Hi,
I've been playing around with the TRF7970A EVM kit and trying to multiplex an array of coils.
According to TI app note SLOA114 (Low Cost HF RFID Multiplexer Examples) is it possible to use the CD74HCT4066 (4x SPDT). These can have quite a high on resistance, so I've used a CD74HC4067 (16way analog mux) instead.
The mux chip seems to work (albeit with reduced read range), but I dont quite understand what it is doing to the circuit. I dont know how long it is going to work for either, as I dont know how far outside the maximum conditions I am pushing the chip. I dont have active probes for my scope, so it is difficult to measure specific points of the filter circuitry without ultimately affecting the operation.
I've got a miniVNApro so I can measure the return loss of the coil both with and without the switch. Typically I get -44dB without the switch, and -9dB with. My characteristic impedance goes from around 50R to around 80R, so its pretty obvious the switch is damping my coil response. Could I retune the coil to take this switch resistance into account?
Reading app note SLOA135 (Antenna Matching for the TRF7960 RFID Reader) talks about having a 10dB RL over the antenna BW and "a rule of thumb is that the antenna 3-dB BW is twice the 10-dB return loss bandwidth" but I dont quite understand how that would fit into my application.
I've gone for the CD74HC4067 because it handles 16 coils pretty cheaply. I dont want to find that my chip stops working after a year, so does anyone have any experience using the mux IC for RFID switching? Otherwise, what cheaper methods are people aware of?
Thanks,
Stephen
