RFID Circuit & PCB Design Notes
It’s usually very important that the voltage supply to an RFID reader is protected from ripple and interference.
Where you have noisy electronics in the proximity:
A simple RC filter of 10R and 1000uF will go a long way to preventing interference. Better still is using a separate power supply from a dedicated voltage regulator if you have the option.
Use a star point for the earthing. Typically this will ideally be the negative pin of a 100 – 1000uF smoothing capacitor, with all the grounds taken back to this single point.
Use thick tracks to high power devices and areas. Airborne interference can result when high value DV/DT is around. This can come from current switching too. The tracks carrying power must be wide. Typically a 0.1” track is about 10nH per cm, so if you switch an amp with a rise time of 0.01uS then you get Ldi/dt = 10/1000000000. * 1/0.01/1000000 = 1V. This 1volt appears on the power supply rail and has several effects. It directly makes a 1volt spike and it makes a large blob of the PCB waggle by 1volt which connects to sensitive bits elsewhere by stray capacitance.
If you are able to have +V and 0V power tracks to high power devices run above and below each other on top and bottom layers this will help cancel out the field they generate.
When using long PCB tracks for a connection to an antenna ideally use a decent thickness and route along the edge of the PCB if possible and away from noisy devices / circuitry. Placing ground plane around and below tracking can be useful to protect the signals. If running tracks to different antenna keep them separated from each other.
Remember, the enemy of any RFID electronics design is noise! Is there anything else you can think of to reduce it?
Solving Interference Problems
If you have +V and 0V cables running in proximity to the RFID reader twisting them together will help cancel out the field they generate. Also make them as short as possible and if possible angle them away from your reader.
Experiment by turning off the RFID reader and looking at the coil voltage with a sensitive oscilloscope. Rotate the coil for minimum interference. If possible move the coil to a minimum interference position. Even if it is impossible to eventually change the angle or possition of the coil, this experiment may identify ‘Hot Spots’ that will help you zero in on the cause of a problem.
Many RFID readers have interference rejection but the interference may be so great that it is saturating the output of the reader detector. It may be that a low Q coil will give a lower output and the reader will not saturate so it may actually read better. A 1k resistor across the coil is extreme but may still be a good starting point. You can plot the range against several values of resistance from 1k to 10k.
The effects of metalwork are often not predictable. If the RFID reader coil is surrounded by metal in such a way that the steel effectively becomes a shorted turn, then this will appear as negative inductance and you could try to compensate by adding 50pF in parallel with the coil. If it helps try other values. A simple way to see if the metal is having an effect is to isolate the reader and measure its supply both in situ and out of situ. If there is a large difference then you may have a problem.
You can switch multiple antennas to an RFID reader using a mosfet relay (e.g. Omron G3VM-353G). The critical specifications are on state resistance (which you want to be as low as possible so it doesn’t kill your antenna current), max AC voltage (so it can handle the high voltage that will be generated in the coil and max current (to handle the current that is passed through the coil). If your using an off the shelf RFID module then this can be an attractive option. However if your designing the reader yourself at the chip level then its often not worth it as mosfet relays with the needed spec’s will often cost significantly more than just using a seperate RFID reader IC for each antenna, with added the advantage that you don’t get the variable resistance problems introduced from the mosfet.