56 CIRCUIT CELLAR • DECEMBER 2015 #305

EMBEDDED IN THIN SLICES

The Internet of Things (Part 4)

Cell Modem Certification
COLUMNS

In the third part of this article series, Bob detailed

how to connect simple devices wirelessly to the
Internet. This month he looks at the requirements
for, the cost of, and some of the problems with cell
modem certification for embedded systems.

By Bob Japenga (US)

Almost every month, I get a call from some The only exception to this is when the cell
budding new entrepreneur with a great idea modem is self-contained and not an integral
for an Internet of Things (IoT) product. Before part of your design. For purposes of brevity,
we get too far along in the conversation, I I will only cover the requirements for North
ask the question: “What is your budget for America. Nor will I go over definitions defined
cell modem certification?” More often than in previous articles in this series.
not, the answer is: “What is that and how
much does it cost?” This month I would like AT&T
to address these two questions as well as If your IoT device is going to use AT&T (3G
address the major issues we have had in cell or 4G), you will be required to pass PTCRB
certification. As always, this is a big topic that and AT&T certification testing. PTCRB (an
we cover in thin slices. obsolete acronym that used to stand for
PCS Type Certification Review Board) is an
WHAT ARE THE REQUIREMENTS? independent certification agency used by
All cell modems are required to be certified some North American cell carriers, including
by cell carriers prior to sale to customers like AT&T. Testing to the PTCRB standard is done
you and me. However, just because the cell by a third-party independent test lab. You,
modem is certified for a particular carrier, the designer, are responsible to contract
you are still required to certify the device that with one of these independent test labs.
incorporates this modem. This makes sense Cetecom (www.cetecom.com) and 7Layers
for a lot of the certification requirements. For (http://7layers.com) are two such labs that we
example, just because the cell modem has have worked with.
an acceptable receiver sensitivity and good After you have passed the PTCRB tests,
robust transmit power, it doesn’t mean that you need to obtain AT&T approval. Once
your design has met these requirements. scheduled, PTCRB testing will take three to
This necessitates that you separately test four weeks. AT&T approval takes another one
your device to the carrier’s requirements. to two weeks. The lab costs depend on the
 circuitcellar.com 57

particular test lab, but it will cost between

$20,000 to $40,000 for GSM modems and
$60,000 to $70,000 for LTE modems.

VERIZON
The process of certification for Verizon 3G
(CDMA) and 4G (LTE) is done directly through
Verizon. This testing can be done through an
independent lab or through Verizon. Verizon
recommends that you pre-certify your
product through its Innovation Center. There
you can work with Verizon test engineers and
technicians to make sure your design is ready

COLUMNS
for prime time before you go to certification.
Verizon provides this service to qualified
companies.
Once you have pre-certified, then you
can contract with an outside independent I discuss EMC requirements. PHOTO 1
certification lab (e.g., Cetecom, 7Layers, and Next is your choice of antenna. We have Old cell phone
Intertek). The cost for a CDMA certification been unsuccessful meeting TIS requirements
will be $15,000 to $20,000 while the LTE without using antennas significantly larger
certification can cost as much as $70,000. than used in our cell phones. We have often
Once scheduled, the pre-certification wondered how all of our cell phones met
timeframe is about two to three weeks with the TIS requirements with their very small
another three to four weeks for certification antennas. I will leave it to your research and
once it is scheduled. your imagination as to how cell phones are
passing the cell carriers TIS requirements
AERIS with such small antennas. In the words of
If you are deploying a GSM modem on Deep Throat, “Follow the money!”
the Aeris network in North America, you will Finally, your antenna should be placed
require PTCRB certification as well as Aeris as far away from any metal as possible and
certification. The cost and schedule are should have a nonmetallic path to the outside
the same as I described earlier. If you are world. One product we had was mounted in a
deploying a CDMA solution, you only require large metal base mounted to an outside wall
Aeris certification (which has the least that shadowed the entire hemisphere behind
stringent requirements of all the carriers, is the product. PTCRB testing of this product
free and takes less than a week). Aeris also required it to meet the TIS requirements
allows you to self-certify for small volumes of completely and evenly around the sphere.
installations. We could not get the test lab to relax this
360° requirement. Instead we removed
TECHNICAL REQUIREMENTS the product from its real world enclosure
Let’s summarize the technical and performed the testing in a nonrealistic
requirements for certification and our environment. This seemed ludicrous to us
experience with these. since we wanted to test it in the real world
Total Isotropic Sensitivity (TIS): All carriers enclosure. This resulted in uncertainty on our
for all radio access technologies require a part once the product passed certification. We
minimum receiver sensitivity. Basically, this were not certain how it would work in the real
test determines how weak a signal from the world when it had this metal box shadowing
cell tower your device can respond to. This the back hemisphere. Thankfully, we have
is one of the situations where certification is deployed more than 50,000 of these with no
your friend—not your enemy. You don’t want TIS problems.
to deploy your great new idea and have a lot Total Radiated Power (TRP): As with
of “Can you hear me now?” problems. TIS, certification testing is your friend
There are three primary ways that we concerning TRP. The carriers have similar
have improved our TIS. First you must stringent requirements for TRP. Here your
make your device whisper quiet in terms of design must carefully place and tune your
radiated emissions in and around the receiver antenna to obtain the maximum TRP. A little
frequencies. If you thought meeting FCC bit of movement of the antenna can make a
Class B EMC requirements were tough, your significant improvement or degradation of
requirements for making your device whisper your radiated performance.
quiet to meet the TIS requirements are much Another critical requirement for your
more stringent. I’ll talk more about this when design is that your power supplies must be
 58 CIRCUIT CELLAR • DECEMBER 2015 #305

capable of instantaneously delivering 1 to 2 through as quickly as possible, each carrier

A of power when a transmission takes place. has data throughput requirements for some
Cell modems have one of the more demanding radio access technologies. This requirement
power supply requirements that we have is strictly a function of your cell modem chip.
worked with. Since your chip is already certified for the
One design flaw we saw in one design particular carrier, it has already passed these
was having the ground plane under the u.fl tests. Unfortunately, some carriers require
connector going to the external antenna. This you to retest many of these requirements that
ground plane was absorbing a significant have absolutely no bearing on your design
amount of both outgoing (TRP) and incoming unless you have modified the cell modem
radiation (TIS). Your antenna connector must chip (which you can do). It is understandable
not be near either the ground or power plane. that the carriers need to protect their
Electromagnetic Compatibility/ network from rouge devices but I feel very
COLUMNS

Electromagnetic Interference (EMC/EMI): We strongly that they need to simplify this area
did a preliminary EMC scan on our first IoT of certification. So chip makers, carriers, and
cell modem design and were very happy that PTCRB board, if you are listening, isn’t there
ABOUT THE AUTHOR
we met FCC Class B requirements for radiated a better way to detect if we have modified the
Bob Japenga has been spurious emissions (EMI) with flying colors. chip’s operation? For example, if there was a
designing embedded What we didn’t know was that PTCRB had its flag in the chip that indicated that the radio
systems since 1973. In own idle mode radiated spurious emissions parameters have been altered in such a way
1988, along with his best requirements which were far more stringent that the carrier/PTCRB certification has been
friend, he started Micro- than FCC Class B. Initially, we were not even compromised, certification could be made
Tools, which specializes close to meeting these PTCRB requirements. much simpler.
in creating a variety of We hired an RF expert to help us. His first A lot of these tests are very complicated
real-time embedded sys- suggestion was for us to rip apart an old and are being performed to moving
tems. With a combined cell phone and tell him what we saw. When standards. We were certifying one product
embedded systems expe- we did this, we saw that the entire circuit that was failing tests that had nothing to do
board was covered with EMI shield cans (see with our design—only with the cell modem
rience base of more than
Photo 1). “That’s what you need to do with chip. What it boiled down to was this: The chip
200 years, they love to
your design.” So, after designing the circuit was tested and passed Version A certification
tackle impossible prob-
with all of the EMI suppression techniques requirements. More stringent requirements
lems together. Bob has
and good layout practices that we knew, we were created later (Version B) which our
been awarded 11 pat- still needed to populate the board with five modem failed. Since we were only required to
ents in many areas of shield cans. pass Version A requirements, we should have
embedded systems and Data Retry: If you were a carrier, you been able to re-run the tests to Version A. The
motion control. You can would not want to have devices tie up band problem was that the certification lab did not
reach him at rjapenga@ width with incessant retries. So each carrier have test equipment that ran Version A tests!
microtoolsinc.com. has its own unique retry requirements. Some Hopefully you see the problem. I strongly
of this retry logic is handled by your cell think this must change as it wastes a lot of
modem (retries connecting to the cell tower). time and money in the certification process.
But in addition, your application software We have wasted several months trying to get
must meet the retry requirements of each this device ready for sale.
carrier. Generally, we are designing systems
that use less than 1 MB of data every month HARMONICS
so we don’t want too many retries at the In 2010, I was at a football game with
application level either. my grandsons and 103,000 other people.
Data Throughput: Remembering that One of my grandsons was not able to make
carriers are trying to get as much data the game, so I wanted to send him a text at
kickoff. Even though I had maximum signal
strength, I could not make the call. When I
RESOURCES looked around the stadium, it was clear that
OET FCC, “Understanding the FCC Regulations many wanted to text or call at the same time.
for Computers and Other Digital Devices,” OET Cell phones must work in close proximity to
Bulletin No. 62, 1996, https://transition.fcc. other cell phones. Most M2M devices do not
gov/Bureaus/Engineering_Technology/Docu- have that requirement. PTCRB certification
ments/bulletins/oet62/oet62rev.pdf for a sum- requires that your device not be transmitting
mary of FCC requirements. on any frequencies other than the frequency
circuitcellar.com/ccmaterials you are licensed to transmit on so as to avoid
  interfering with nearby cell phones. The
PTCRB, “Who Does Testing?,” www.ptcrb.com.
first device we took through PTCRB testing
Verizon Innovation Center, www.innovation. failed these tests at a couple of points. What
verizon.com. we discovered was that every diode in your
 circuitcellar.com 59

design acts as a re-radiator of the radio signal

you are transmitting. And it radiates at one
of the harmonics of the transmit frequency.
This must be squelched or you will fail your
Harmonic Radiated Spurious Emissions (RSE)
tests.

WAIVERS
Even after doing another spin of the board
with small capacitors around every diode, we
were still failing Harmonic RSE at a couple of
frequencies by a few decibels. The product
was already several months late. Should we

COLUMNS
do another spin of the board after we find
the diode we missed? At this point, I pushed
through a waiver. This was a formal request
to the PTCRB board for an exception to the
requirements. Our unit was stationary. Our
unit did not operate in the presence of other
cell phones. Come on, we are talking about
only 2 db! Thankfully and quickly, the waiver
got approved. We had our first cell modem-
based IoT device ready to ship. So the moral of
the story is: Work with the certifying agency.
Some requirements that apply to cell phones
do not apply to M2M products. Sometimes the
certification process is our friend but a lot of
time it is just a pain in the neck.

CERTIFY FIRST
You have a good IoT idea that will make
this world a better place. But before you bring
it to fruition, you will need to pass the
necessary certification tests imposed on you
by the cell network carriers. This article gives
you a thin slice as to what’s involved and what
it will cost.