Silicon Labs’ RF chips

Just a few notes about Silicon Labs chips and HopeRF modules. All the info is in the datasheets or online, but I like to keep use the blog to keep track of my findings and thoughts.

The HopeRF modules use chips from Silicon Labs. The RF12B uses the  Si4421 (datasheet). Jeenodes from Jeelabs uses these modules and they work well. These chips are from Silicon Labs’ EasyRadio set of chips. I believe Silicon Labs purchased Integration which made the original chips, in June 2008.

Silicon Labs also makes several other interesting modules which HopeRF have turned into modules. Mainly, the RF22B which has up to +20db output power @ 89 mA TX. These come from the EasyRadio PRO chips. The Si4430/31/32 chips looks very interesting. Some people have done some work on drivers for the Arduino, but they may not be as used as Jeelabs RF12 library. Specifically, mikem seems to have done a lot of work getting a good driver for the RF22B. Read about it here.

Silicon Labs has the RFM23B HopeRF modules listed on their website here — one interesting aspect is that they are pre-certified which would help in getting any product certified. As with a lot of wireless technologies they are aiming at the green energy market (smart meters, solar, etc…) These use the Si4431 chipset. (Press Release)

It seems the chips can do a wide range of frequencies, but the 433MHZ need different supporting resistors and capacitors. The 833MHZ and 915MHZ use the same supporting parts.

Digikey sells the Si4421-A1-FT for $2.88/each, $2.77/25s or $2.69/100s. Mouser has the Si4421-A1-FT for $5.03/1 or $3.97/25s or $2.94/100s. Interesting that digikey has the price of 1 less than the price for when buying a 100 from Mouser. Pays to comparison shop. The crystal is about $1. That already puts it above what the module costs direct from HopeRF.

That said, it would be neat to make a small board with all the necessary components and with a SMA or RP-SMA connector for a more robust antenna. (I’ve had 3 of the 4 Jeenodes’ antennas break-off since I’ve had them and I’m relatively careful with them — the antenna that comes from Modern Devices is stranded and seems to be relatively weak. I replaced them with a solid core wire and it seems a bit better. Not sure which is better for RX/TX but they are working fine with the replacement antennas.

Of course, the downside to building the chips onto the same board is that the supporting caps and resistors are tuned for the frequencies so you’d have to keep 2 versions stuffed with different chips instead of just shipping out a different module.

The above is an older version of the module. It has 9 components and the crystal, plus the main chip of course. Interestingly, the newer version below has 8 components as well as the different main chip format. You can see the area at the bottom center that was left unstuffed. (Note: I rotated the image so it aligns with the pinout diagram below).

Here is the pinout for the module:

The datasheet shows a typical application shown below:

The Si4221 needs:

  • X1 – 10MHZ Crystal (can share with the microprocessor – but that would make it not a normal Arduino I think)
  • C1 – 2.2μF
  • C2 – 10nF
  • C3 – different for each ISM band (433MHZ – 220pF, 868MHZ – 47pF, 915MHZ – 33pF)
  • C2 & C3 should be 0603 ceramic capacitors
  • C4 is listed as optional and is connected to the ARSSI pin

The rest of the components on the RF12B chip must be for the matching network for the antenna. From what I can understand, from my non-existent knowledge of antenna design, the matching network transforms the high impedance of the antenna to the low impedance needed for the transceiver to limit any power loss/reflectance.

In the Si4421′s datasheet it shows a schematic of their evaluation board (page 39) with a 50 ohm matching network. The parts of the matching network are:

  • 4 ceramic inductors  (L1-L4) in nH (nanoHenrys)
  • 4 capacitors (C8-C11) C11 listed as optional

different values for each depending on what frequency band you are after. The three RF inductors’s SRF (self resonant frequency), DCR (DC resistance) and Q should be similar. A high Q has low insertion loss which minimized power consumption.

Hmmm, on the HopeRF board there are 8 or 9 small components where as the reference design has 10 or 12, depending if you include the optional parts. Guess the reference design’s matching network is more complex.

The Jeenode connects only the 5 required connections: SCL, SCK, MOSI, MISO and IRQ. There is not a connection to the ARSSI or any of the other optional pins such as FFS and FFIT.