oscillator and sensor control, all provide low current consumption for long battery life time.
Bill of materials
External components in many applications can add up to dominate the bill-of-material (BOM) cost. For those applications trying to achieve long range, an external PA is often used, which can cost in excess of US$0.50. Integrating the +20dBm PA significantly reduces system costs compared to other solutions relying on an external PA. If +20dBm is not required in the application, the extra output power can be used to compensate for a small, low-cost, low-EIRP antenna while still achieving the same range at a lower cost, compared to the +10dBm integrated PAs commonly available in solutions that require highly tuned antenna.
The highly linear front-end of the radio, in conjunction with its digital channel selection filter, circumvents the need for a dedicated SAW filter in the receive path, as often found in other ISM receivers. Many competitive solutions require an external SAW filter to meet regulatory requirements for spurious performance; these external SAW filters significantly boost their BOM cost.
Instead, a limited number of common passive components (inductors and capacitors) can perform all required filtering and matching. Several features are integrated to further reduce system cost, such as: low battery detection with programmable threshold, general purpose 8-bit ADC, temperature sensor with programmable trimming, ultra low power wakeup timer, and a low duty cycle mode for automatic periodic receive scan. Three GPIO pins are available for numerous applications such as antenna diversity, programmable clock output, interrupt input, valid packet output, etc.
The architecture and block design were carefully chosen for high performance with low current consumption. The transceiver architecture is outlined in Fig. 2.
The receiver architecture is optimized for high sensitivity and high immunity from interferers. For narrow band applications, sensitivity as low as -118dBm can be achieved without an external LNA. Immunity to blockers of 70dB is obtained without using an external RF pre-selection filter. While the high sensitivity is enabled by highly optimized analogue design of the LNA, quadrature mixer, programmable gain amplifier (PGA) and ADC, the channel selectivity and modem sensitivity are performed in the digital domain. This partitioning allows for the best optimization of performance, power consumption, and system cost. The dynamic range is further extended by several gain steps in both the LNA and PGA. These gain steps are controlled by the Automatic Gain Control (AGC) circuit and extends the radio signal strength indicator (RSSI) range up to 120dB.
The main functions of the digital modem are: channel selection filtering, modulation and demodulation, AGC, antenna diversity algorithm and packet handling. The channel selection filter, modulation and demodulation blocks are programmable to cover a wide range of applications. The channel selection filter can be programmed from a minimum bandwidth of 2.6kHz to a maximum bandwidth of 620kHz, supporting both narrow band and wide band ISM applications.
When two antennas are applied the antenna diversity algorithm will automatically select the antenna that receives the strongest signal, recovering link budget in the order of 8-10dB. The integrated packet handler contains numerous programmable features, reducing the required application development time and allowing for use of a low cost MCU. Preamble length, synchronization word, header, packet length and CRC are all extensively programmable from the SPI interface. Separate TX and RX FIFOs, 64bytes each, simplify the communication with the external controller. A retransmit feature is added to repeat a packet transmission without the need to reload the data into the TX FIFO. The header is programmable in length up to 4 bytes and allows for comprehensive packet filtering and broadcasting. The packet handler also supports data whitening/de-whitening and Manchester coding/decoding.
Overall this transceiver pushes technology by providing more performance and features while still maintaining low power consumption and low cost.
Nick Dutton is Marketing Manager at Silicon Labs in Austin, Texas, USA. www.silabs.com