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RFID Technologies Laboratory III

Alberto Toccafondi

Alberto Toccafondi

Capacitance matching circuit Ø  Connection between the reader board and the antenna coil using 50 Ω

coaxial cable.

Ø  Parallel/series capacitor matching circuit

322 11 READERS

11.4.2 Supply via coaxial cable

At frequencies above 1 MHz, or in the frequency range 135 kHz if longer cables areused, the HF voltage can no longer be considered stationary, but must be treated as anelectromagnetic wave in the cable. Connecting the antenna coil using a long, unshieldedtwo core wire in the HF range would therefore lead to undesired effects, such as powerreflections, impedance transformation and parasitic power emissions, due to the wavenature of a HF voltage. Because these effects are difficult to control when they arenot exploited intentionally, shielded cable — so-called coaxial cable — is normallyused in radio technology. Sockets, plugs and coaxial cable are uniformly designed fora cable impedance of 50 ! and, being a mass produced product, are correspondinglycheap. RFID systems generally use 50 ! components.

The block diagram of an inductively coupled RFID system using 50 ! technologyshows the most important HF components (Figure 11.17).

The antenna coil L1 represents an impedance ZL in the operating frequency rangeof the RFID system. To achieve power matching with the 50 ! system, this impedancemust be transformed to 50 ! (matched) by a passive matching circuit . Power trans-mission from the reader output module to the matching circuit is achieved (almost)without losses or undesired radiation by means of a coaxial cable.

A suitable matching circuit can be realised using just a few components. The circuitillustrated in Figure 11.18, which can be constructed using just two capacitors, is verysimple to design (Suckrow, 1997). This circuit is used in practice in various 13.56 MHzRFID systems.

Figure 11.19 shows a reader with an integral antenna for a 13.56 MHz system.Coaxial cable has not been used here, because a very short supply line can be realised

L1

ZL

P1 P2Coaxial cable 50 !Reader

PA(poweramplifier)50 !

Zin = 50 !

Matchingcircuit

Figure 11.17 Connection of an antenna coil using 50 ! technology

C1s

C2p XLs

RLs

ZA

Matching circuit Antenna coil

50 !

Figure 11.18 Simple matching circuit for an antenna coil

Rcoil

Lcoil

RparC2

C1

Alberto Toccafondi

Keysight* ADS

*formerly Agilent

Alberto Toccafondi

Creating workspace

Ø  Create new Workspace and click on “next”

Ø  Open ADS

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Ø  Simply click “next”

Ø  Rename workspace and click “next”

Creating workspace

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Simply click “next”

Ø  Rename workspace and click “next”

Creating workspace

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Then click “next”

Choose 2nd option

Createing workspace

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select workspace

and create a new schematic

Creating schematic design

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rename cell (optional)

choose S_Parameters template (important) and

click “OK”

Creating schematic design

Alberto Toccafondi

A new schematic page with two 50-ohm terminations and a S-parameter controller placed on it with default frequency settings should be visible

Creating schematic design

Alberto Toccafondi

To create a circuit, first we can delete Term2, since our circuit have only one port....

Creating schematic design

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....then go to Lumped-Components, select and place SRL, R and C components on the schematic, and set the correct values

Creating schematic design

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Double click on the S-Parameter controller and set the parameter for simulations, then “apply” and “ok”

Creating schematic design

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Click on Simulate icon (or press F7) to start the simulation and once done, data display window appears showing the simulation results

Simulation and results

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double click on “Reverse Transmission graph”, delete dB(s(2,1)) ....

Customize results

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and add “S(1,1)” choosing “dB” (rename the title)

Customize results

do the same with

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insert a marker on the trace

Customize results

marker

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to make component values tunable, double click on the component and click on “Tune/Opt...” button, then choose “enabled” in tuning status

Tuning the components

Alberto Toccafondi

to make component values tunable, double click on the components you want to see the impact on circuit performance and click on “Tune/Opt...” button, then choose “enabled” in tuning status and click ok. (you can change limits if needed)

Tuning the components

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Click on the Tune Parameter icon in the schematic page....

Tuning the components

... and a Tuning slider window appears

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Tuning the components Put Tuning slider window and data display side by side and start to move the slider of component values and see the corresponding graph changing with the component values

Please note there are many other features of Tuning wizard