In 2003, a big automatisation manufacturing company* asked us to research and develop a radio modem for some home equipments.The radio modem has to be incorporated into gauge meters and has to be bi-directional in order to accept remote disconnect commands for the in-debt customers.
The main challenge was the price. It has to be under 5 EUR/unit at industrial scale, able to send secure data at 19 kbps on 432 MHz ISM band and no more than 20 x 20 x 4 mm.
The second main problem was the BER in real life environment.
Our approach was in 3 steps:
-Evaluation;
-Prototyping and testing;
-Final report.
* - We still are under NDA.
Evaluation
An extended survey on "transceiver-on-a-chip" was made. Then, we put all the chip with all their features into a table and give them a "score" based on performance, internal configuration and price.
Then we choose the first three integrated circuit transceiver as the most appropiate circuits for our project: XE-1202, AT-86RF211 and TDA-5255.
Prototyping and testing
We made prototype boards for the selected chipsets; the PCB were made in Romania. We ordered sample for two transceivers with each IC.The next step was to choose real life environment for testing; we choose a large building, a dense foliage area and a LOS (line of sight) area.
For each IC we made tests on each "benchmark".
We used a dedicated software (under Windows) to measure the BER. The BER was measured by sending a packet via first data unit, the packet was repeated by the second unit which was configured only to repeat what was received. Then, the first unit sent the received packet back to the PC software for comparison with the initial packet. Final report
The tests shows that AT-86RF211 and TDA-5255 transceivers have a linear relation between range and BER and XE-1202 have an exponential one which means that XEMICS implemented some tricks on the radio path to reduce BER. We didn't follow this path because was not our goal, we just take it into account.The XE-1202 was "the chosen one".
The RF output power of the XE1202 can be dynamically modified. Four levels ranging from 0dBm to +15dBm was available, and with a receive sensitivity of -113dBm (at 4.8 kbps data rate, 5kHz frequency deviation, 0.1% BER).
Interesting, the LOS tests shows that the modem performed extremely well (less than 1% BER) at more than 2 km with a quarter wave antenna on the ground level at +10 dbm Tx power.
More tests was made in real environment to ensure the validity of the data.
We also redesigned the final PCB to meet the dimension requirements.The final report (incl. the mass production details and samples) was sent to the client.
The price for the modem was less than 5 USD per unit and all other requirements was fulfilled.
PAT (Project Allocated Time)
Evaluation was made in 1 week from the first meeting with the client.
Prototyping and testing was made in 5 weeks from acceptance on evaluation.
Final report was received by the client in 8 weeks from the first meeting.
Bonus
Because XE1202 was frequency agile we was able to derive a radio modem for radioamateur's use. We modify the concept to use a motherboard for the controller unit and a RF daughterboard with the TRx chipset because we want that to be able to use other chipsets for the radio path.We maintained the XE-1202 design due to the frequency agile capability which was very good for hamradio applications.
Adi
