Semi-passive tags, also known as battery-assisted passive tags, have a battery to power the local circuitry and use the signal sent by the reader to initiate a tag-to-reader communication just like passive tags. Dobkin in the book The RF in RFID UHF RFID explains semi-passive tags achieve a much higher read range and are more reliable to a valid interrogation. It is because of this that semi-passive tags are now being used in applications such as road-tolling, tracking on high-valued items, tracking of temperature & humidity sensitive items with respective sensors.
Semi-passive tags operate at much lower power levels than passive tags. The activation power (turn-on power) of passive tags is given by the minimum power that is required by the tag chip to operate. It is the tag IC’s power consumption, which is the limiting factor for the passive tag sensitivity. On the other hand, semi-passive tags are powered from a battery. The IC operates regardless of the RF field strength, but the input signal might become too weak for the detector to get the reader data. The minimum power level at which a semi-passive tag can detect the reader signal is typically lower than the turn-on power of passive tags. To overcome this, testing of semi-passive tags should be done at lower power levels using test instruments with high sensitivity.
CISC RFID Xplorer can measure the activation power and the backscatter power of semi-passive tags as required by these tags. The recommended setup for such a test is the bi-static setup as the bi-static setup provides better accuracy in comparison to monostatic setup. As the goal of the tests is to measure the sensitivity and the backscatter power of semi-passive tags (i.e. the minimum signal level that the tag can decode and the strength of the tag response, respectively) and not the limits of the reader (the test instrument in this case), it is also recommended to test the tag at the default distance and to use an attenuator to shift the TX power to the intended range.
The importance of the backscatter range
The backscatter range is the theoretical range over which the tag answer is attenuated to the smallest level that can be received by the reader. For passive tags, when read by standard commercial readers, the backscatter range is usually greater than the read range. The situation is different for semi-passive tags. In general, on some frequencies the read range can be greater than the backscatter range and on other frequencies smaller. The range over which the tag can be read (or better say operated) in application setups is the minimum of the read range and of the backscatter range. Therefore, Xplorer provides an important metric called “Operating range” which displays the real range of the tag in the application setup with the specified reader (called reference reader). The operation range is the minimum of the read range and the backscatter range. This is particularly useful when we want to understand how the tag and the reader behaves in real applications.