Part 1 of this blog series reviews the wM-Bus protocol standard in the 868MHZ ISM band in Europe. Now, let’s have a closer look at the wM-Bus protocol version optimized for the 75 kHz narrowband at 169.400 MHz band, as defined in the European Norm (EN) EN300 220 v2.4.1 standard for tracking, tracing, data acquisition and meter reading applications. The maximum radiated power of +500 mW (or +27 dBm) and the allowed duty cycle of <=10% enable a wide area network (WAN) approach, covering a few kilometers even in dense urban environments. The idea behind it is to have as few as possible but highly-sophisticated data collector (DC) units, each of those serving up to 1000 end nodes (or smart meters) with no repeaters in between. The EN13757-4:2014-2 defines the wM-Bus N-modes Nabcdef and Ng by splitting the 75 kHz total available bandwidth into six narrowband channels of 12.5 kHz each. Four channels (Nabef) can carry data rate of 4.8 kbps while the two (Ncd) others run at 2.4 kbps with 2-GFSK modulation. A higher data rate channel (Ng) has been also defined, utilizing 4-GFSK modulation to achieve data rate of 19.2 kbps and occupying a channel bandwidth of 50 kHz (see Figure 1).
Figure 1 wM-Bus modes N-modes and ETSI 300 220 v2.4.1 relationship
This wM-Bus N-mode has been adopted as the RF communications protocol for residential gas meter deployment in Italy and France as well as for the water meters in France. In N-mode, the highest possible receiver class (Hr) should meet ETSI Category 2 receiver blocking requirements. In practice, designers are well advised to target the significantly more challenging ETSI Category 1 receiver system, due to known interferers such as digital video broadcasting (DVB) or FM radio transmitters which do run on other frequency bands but due to the many kilowatts of RF power, they are causing some interference even in the 169 MHz ISM band.
Now, what actually is ETSI Category 1?
The short answer is that it is the strictest RF receiver specification under EN300 200 v2.4.1 for “Highly reliable SRD communication media; e.g. serving human life inherent systems (may result in a physical risk to a person)…”
One of the most challenging requirements in Category 1 is the adjacent channel rejection/selectivity. This is a measure of how robust the receiver is to an interferer that is only +- 12.5 kHz away. Because of its closeness to the wanted signal, it is not possible to filter this signal out with an external SAW filter. Another very challenging condition to meet is the spurious response rejection that needs to be 60 dB from 0.1% of the RF frequency. At 169 MHz, this is only 169 kHz, so RF transceivers with IF frequency above 85 kHz will get the image frequency above 169 kHz and has to show better than 60 dB rejection.
TI’s high-performance Sub-1 GHz CC1120 RF transceiver family is meeting and exceeding all requirements of the N-mode wM-Bus standard (EN13757-4), including all RF requirements in the Italian and French gas meter specifications. The transceiver fully supports reception of all N-mode telegrams with 16 bits preamble (including the 4-GFSK sub-mode) without packet loss due to its WaveMatch feature. The extremely fast automatic gain control settling needs only 4 bits; if combined with RX Sniff mode, it keeps the CC1120 sensitivity at the maximum and reduces the average current while searching for a preamble, as shown in TIDC-WMBUS-169MHz. By applying an optimized set of CC1120 register settings, called “best blocking”, an ETSI Category 1 receiver system performance is achievable without needing to add a costly external surface acoustic wave (SAW) filter or LNA component. The CC1120 RF transceiver was the first integrated transceiver in the industry that can achieve ETSI Category 1 compliance without an external SAW filter and is still market leading in RF performance and robustness. More technical information on CC1120 in wM-Bus N-mode is found in the TI Designs reference designs TIDC-WMBUS-169MHz and TIDC-MULTIBAND-WMBUS . The ability to tweak the performance of the CC1120 for “best sensitivity” or “best blocking” with a few register changes enables a flexible wM-Bus N-mode solution, which can dynamically adapt to a changing RF interference in the field.
Besides the RF sub-system itself, TI has also introduced an innovative battery management solution such as the Energy Buffering for Long-Life Battery Applications Reference Design (PMP9753), which eliminates the costly hybrid layer capacitor (HLC) component and enables the use of multiple battery types from different vendors.
Combining the battery management, the N-mode compliant RF system with a complete CIG compliant wM-Bus stack for Italy and advanced ultra-low power metrology (see EVM430-FR6989 for rotation sensing or the ultrasonic TIDM-ULTRASONIC-FLOW-TDC), TI is offering multiple optimized platforms for implementing smart flow meters with 169 MHz RF communication.
The next blog in this series talks about how a new ultra-low power wireless microcontroller enables next generation single-chip wM-Bus solution for 868 and 433 MHz; stay tuned for an upcoming On the Grid post.
