simple rf id reader module design

Posted on Feb 6, 2014

Used a coil of about 1mH for both sides. Since my chosen frequency was 125 KHz, my capacitor should be 1. 62nF according to the following equation; I picked 1n5 standard value. L1 is driven via a low-impedance 125 KHz oscillator, can be a sine or a square wave since theLC circuit will filter out the unwanted harmonics that are presented in a square wave. If the Q of the inductor is high, then a

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voltage that is greater than the oscillator`s output is going to be present in the Out . I`ve seen 100 Vpp when I fed the LC circuit with 5Vpp! So, the Out waveform at the top of the C1 is a sine wave of a 125 KHz frequency. Now, the fun thing begins when we put the tag near the reader. L2, C2 pair picks up the 125KHz waveform via L2. So, if you scope C2, you will see 125 KHz sine wave. Now, if you scope Out , you will see that Vpp at C1 will drop when we close the switch SW1. That is because we load L1 ²s magnetic field via L2. Now, push the button like you are sending aMorsecode and watch the Output waveform on the scope. Aha, modulation! Simple! That is how real RFID passive tags work. However, instead of sending Morse code, they modulate the signal with their specific modulation scheme. I am going to work with EM4100 protocol since it is widely used. OK, L1 and C6 are our main guys. They are the components that are mentioned before as L1 ³ and C1 ³ in Figure 1. The circuitry on the left side of L1 is used to drive this LC circuit, and right side of C6 is used to read the changes in the signal. C1 AC couples the clock signal of 125KHz to the circuit. R1 and R2 biases the transistor Q1. R4 limits its base current. Q1 drives the input of push-pull follower formed by Q2 and Q3. A push-pull follower will drive the signal at low output impedance. D2 and D3 prevents distortions at the cross-overs from zero level. Of...

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