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UHF RFID Chip impedance with 50 ohm

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shanmei

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Is there any UHF RFID output impedance close to 50 ohms, and the imaginary part is very low? Thanks.
 

I don't know why not design the input impedance of the UHF RFID chip to be close to 50 ohms. Then the antenna is easy to design.
 

Not feasible for a sensitive RFID chip. -20 dbm input level @ 50 ohms is only 22 mV, too low to operate the rectifier. LC matching network required.
 
One typical RFID has the input impedance of 20-j*200 at 900MHz, then R=20 ohms.

V=P^2/(2*R). With a smaller R, it gets larger V. which benefits the operation of the rectifier. Thanks.
 

V=P^2/(2*R)
Completely wrong. V=√(P*R)

Please consider that the impedance is expressed as series equivalent circuit. The real chip can be more realistically modelled as C parallel R circuit.

20-j*200 turns into 2020 || -j202
 
20-j*200 is already the series equivalent circuit, and its impedance z=sqrt(20^2+200^2)=201 ohms.

-20dBm=0.01mW=0.00001

V=sqrt(2*P*R)=sqrt(2*0.00001*201)=63mV.

Should the V=sqrt(2*P*R) or V=sqrt(P*R) here?

Thanks.
 

Should the V=sqrt(2*P*R) or V=sqrt(P*R) here?
Considering that the source impedance is outside the circuit of interest, I vote for V=sqrt(P*R).

Anyway, the series equivalent circuit isn't real. Inside the chip you have load capacitance in parallel to the real "energy harvesting" load. So if you want to get an idea of expectable voltage levels, you'll refer to parallel equivalent circuit.
 
If -20 dBm is the input level at the antenna of impedance 50 ohms, and if its matched, then you only get -23 dBm at your load, and then its only 15.8 mV.
 
If -20 dBm is the input level at the antenna of impedance 50 ohms, and if its matched, then you only get -23 dBm at your load

You seem to be confused with the concept of matched sources. There is no such power loss in the antenna.
 
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