April 22, 2010

Tech Talk – The gap between Active and Passive Transponders

It can be confusing these days deciding on a technology to adopt for timing your next race. Some technologies have come and gone whilst others like the Texas Instruments half duplex low frequency transponders developed in the mid 90s continue to live on. Active tags use an internal battery to transmit their signal when woken up by a signal from the RFID reader. The transmitted signal from the tag is strong and thus read performance tends to be very close to, if not 100%. The drawback with active tags is their cost – ranging from USD $25-$100 – and their limited lifespan based on the coin cell battery used. The passive tag uses no battery to transmit the return tag code but uses the energy sent from the reader to either charge up a little on board capacitor for power, or reflect the energy in the form of a modulated signal. The first instance is used in many low and high frequency passive tags. The second instance is used in Gen 2 Ultra High Frequency systems and is called backscatter. The read performance of a well designed passive RFID system approaches 100% but there is always the odd chance that a tag is not energised in the short time it is in the read field. The big advantage of passive tags is the cost factor which is many times less than for active tags. This is because the circuitry is simpler and there is no need for a relatively large power source on board the actual tag. These tags can cost as little as USD $0.50 in volume.

A hybrid passive tag is the Battery Assisted Passive (BAP) tag developed by the company PowerID. The thin film battery only has a small amount of energy stored relative to a coin cell but is cheap to manufacture and has no harmful chemicals to the environment. The BAP tag uses this energy source to wake the tiny integrated circuit onboard the tag so that almost 100% of the energy sent from the reader is backscattered back to the reader from the tag. The improvement in performance is clear both in read range and readability next to bad substances like water and metal. RFID Race Timing Systems decided to adopt the new PowerID tags because the BAP significantly narrows the gap between active and passive RFID tags used in sports timing keeping.

Tech Talk, Uncategorized

Leave a Reply