PN532 Sheild

[codedawg82]

In the PN532 schematic the antenna matching circuit seems to come down to one point between R5 and R6. Is this the lead to the antenna loop? Could this lead be tied to an SMA connector and then hooked up to an external antenna for remote use? For example, this way, the host board could be stored somewhere else, while the antenna is located on the outside of some enclosure?

[adafruit_support_bill]

This is the designer's response to a similar question: viewtopic.php?f=19&t=31702&p=158648

[codedawg82]

I don't understand what he means when he says ---

You would have to physically cut the traces go from the PN532 out to the on-board antenna, including the matching networks of parts.

Does he means that the matching network would be on the remote antenna's board? or the matching circuit would be wherever the PN532 resides?

[ktownsend]

There's no realistic way to put an external antenna on this board ... the matching network that exists has been carefully calculated to provide optimum performance for the antenna that is already present, and if you remove the discrete components in the matching network you would see the range drop to about 1/3 to 1/5 what it is right now. I say that just to illustrate the importance of having something that is properly tuned, so unless you have access to a vectored network analyzer (they start around $15K) your results are likely to be very poor.

There are also two distinct traces requires for the type of antenna used with NFC devices (TX1 and TX2), so a single SMA connection isn't appropriate in this case.

If you wish to use an off-board antenna, you'd be better designing a custom board for this, but you'll still have the issue of impedance matching and you'll need access to the tools to do this if you want to get decent performance out of the board.

Kevin

[Arctic_Eddie]

Since this device operates at 13.56MHz, you will need an antenna approaching 1/4 wavelength. This is 18 feet long and requires an even larger ground plane. If you can't approach these dimensions then no gain will be realized. Matching networks are critical on any antenna system as it controls the power radiated, the received signal level, and the SNR. If it's not done right then it's a net loss.

Also, the R5/R6 junction is a return to ground for the TP points.

In addition, this frequency is one of several reserved for industrial use. A common usage is multi-thousand Watt plasma evaporator systems. These systems are shielded but if your receiver becomes too efficient then you may get a signal from one of these systems. If that happens then the data can be corrupted in an unknown fashion.

[codedawg82]

Also, the R5/R6 junction is a return to ground for the TP points.

Are the return to ground points necessary? I ask this because looking at my Nexus S (which also uses NFC), I can see it has an NFC antenna on the back of the cover but only TWO contacts. This leads me to believe this just uses TX1 and TX2 and ignores having to use a junction for ground. Why was this junction necessary in the Adafruit board?

FYI: I'm assuming the board has three contact points for the antenna denoted by TP4, TP5, and TP6 in the schematic...

[Arctic_Eddie]

The R5/R6 junction connection to ground is essential as it affects both matching and measuring. Note the terminology on the TP names. The ones on the left are -PRE and those to the right are -POST. Also note that some component values are marked NC. These component values are probably selected at final test to maximize performance base on measurements at some of the TP points. In addition, the RX pin has no matching network. This is because receiver gain and SNR are fairly insensitive to network tweaking in short range communications. There is also potential damage to the transmit circuits if the antenna equivalent load is not matched to the driver amplifiers. The receiver doesn't really care other than performance factors, gain and SNR. If you extended the transmit antennas you could actually lower the radiated power because the mismatch loss is greater than the wire length gain.

[codedawg82]

How would I prototype something like this --- using a breadboard from Adafruit would probably cause massive performance loss, possibly even lack of functionality with through hole components. Especially when we're talking about an RF circuit --- we'd have lead inductance, parasitic capacitance from trace routes and all sorts of tuning issues. I want to be able to swap out caps/resistors to test an external antenna but I'm not sure how to proceed. Am I left with just having different boards fabricated for me? Or is there a more flexible testing route?

[Arctic_Eddie]

It will be very difficult to build a prototype on a solderless breadboard for the reasons you stated. Your only chance will be to get a board with a ground plane on the underside and a grid of pads on top. Arrange your parts so that the one to be determined bridges two adjacent pads. You can then hold a chip part down with a weighted wooden stick while you measure the performance. When you're satisfied that it's now the right value then solder it down and move on to another component. You may have to buy a large range of values in chip caps. Be sure they're NPO or a type that is temperature stable. Most of the common ceramic caps will not work well in matching networks. With a longer antenna wire you'll find the first coil to reduce in value and the capacitor divide ratios change. It can be a very frustrating task so good luck.

[dmpyron]

To hop in a little late on this, I saw a presentation last year or the previous at a conference in Las Vegas in the July - August time frame in which the guys attached a yagi to their system. I'd have to dig up the stick with the audio, but the comments were something along the lines of "we built 6 or 7 boards and threw away a couple of chips before we got something to work, and went through several more revs to get what we wanted." So decide how badly you need one.