Incorporating wireless connectivity into a product is now a standard design requirement for Wi-Fi, IoT, and other applications. And the good news is that it’s much easier than it was just a few years ago. While a custom design may be the best or only choice for very low-end applications (a simple point-to-point link signaling a door has been left open, for example) and for high-end challenges (such as mil/aero installations), the overwhelming majority of RF designs are better satisfied by using a nearly complete wireless module or IC, or a vendor-supplied reference design.
The key phrase here is “nearly complete.” These solutions may be highly integrated and include the system processor plus operating system, layers for format/protocol, and even analog front end (if any), or they may provide only RF portion of the system. Regardless of the level of functionality, it’s unlikely that what is called a “canned” module will include the antenna and power supply. Of course, the IC certainly won’t have either one, while a reference design is just that: a starting point with detailed documentation.
Regardless of design approach, the final shippable product must meet various regulatory mandates, which are primarily related to transmit power level, the operating frequency, and the operating region of the world. Deciding which approach to use for providing that RF connectivity includes tradeoffs in time to market, design and performance risk, BOM cost, and regulatory approval standards. Most design teams will look first to a module and IC approach, then look next to a fully documented reference design, rather than try to go it alone and develop a custom wireless link.
Highly integrated does not mean absolutely complete
It may seem that using a standard module or IC is a simple solution with no problems, and that a well-documented and tested reference design is almost as problem-free. That’s not quite correct. Although they may be relatively simple, all wireless approaches still have potential issues. They all have several factors in common: they need a DC power supply, they may need RF bandpass filtering, and they need an antenna. These are areas where the completed design can have performance and regulatory issues, even if the module is certified and approved, or the core of the design has been used in an approved system.
Reality is that a module or IC which is approached and certified for compliance with various regulatory mandates is a major head-start to the process, but it’s the final system design which must still meet standards. A certified module is a necessary but not sufficient condition for approval. Of course, for ICs and reference designs, there is no approval except possibly in the particular implementation that was built by the vendor — but only if it was submitted for approval; “paper designs” or even demonstration-unit breadboards are not the same as the completed product.
A look at a highly integrated, high-performance IC shows the scope of the issues. The CC2530 from Texas Instruments is a second-generation, extreme low-power system-on-chip (SoC) supporting 802.15.4/ZigBee/RF4CE operation at 2483.5MHz. It targets remote control s (replacing the classic IR remote) as well as Zigbee designs in applications such as home/building automation, lighting systems, industrial control and monitoring, low-power wireless sensor networks, consumer electronics, and health care.
The internal block diagram (Figure 1) shows the high level of functionality that this IC offers, with MCU, multiple memories, an eight-channel/12-bit ADC, a 2.4-GHz RF transceiver, and USB Controller, among its many features. The design is optimized for high receiver sensitivity and is also designed for robustness to interference, the transmit side has programmable output power up to 4.5 dBm. Power saving is also a focus, with a variety of power-on and power-down modes (idle, sleep, interrupt-only) modes to maximize battery life.
While the internal block diagram is impressive and clearly shows the level of functional integration and capability, the system-level interconnect diagram (Figure 2) shows some of the additional challenges designers must overcome when using this IC (or any one of the many that are similar to it). Note that the figure does not show the decoupling capacitors for the 2.0 V to 3.6 V power-supply rail, which are mandated by TI (with good reason). Of course, even a few centimeters of wire – whether as a discrete wire or a PC-board track – can also become an antenna and thus pick up ambient RF, or radiate it to nearby circuitry.
Start with the DC supply, since a solid rail is the basis for reliable and consistent system performance. The supply is most likely a single-cell battery or multiple batteries with a DC/DC switching regulator to ensure stable DC even as the battery output drops or the load demand changes. Even a few millivolts of RF noise on the DC rail for the RF section can cause subtle problems which result in non-compliance or inadequate performance. This noise can come from the regulator or from other components using the same rails, from nearby digital noise being picked up by the supply rail tracks, or as EMI/RFI from the transmitted RF itself.
Ferrite beads on the supply lines, whether these lines are discrete wires or PC board traces, provide a simple, effective, and low-cost solution. For example, the HZ0603B102R series of ferrite beads from Laird PLC is optimized for differential-mode EMI suppression (Figure 3). These low-cost, tiny components – just 1.6 mm long, with a 0603 body style– are effective to 100 MHz, can handle up to 200 mA, and have no technical tradeoffs or downsides. Don’t be fooled, though: despite their ease-of-use and functional simplicity, they have well- defined characteristics and the vendor even offers Spice models for these modest devices.
There are often other filtering situations, but far away from the DC end of the electromagnetic spectrum. Bandpass filtering prevents unwanted out-of-band emissions, and is often needed for compliance with regulatory standards. As a complementary benefit, these filters also prevent out-of-band interference with the receive side of the wireless link, which could lead to cross-modulation, unwanted interactions, and even saturation in some cases.
For example, for simple RF remote control links at 433.92 MHz (usually done as a limited-function custom design to avoid the power dissipation and cost of a standard IC or module), TDK’s B3780 series of SAW bandpass filters is an option. These filters – just 3.8 × 3.8 × 1.5 mm – have a typical 3-dB bandwidth of 780 kHz, with minimum/maximum bandwidths of 850 and 920 kHz, respectively. In order to provide a 50Ω impedance match, the user may have to add matching circuitry, which can be accomplished with L and C elements created within the PC board layout, or by using discrete components (Figure 4).
Antennas: discrete or PC board?
Every wireless system needs an antenna, and there are two very different ways to implement one at the 500+ MHz frequencies of these designs: use a discrete antenna, or one which is fabricated as part of the PC board’s “real estate”. There are distinct tradeoffs associated with each approach.
Discrete antenna pros:
- can be located away from RF circuitry;
- no unintended interaction with RF circuits and its location is not affected by, nor has effects on, that circuitry;
- has minimal impact on product-packaging arrangement
- requires no PC board space except for connector (if used);
- requires no design expertise to create or use: just connect it and it is ready.
Discrete antenna cons:
- has obvious cost as an item on the BOM;
- may add to overall product size if located internal to enclosure;
- if external, can be misplaced or damaged by user.
PC board antenna pros:
- has no apparent cost on BOM, except for a slight increase in PC board size;
- it is one less item to source;
- can be custom-tailored for unique frequencies, bandwidths, and polarizations;
- can be designed to attenuate specific undesired signals, in band or out-of-band.
PC board antenna cons:
- requires some PC board space, which may be limited;
- is sensitive to placement of nearby components, and may constrain layout options;
- while there are many standard designs available, they still require skill to design into the system;
- require skill in embedding into the board layout, even if using a standard design;
- are inherently hard to debug, and inflexible if there are problems;
- may have issues related to impedance matching, and require additional components or redesign;
- may require extra-tight tolerances in the PC board manufacturing process.
If the decision is to use a discrete antenna, vendors offer a wide range of standard options, including single-band units to antennas which support multiband operation. One example of a simple-looking yet high-performance unit is the 1052630001 from Molex (Figure 5) a tiny six-band unit with 3 dBi gain (omnidirectional pattern, linear polarization); see Table for band listing. This compact patch antenna, just 107 L × 13 W × 0.1 mm deep, is well-suited to supporting the latest generation of cellular links, which are multiband designs. The antenna comes standard with a 50Ω microcoaxial interconnection cable that is 100-mm long; 150 and 200-mm lengths are available as standard options for maximum flexibility in installation and use.
Using a simple, one-band, off-the-shelf 50Ω antenna does not mean that the designer of the wireless system just connects it to the IC and the job is done. Looking again at the interconnection diagram for the Texas Instruments CC2510, there are three inductors and six capacitors between its RF port and the 50Ω antenna. These passive components are needed as a balun to transform the balanced RF interface of the IC to a single-ended topology compatible with the antenna. Another option is to use a balanced, folded-dipole antenna, but impedance matching may still be needed. The components also implement impedance matching between the two points to maximize energy transfer and minimize signal loss and VSWR, which can result in undesired effects such as excess dissipation and even amplifier distortion due to energy reflecting back to the source.
Regardless of the wireless-implementation approach chosen (module, IC, reference design, or custom), meeting regulatory requirements can be a challenge ranging from modest to severe. Rather than go directly to final, formal approval tests, many design teams choose to run a detailed suite of pre-compliance test using in-house capabilities or a convenient facility. This minimizes the high cost and disruption of the formal test process, as well as risk of failing, having to go back to the bench to find the problem, developing a solution, and then going for a retest. Relatively small passive components are often significant parts of the compliance solution
Keep in mind that regulatory compliance is not about wireless performance parameters such bit error rate (BER), RF sensitivity, throughput, accuracy, battery life, or dissipation. Instead, it’s entirely about unwanted emissions. There are specialized compliance consultants and experts who can review a design concept, BOM, and final physical implementation, advise on the pre-compliance test cycle, and even help with the actual certification process.
About the author:
Bill Schweber is an electronics engineer who has written three textbooks on electronic communications systems, as well as hundreds of technical articles, opinion columns, and product features. In past roles, he worked as a technical web-site manager for multiple topic-specific sites for EE Times, as well as both the Executive Editor and Analog Editor at EDN. He has an MSEE (Univ. of Mass) and BSEE (Columbia Univ.), is a Registered Professional Engineer, and holds an Advanced Class amateur radio license. Bill has also planned, written, and presented on-line courses on a variety of engineering topics, including MOSFET basics, ADC selection, and driving LEDs.
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