isolant rx
I'm currently designing UHF passive reader and having a problem in TX/RX isolation. As we know in backscattered tags the continiuos RF signal must be presented at the time of backscattering. This causes the likage from transmitter to receiver and interfering with receiving signal. One of the solution is to have two separate antennas for TX and RX. Another solution coluld be circulator with high isolation, but I couldn't find it so far. Even if I find it's going to be very expensive.
Does anyone know other solutions/circuits for this problem?
Regards, Alex
u can put pi diode to gnd at the rx path, and to give positiv current at tx time
eladg: If I do this I block the signal from the tag, since transmitting and receiving occur at the same time
There is a thing called an ASH receiver where you chop the rx signal. You use the time delay in a saw IF filter to do it. If you do it right, you could be muting the RX as you are Transmitting. I think SAWTEK talks about it on their website.
biff44: that means it will required complicated radio structure with downconversion to IF frequency. I'm trying to build derectconversion radio. Besides the TX need to be muted not the RX. Thanks for your reply
Regards
I never worked with RFID, however as far as I understand, you will receive anyway signal similar to the one transmitted by reflection from the other objects in the environment.
So in the RX channel of the reader you will have anyway probably a large component, unmodulated (or left unchanged) reflected back from other objects and a smaller component that comes from the tag and it is modulated with the tag info. Well, dependends on the actual application and environment...
Is this, environment reflected signal received, interfering with the leakage from TX?
fekete: the reflected signal will be much smaller than the one that leaks from the transmitter.
Alex,
Then your isolation is really bad, after all in a general environment you can have people moving around, metallic objects like carts and machines that move your tagged objects and all this can reflect quite a lot - and your leakage is higher than that ? (Assuming you already did whatever possible to reduce the leakage ? like with a circulator even if it isn?t perfect)
Even if you would have ideal isolation, the receiving part will get a mixture of useful signal and spuriously reflected one, which in some conditions could be bigger than the useful one. And that is true also for a two antenna system.
fekete: I'm planing to use IQ demodulator which splits the signal 90 degrees apart. But even with this solution 50-60 db of isolation is still required.
Alex,
That is an important ?detail? you didn?t mention before. After all, most RFID systems use simpler modulation schemes.
try this
http://www.ctsystemes.com/zeland/publi/p1147.pdf
Thes same technique also works for hybrids and circulators allowing high
isolation for TX RX operation.
Similar technique using feedforward technique with circulator
71dB isolation tx/rx
http://mwlab.inchon.ac.kr/003.pdf
Is there some way to separate the forward and reflected signals in the frequency domain? If you send out a transmitted signal with, say a 1 KHz data word on it, could you remodulate it with a 20 KHz data word before retransmitting. In the IF of the 2nd receiver, just highpass filter out the unwanted TX data in the baseband IF signal.
The 2nd receiver will have to have a high IP3 capability, so that you do not get crossmodulation.
Hi
I am sorry for disturbing your conversation.
but as you are designing a passive UHF RFID reader , may support me with some documentation or application notes about the RFID Readers?!
Best Regards
Hossam Alzomor