Nordic ID

For any battery-powered device, including RFID readers, power consumption and operating time are critical parameters. This is especially true for UHF RFID readers, because they consume relatively large amounts of power when operating at full RF power. By reducing power consumption, you can extend the battery life and operating time of your devices.

This article covers three ways you can decrease power consumption, using test results from a Nordic ID Stix USB RFID reader as an example. Each method has its benefits, drawbacks, and effectiveness, but you can implement one or more of them simultaneously as needed:

• Reduce the reader's actual operation RF power level, which also reduces the maximum read range
• Use the integrated power save modes to decrease power consumption when there are no tags in the reader's field while reading is on
• Apply a duty cycle, either within the host application or through the RF off-time parameter in the device firmware

1. REDUCe THE RF POWER LEVEL

Reducing the RF power level is a very straightforward way to reduce power consumption, but the reader is no longer working at maximum read range. This is not always an efficient way to decrease power consumption, because halving the RF power does not correspond to an equivalent reduction in power consumption, in fact far from it. RF power amplifiers are usually tuned to provide maximum efficiency at maximum output power, and efficiency drops greatly with lower RF power levels.

Figure 1 shows the average power consumption of our test reader at various RF power levels. Note that the test was carried out with tags in the reader’s field to ensure that the reader's power save mode does not interfere with the test results. Only when the RF power level is set to the minimum value is the power consumption actually reduced by half.

Advantage

• Easy way to save power if read range is not an issue

Disadvantage

• Inefficient way to gain power savings

2. Use pOWER SAVE MODES

In our test, the battery operated RFID reader has integrated power save modes. When there are no tags in the reader’s field, reading is stopped for a set period. The test device supports three different period lengths: 100 ms, 500 ms, and 1000 ms. If a tag enters the reader’s field, reading is switched on immediately and continues until there are no tags again. 

With this method, reading performance is not decreased, but power savings are only achieved during the times when there are no tags in the reader’s field.

Figure 2 illustrates the measured power consumption at each period length option. Power consumption is unaffected when tags are within the reader's field, but drops dramatically when no tags are in reading range. This method is especially effective for use cases where reading needs to be on and available most of the time, but where tags enter its field infrequently, such as in access control applications.

Advantages

• Very efficient way to reduce power consumption without reducing the read range
• No effect to read performance

Disadvantage

• Saves power only when no tags are in the reader’s field

3. APPLYING a DUTY CYCLE

The reader used in these tests supports two different ways to apply duty cycles:

• In the application layer. In practice this means that the host application will have off times between separate reads. 
• Using an RF off-time parameter, which turns off the transmission and powers down the RF power amplifier for the defined time period. When this happens, average power consumption decreases regardless of whether there are any tags in the reader's field. Maximum read distance is unaffected but read speed is reduced.

Our test implemented the RF off-time approach, which is more efficient.  This test was carried out with tags in the reader’s field, to ensure that duty cycle results are not impacted by the device’s power save mode.

Figure 3 illustrates the measured power consumption with different RF off time parameters and at two RF power levels.

Figure 4 shows the effect of RF off-time parameters on the reading speed.

Advantage

• Decreases the power consumption without decreasing the maximum read range even when there are tags in the reader’s field.

Disadvantage

• Read speed is decreased, although this is not an issue in most circumstances.