Differences Among RFID, Extending Bluetooth's Range

RFID Frequencies

Are all RFID systems able to work in the same frequency range?

Signed, Range Rover

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Wise Guy: No, RFID systems operate at various frequencies. Passive tags tend to work at lower frequencies while active devices operate at higher frequencies because they have more information to communicate. In general, the higher the frequency, the higher the bandwidth and the smaller the form factor for the device (because shorter wavelengths allow for shorter antennas).

Figure 1. Alien Technology s Squiggle RFID tag works in the 902–928 MHz range
Figure 1. Alien Technology s Squiggle RFID tag works in the 902–928 MHz range

Unintentional Tag Reading

If I deploy an RFID reader in our plant for process tracking, will it also read the little tags that Wal-Mart is putting into my employees' shirts?

Signed, Stuffed Shirt

Wise Guy: Most of the Wal-Mart tags are passive, but they do conform to standards. Therefore any RFID reader looking for standards-based tags may read the shirt. (And this is why privacy groups have concerns about the "Big Brother" aspect of ever-present RFID systems.) But the real question is whether or not the plant's application would be able to use the information carried on the shirt tag. And unless the software is programmed to do so, this is really a non-issue.

Are RFID Tags Sensors?

Are you saying that RFID tags are just wireless sensor devices?

Signed, R.U. Sensor

Wise Guy: Well, they can be. But only in the case of active tags.

Going The Distance with Bluetooth

I have a pair of Lemos Bluetooth SD202 serial wireless adapters. I'm trying to use their best antennas to communicate throughout a building that has lots of metal ducts and ceiling panels. Try as I might, I haven't been able to get the distance I need. The long-range patch antenna has an 8.5 dBi gain. Is there anything I can do to improve my system's signal?

Signed, Distance Runner

Wise Guy: Given that you are attempting to transmit the signal through a maze of structures, you are up against the proverbial brick wall. But here are some things to try:

1. First, determine whether the cables and connectors in your antenna system are "leaking" strength. A simple attenuation test will do the trick. Here's how:

Grab a spectrum analyzer and tune it to the 2.4–2.5 GHz range. Set it to perform an integration of the amplitude readings (think fancy summation) across this range for 1 s. Within that time frame, a "compliant" Bluetooth transmitter will have frequency-hopped 1600 times—all within the spanning range. (If your spectrum analyzer can't do this, use an RF power meter with a sensing unit that is good at 2.4 GHz. It will give you a simple reading of the total power detected across that frequency range.)

Now plug the output of the radio into a 10 dB attenuator (to protect the sensitive input stage of the analyzer), and plug the other end of the attenuator into the analyzer's input stage. Then turn on the transmitter and let the analyzer average 10 readings. Write down (yes, really) the average (integrated) received signal strength.

Next, grab the cable that you are using with your higher gain antenna and repeat the test using it between the attenuator and the analyzer input stage. Repeat the measurements and write down that number too. If the cable is fine, then these answers will be within 0.1 dB of each other (note that you're looking for relative mea-surements, not absolute values). If not, trash the cable. And then repeat the procedure for each of your connectors to verify their performance.

2. Put higher gain antennas on both ends of the link. This, though, introduces the problem of negating some of Bluetooth's ad hoc networking benefit since you'll have to be aware of the devices pointing to each other.

3. Verify that the transmitters are currently delivering their maximum output power. To do this, set the output of your Bluetooth radio modules to the maximum level using the support/control devices or software that came with them. Repeat the process outlined for determining the integrity of the cables and connectors, measuring the maximum integrated power over a 1 s interval. Keep in mind that a milliwatt is formally a mJoule/s, so if the spec says the radio is pumping out 10 mW, that value is averaged over 1 s (hence the units). Thus, the same duty cycle "issues" are in their spec as in your measurement.

4. Place power amplifiers into the output stage, before the antennas.

Wise Guy is the problem-solving persona of WINA, the Wireless Industrial Networking Alliance (www.wina.org). Send your questions to [email protected].

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