Radio Frequency Identification (RFID) technology is increasingly being used to track just about everything. Indeed, applications of the technology include diverse areas ranging from business inventory management to crowd control at large public events. As we delve further into the era of the internet of things, the use of RFID is expected to rise even more. There are a number different types of RFID technologies in use today, differentiated by the electromagnetic frequencies they use and whether their tags are passive or active. One type of RFID tag technology that has been growing in use more recently is that which uses the Ultra High Frequency (UHF) band of the electromagnetic spectrum. The UHF band encompasses radio frequencies from 300 MHz to 1 GHz, but the UHF RFID tags and readers that are making the most waves recently are those that use the 860 MHz - 960 MHz portion of the spectrum. But how exactly do UHF RFID tags work?

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When it comes to UHF RFID, we are usually talking about passive tags i.e. those devoid of their own power source, relying on received radio waves for energy. RFID readers emit those radio waves in the UHF range which activate compatible RFID tags. When the radio waves hit the antenna of the RFID tag, they set in motion a number of mechanisms that allow the tag to respond and, if required, send back the information contained within it.

So what are those mechanisms? First and foremost, an RFID tag needs power to operate. Once it has power, the tag can read the signal sent to it by the RFID reader. Subsequently, the tag interprets the request and executes an action, often retrieving data from its memory. Finally, the RFID tag has to relay any information it has retrieved back to the reader to complete the process.

Power

Power is the first priority for any RFID tag to operate. The interrogating radio waves of RFID reader provide the power that passive tags need. The RFID tag captures these radio transmissions and stores their energy in an internal capacitor. Finally, the capacitor functions as a small battery, distributing power to other tag components as required.

RFID tag components

A passive RFID tag derives its power from the RF transmission sent by the reader. The tag stores this energy in a capacitor on the tag and releases it to other tag components as needed.

But how exactly are radio waves converted into energy that can be stored in a capacitor? Radio waves come in the form of a sinusoidal wave, inducing Alternating Current (AC) in the RFID tag antenna. The tag's antenna is connected to the internal capacitor, however, the capacitor cannot use AC to charge it directly. Instead, the AC must first be converted to Direct Current (DC). To do this, a rectifier circuit in the RFID tag converts the incoming AC into DC by allowing the current to flow through it in one direction only.

RFID tag AC to DC to capacitor

Radio waves are converted into direct current which charges a capacitor in the RFID tag.

Perhaps unsurprisingly, radio waves only provide a very limited amount of energy. Therefore, optimising all components of an RFID tag is necessary to minimise power consumption.

Receiver

Once the RFID tag has power, it is able to perform its other functions. The first is to receive and interpret the signal sent from the RFID reader. Achieving this involves parsing the amplitude of the incoming radio waves. The wave amplitude is varied to encode the signal, a process known as Amplitude Shift Keying (ASK) modulation. Using the information contained within the signal, the tag can confirm that the reader is communicating with it and identify what has been requested.

RFID reader ASK modulation

The RFID reader uses Amplitude Shift Keying (ASK) modulation to transmit information.

The Controller and the Clock

The co-ordinating component of the RFID tag is the controller. It interprets the incoming signal and activates the tag's various elements at the right time. Aside from controlling the tag's activity, the controller is also responsible for generating a clock signal. This is primarily needed for the transmitter function of the tag (see next). Normally, electronic circuits use a crystal oscillator and a phase-locked loop (PLL) to generate a clock signal and an output RF signal of the correct frequency. However, these components are too power hungry to use in an RFID tag. Instead, the tag contains a simple low-powered oscillator. It synchronises with the frequency of the incoming RFID reader signal to generate a clock the tag can use.

RFID tag oscillator uses the reader's signal frequency to establish a clock signal for the tag

An oscillator in the RFID tag uses the RF signal from the reader to synchronise the tag's internal clock.

The Transmitter and Backscatter Modulation

Finally, we have the transmitter component of the RFID tag. Contrary to its name, the transmitter does not actually generate its own RF transmission. This is because the electronic components required to do so would consume a lot more power than a passive RFID tag can ever hope to generate. Instead, passive RFID tags use a process known as Backscatter Modulation to send back information to the RFID reader.

Backscatter modulation works by inducing changes in the signal the tag is receiving from the RFID reader. This can be explained, in simple terms, by the RFID tag having a switch connected to its antenna. This switch signals '1' when closed and '0' when open, allowing the transmission of binary information. Due to the electromagnetic coupling between the RFID tag antenna and the RFID reader antenna during communication, a small change in the tag antenna induces a corresponding change in the RFID reader antenna. This subtle alteration to the RFID reader's signal can be detected by the reader, offering an ingenious method to transmit information back to the reader without the need to transmit a discrete RF signal.

The RFID tag transmitter uses backscatter modulation to signal the reader

Backscatter Modulation: The RFID tag modulates the incoming RF transmission in order to signal the RFID reader.

FSK and PSK

Example of RFID tag FSK modulation

Example of FSK encoding for a 125 kHz RFID system

As you might expect, backscatter modulation is very weak making it very vulnerable to background noise. To overcome errors in RFID tag return signals, a couple of RF modulation techniques are employed. These include variants of Frequency Shift Keying (FSK) and Phase Shift Keying (PSK) modulation.

The variant of FSK that RFID tags typically use involves changing the period of the amplitude-modulated clock cycle. This allows two different frequencies to represent the '0' and '1' bits of the binary code. RFID tags of the 125 kHz variety often use this modulation type to enhance the signal received by the reader.

However, UHF RFID tags do not typically use FSK. Instead, 860 - 960 MHz UHF RFID uses PSK to encode its backscatter signal. Here, the carrier frequency remains unchanged but the phase of the signal shifts to differentiate between binary code bits.

Specifically, 860 - 960 MHz UHF RFID tag signalling uses FM0 or Miller PSK encoding to modulate the signal. Notably, these PSK variants change the signal's phase mid-clock cycle to differentiate one binary logic bit from the other. However, FM0 differs from Miller encoding in when and under what conditions these mid-cycle phase changes occur. The rules governing each type of encoding regimen are outlined below:

PSK: FM0 Encoding
Example of RFID tag PSK modulation using FM0 encoding

Rules for FM0 encoding:

  • If the data bit is 0, invert the signal at the middle of the bit
  • If the data bit is 1, do not invert the signal at the middle of the bit
  • Invert the signal at the edge of all data bits
PSK: Miller Encoding
Example of RFID tag PSK modulation using Miller encoding

Rules for Miller encoding:

  • If the data bit is 0, do not invert the signal
  • if the data bit is 0 and the previous bit is also 0, invert the signal at edge of the bit
  • If the data bit is 1, invert the signal at the middle of the bit

With these types of signal modulation, RFID tag data can reach the reader with greater fidelity and fewer errors.

RFID Standards

GS1 logo

RFID technology has become more standardised in recent years. Indeed, certain international organisations have taken on the task of implementing those standards. GS1, best known for its standardisation of barcodes, has become the key organisation that sets standards for RFID specifications globally.

RAIN alliance logo

For UHF RFID readers and tags operating in the 860 MHz - 960 MHz range, the specifications are covered under the GS1 Gen2 Air Interface Protocol. Currently, version 2 of this protocol defines the latest specifications that 860 - 960 MHz UHF RFID equipment manufacturers should adhere to.  Another organisation, called the RAIN Alliance, has taken on the role of marketing this GS1 Gen2 Air Interface Protocol, giving rise to the notion of 'RAIN-compliant' devices.