Tips for FCC certification on the EM250 or EM260

Categories:

FCC testing is an important milestone in wireless product deployment. The Federal Communications Commission (FCC) is responsible for regulating communications equipment in the United States, including acceptable use of frequencies in the licensed and unlicensed bands. All wireless products, such as those using 802.15.4 or ZigBee, must undergo FCC testing prior to being sold in the North American marketplace.

If your are considering FCC certification on your EM250 or EM260 device, the FCC testing report (from Curtis-Straus), a PDF linked at the bottom of this article, for our EM260 RCM may be useful to you as a reference. Note that while Ember’s EM250 and EM260 RCMs (from the development kits) have completed the necessary FCC test conditions for certification, the RCM itself is not an end product with an FCC ID (since it is intended for development), so it can only be used in development & demonstration context. Even if you build a replica of our RCM design, you will still need to undergo FCC end-product certification testing for your hardware.

Below are some notes and tips relevant to the testing:

If you encounter difficulties passing the Radiated 2nd and 3rd Harmonic Level testing on the EM2xx platform (measurement of signal energy at frequency*2 and frequency*3), it may be necessary to use a 27% Duty Cycle correction factor (effectively a -11dB reduction) as the worst case timing (amount of time in TX Mode in any 100ms window). Note that you do not need to use the Duty Cycling correction factor if the output power from the EM2xx is reduced to +1dBm instead of +3dBm (and no Boost mode or PA is enabled), but reducing power will, of course, have an adverse effect on your achieved communication range between devices. In case your test house needs the duty cycle calculations (which are consistent across any device employing an 802.15.4 MAC layer implementation), they are attached here as an Excel 2003 spreadsheet.

If using an external PA (power amplifier), you may notice some noise or other spurious emissions in the transmitter waveform. If you see this, may be an indication that the PA is in compression, so you’ll want to back the TX power down a bit (in software), to just before the amp hits compression, so that you still get the maximum output from the amplifier without the added distortion. (This will require some trial and error in order to determine the optimal point where compression is eliminated with minimal sacrifice to output power.) Proper radio circuit performance should always be verified before (and after) applying the PA to the circuit, as using the PA to compensate for poor transmission power can cause adverse side effects.

Other spikes in the output, excessive harmonics or abnormally low output power generally indicate manufacturing or design issues with your hardware, so you should check to ensure that the design is laid out according to Ember’s reference designs and design guidelines (see “PCB Design with…” documents among references list at the end of this article) and that the manufacturing quality is satisfactory. This includes the following areas:

proper grounding and capacitive decoupling of the chip (coupling of noise can produce harmonics), which includes following Ember’s reference design to prevent noise from coupling into the RF by properly routing traces concerning placement of vias, then decoupling capacitors, then IC pads. (In general RF component pads should be placed in line with the traces and not branched off the signal path. Crystal circuits should be kept as short as possible, and crystal shunt capacitors should be tied to the same ground via. More details are contained in Ember’s reference designs.)
proper seating of the EM2xx on the solder and vias underneath the chip: Design wise, too many vias can reduce the temperature at the ground pad of the EM2xx during reflow, causing it to have a cold solder joint; large paste stencil openings can cause too much solder to be placed under the EM2xx, causing some outer pins not to be soldered properly; insufficiently large solder openings in the paste stencil for the EM2xx can cause a poor ground connection to the chip; manufacturing reflow temperature profile could be incorrect, causing the solder under the EM2xx not to flow properly, thereby resulting in a cold solder joint on the ground pad. Be sure to follow Ember’s footprint recommendations to avoid manufacturing defects. (Traces and vias should not be placed under the corners of the EM2xx chip.)
proper antenna matching (to 50 ohm impedance with respect to nearest ground plane), including any inductor tuning that may be necessary from changes to board housing (such as adding an RF shield)
adequate transmitter ramp-up at the PA, if one is used

The top end of the 802.15.4 channel band (Channel 26 and occasionally Channels 24 an 25 at very high output powers) usually pushes up against the limits of the band, encroaching into the reserved area in the adjacent (higher) frequencies. Therefore, it’s often necessary to artificially limit (with software) output power on these upper channels (as much as -9 or -10 dBm) in order to pass FCC TX testing.

Depending on the amount of harmonics output and packaging design, you may want to consider a shield around your RF circuit. While this is not required to pass FCC certification, some customers find it helpful in suppressing 2nd harmonics on the circuit.

Required FCC testing for 2.4GHz DSSS devices such as the EM260 should involve tests 15.209 and 15.247 from FCC CFR 47 Part 15. These are RX testing and TX testing, respectively. Other tests shouldn’t be required, provided you’re staying within the usual limits of +20dBm (100mW) output power.

As far as performing the necessary tests, the radio test modes for the EM2xx can be accessed using the special EM250 or EM260 Rangetest firmware, provided pre-built in the EmberZnet software releases, or by using the Manufacturing Test Library (“mfglib”) command set in EZSP (the serial protocol that normally runs on the EM260) or the “mfglib” library found in the EmberZNet “build” directory (described in stack/include/mfglib.h). See app/mfglib-host in the EM260 releases or app/mfg-sample-app in the EM250 releases for an example of an application that utilizes this mfglib command set / library to perform low-level functional testing. Additional details about low-level functional testing can be found in Application Note 5031/5041 (Bringing Up Custom Devices for EM250/EM260) and Application Note 5016 (Manufacturing Test Guidelines), both referenced below.

References: