Miniature radio tracking tags built on CoolRunner-II features can transmit environmental data hundreds of feet.

Whether underwater, in the air, or below ground, radio tracking tags bring sensors to where they’re most useful, transmitting local data back for logging and analysis. To ensure data accuracy in challenging, noisy environments, the tags transmit error-corrected digital IDs to represent varied sensor data.

Biotags developed the Micro-ID family of radio tracking tags around the special features of the Xilinx CoolRunner™-II CPLD, which provides the noteworthy combination of microampere quiescent current draw, nonvolatile ID storage, JTAG configuration, micro-ball grid array (BGA) packaging, and high-speed output drive.

In addition to the 1.8V CoolRunner-II device, Biotags’ Micro-ID core functions require just a watch-crystal oscillator for time base and a gated-crystal oscillator for radio frequency (RF) carrier generation – an integration that results in a functional single board data transmitter contained in a 0.8-cm square (8 mm by 10 mm).

Customizing Micro-ID tags to different applications often requires only source code changes to the CPLD programming, providing a significant cost advantage over ASIC-based radio tags.

Radio-Tagged Salmon
In fisheries research and population studies, biologists often trace downstream salmon migration by placing radio tracking tags in the smolts and tracking the tagged fish as they pass around dams. Because salmon smolts are typically under 20 cm long, they require a tiny radio tag. As shown in Figure 1, the smallest version of the Micro-ID radio tag measures 27 mm long and 10 mm wide, and has a mass of less than 2.5 grams.

To monitor migrations from river to ocean, researchers set up antennas and receivers at crucial areas around dams where salmon and predators are most likely to meet. Typically, the receivers can detect a tag at a distance exceeding 1,000 feet, even when the tag is at a water depth of 6 feet. Because each tag has its own set of custom IDs, researchers can track the route of each individual salmon through miles of river.

Location tracking is only part of the story, however. Sensor data provides far more specific information on survivability. For this application, the sealed Micro-ID tag continuously monitors salmon body temperature for a period of three days. If the temperature exceeds 80°F (the digital switch point), then the salmon didn’t survive and likely served as prey for a mammal or bird.

To differentiate these two environmental states, the Micro-ID tag stores the change and transmits a different ID after the temperature switch point is triggered, allowing researchers to review the transition moment in the data log. In this way, radio tracking tags monitor and respond to their environment.

Beyond underwater radio transmission applications, the Micro-ID radio tracking tag can transmit through building materials and masonry, and thus track pipes in buildings or even under a street. In another industrial application, placing a Micro-ID tag within raw materials aids in the continuous identification of the source and movement of those materials.

Extend Battery Life
Because Micro-ID tags include a rechargeable battery, they’re reusable. But when an RF ID tag includes a battery, it’s defined as an active RF tag. Let’s consider how the brief RF transmission time of the tag programming extends battery life.

Low-power active tag operation depends on a few crucial specifications of the CoolRunner-II CPLD: low quiescent power draw and low I cc vs. frequency. Tracking tags typically have a tiny duty cycle (a ratio of 1:32 or less) so that the higher current draw of the RF transmission portion of the cycle results in low average power consumption.

By driving the CoolRunner-II CPLD during the tag’s quiet period with a low frequency clock and minimizing the timing logic, it is quite easy for a programmed 64-macrocell CoolRunner-II device to draw under 40 µA in this application.

With a judicious review of the I/O design as well as careful mapping of I/Os in Xilinx WebPACK™ software tools, you can optimize power consumption to microampere levels, allowing low-mass watch batteries to power the CoolRunner-II CPLD for days.

WebPACK Software Compacts ID and ECC Data
Although the 56-pad micro-BGA package for CoolRunner-II CPLDs contains at most 64 macrocells, you can implement at least four codes, 32 bits in length, and still have room for control logic, because WebPACK software compacts the 128 bits of serial ID data during logic reduction.

Let’s consider how to build custom data codes for ID transmission at the bit level. I often construct 32-bit data codes to include synchronization (occupying the first four bits) in the error correcting code (ECC) calculation. After the synchronization bits come the ID code for the tag (14 bits), followed by 14 bits of ECC data.

The large number of ECC bits in this application provides for excellent data reliability. The type of error correcting codes and modulation techniques depends on the tag application.

Most importantly, ECCs and modulation techniques are defined in the source file for CPLD programming. With the serial ID data defined, the next step is to transmit the data via an RF carrier.

Flexible RF Modulation
Biotags’ Micro-ID tags implement a data modulation scheme that makes the most of its dual-oscillator setup: a low-frequency 32 KHz watch-crystal oscillator to provide a reliable time base over a wide temperature range and a higher-frequency crystal oscillator that forms the basis for harmonic generation of the RF carrier.

Because of the variety of wireless data transmission methods possible with this two-oscillator design, a programmable logic-based RF tag provides an excellent platform for the development of sensor and ranging applications. By using a CPLD, you can easily reconfigure the dual-oscillator setup to support many modulations, including AM, FM, and sideband. Given that you have serial bitstreams to transmit, let’s consider how to implement each type of modulation.

The simplest technique is amplitude modulation (AM), where the base frequency for the RF carrier, typically in the 10 MHz-60 MHz range, is gated by the data stream. For the period of time when the tag is transmitting a “1,” the CPLD enables the RF base frequency output on one or more of the CPLD’s pins, generating the RF carrier via a tuned circuit. When the tag is transmitting a “0,” no RF carrier is transmitted.

A popular variation of the basic AM technique provides for self-clocking of the data, which adds 1-0-1-0 transitions at twice the data clock frequency when transmitting a “1.” To decode the self-clocking data, I use a known data frequency and check the ECCs. This type of data code is displayed in Figure 2.

Now that we’ve considered AM, digital frequency modulation (FM) is easiest to comprehend if you recall a basic premise of Fourier analysis: when modulating a high-frequency carrier with a low-frequency signal (for example, 100 MHz and a 20 KHz signal), the harmonic content of the signal shifts. It no longer includes the 100 MHz carrier. Instead, the harmonic content of the carrier contains the sum and difference of the two frequencies: 99.98 MHz and 100.02 MHz. An RF spectrum analyzer will show these two peaks as the lower and upper sideband.

To receive FM-encoded data, assume 20 KHz below the RF carrier to represent a “0”; the RF carrier represents a “1.” Then tune the FM receiver midway between these two frequency ranges – 99.99 MHz – and specify a 10 KHz FM modulation.

Because multiple tags usually transmit on the same RF carrier frequency, RF carrier generation requires the use of a stand-alone gated-crystal oscillator – gated to minimize power draw during the tag’s quiescent phase. Finally, as shown in Figure 3, a circuit’s physical size influences RF carrier generation, where the small physical size and minimal circuit board parasitics of the 56-pad micro-BGA package excels in radio spectrum accuracy.

Conclusion
Biological research with small juveniles of a species requires low-impact, self-powered, implantable packages with exceptional reliability. The reconfigurable featues of the CoolRunner-II CPLD enable 100% testing of each Micro-ID radio tag, an essential requirement for wild release population studies of endangered species.

Customized through CPLD programming for each biological application and local environment, Biotags’ Micro-ID integrates advanced packaging, low-power operation, and wireless telemetry into a tiny battery-powered unit. One just might be monitoring a salmon swimming in a river near you.

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