Initial bring-up and tuning

[amcewen]

With a couple of boards fabbed we're onto bring-up and tuning of the antenna.

Initial key capacitor and resistor values are:

• C303 = C310 = 22pF
• C306 = C309 = 15pF
• C300 = C313 = 220pF
• C315 = C316 = 150pF
• R300 = R302 = 2R4

[amcewen]

On the first board I've been trying:

• We can talk to the chip over SPI to read its ID
• I'm not getting anything from it in software when I present a tag.
• Trying the "field detector" (a butchered MiFare tag with an LED soldered across the coil) there doesn't seem to be anything coming from the antenna
• Checked and found no shorts across the varicaps
• Checked and found no bridges on the chip
• Resoldered all the 0603 parts (just in case they'd not reflowed successfully, although they looked alright)
• Comparing our board to the NFC-08 dev board, I'm not seeing any voltage across J411 - whereas the NFC-08 periodically shows a small AC voltage - and nothing if I choose the "frequency" measurement on my meter - whereas the NFC-08 shows a steady 87.2kHz (presumably some harmonic of the 13.56MHz it's actually sending out?)

Things to try next:

• Looking through the software to see if it's failing anywhere
• Check the AAT section of the software to see if we can (maybe need to?) set it to either extreme
• Solder up the final components on the second board we've got soldered, and see if that works any differently

[amcewen]

We tried the second board on Friday afternoon, and that was much more successful, the board could read a tag!

Then we were on to tuning. Using the antenna-tuning-util sketch and an oscilloscope with a simple RF probe:

We worked out that the ST25R3916B has an option to run a field amplitude test, so added an option to scan through all values of aat_b and print out the amplitude. We could then try different values for C306/C309 and measure the results:

• 15pF: B value of 0 is the highest amplitude: 47, it then drops slowly to 24 at a B value of 255
• 33pF: the amplitude runs from 83 (at B of 0) to 36 (at B of 255)
• 56pF: from an amplitude of 32 (at B of 0) to 92 (at B of 255)

Each of those the highest value was at one end of the scale, whereas we'd like to have a peak value roughly in the middle of the range.

• 47pF: now we see the peak. It's running from 44 (at B of 0) to 73 (at B of 255) but with a peak of 92 (from B of 161 to 168)

Now we can look at tuning the aat_a value. For that, I added a "full scan" option to the antenna-tuning-util sketch, that just iterates over all combinations of aat_a and aat_b. It prints out the values in a CSV format, and then that can be fed into xyz to produce nice heatmaps like this:

[amcewen]

Modifying the resistors R300 and R302 would let us adjust the damping of the antenna (IIRC) and tune the effective range for reading tags. Swapping from 2R4 ohms to 1R5 ohms gave a useful increase in range. We also re-ran the antenna amplitude scans inside the enclosure (which made more difference than I'd expected).

Those results are all in the tuning-test-data folder now.

Inside the enclosure the amplitude peak is getting close to the edge of the on-board tuning parameters, so it would be useful to pull them closer to the centre of the range of both aat_b and aat_a.

Swapping C306/C309 to 39pF pulls the peak to ~110 for aat_b which is nice. It skews towards 50 or 60 for aat_a, so nudging the values of C303/C310 should improve that.

[amcewen]

Switching C303/C310 to 33pF centred things on the aat_a axis nicely. It pulled the aat_b peak down though, to around 75. Swapping C306/C309 back to 47pF moves the aat_b peak higher again, to around 150 when in the enclosure, but up to the edge of the tuning range in free air.

Feels (to me) like we're getting near to a sweetspot for tuning things now.

[huffeec]

I've updated the schematic with:

C303 = C310 = 33pF
C306 = C309 = 47pF
C300 = C313 = 220pF
C315 = C316 = 150pF
R300 = R302 = 1R5

[amcewen]

For the C3xx caps we'll want C0G or NPO dielectric, rather than X7R, etc. and 50V or better voltage rating.

C306, C309, C301 and C312 should be 2% tolerance or better (and so C213 and C214 should be too as they're 10pF, the same as C301 and C312, and there's no point having two different 10pF parts in the BoM)

[amcewen]

Need to double-check these changes have made it into the BoM / schematic, and then we can close this.

[huffeec]

Closed with 4cfd301