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Even though wireless technology continues to proliferate faster than most of its engineering counterparts, an OEM's design is still not complete by buying an off-the-shelf radio chip and connecting an antenna to it. Hardware integration such as impedance matching and optimum PCB layout still makes RF design a significant challenge. Wireless IC manufacturers spend time and money addressing these various design challenges at the silicon level during chip development. However there is a clear missing link between a well designed RFIC and its successful integration into the OEM's product. This is where RF reference modules step in and bridge the gap thereby giving the OEMs the much needed head start for designing their application around the chip.
Preliminary testing
Before making a significant R&D cost and time commitment to a radio chip, the first step an OEM wants to take is to evaluate the performance of the RFIC. The RF reference modules provide this platform typically connected to a development or evaluation kit with the necessary firmware. Indoor and outdoor range testing can be done using the reference modules. Instead of relying on the datasheet specs the OEM gets a glimpse of the real world performance of the radio chip.
Figure 1. CY3630M reference module with PCB trace antenna
Since the wireless IC manufacture has invested considerable time and resources on optimizing the reference module, what the product developer is evaluating is a benchmark performance of the radio. This obviates the need for the OEM to start designing the chip based solely on datasheet specs and not actual performance. Thus the risk on ROI is reduced considerably.
Detailed testing
A second version of this RF reference module can be offered wherein the antenna is replaced by an SMA or micro-coax connector (See Figure 2). This facilitates taking conducted RF measurements of the circuit up until the antenna which is typically matched to 50 ohms. This also gives the OEM the flexibility to use commercial off-the-shelf Rubber duck antennas.
Figure 2. Reference module with SMA
Such a module also makes it possible to take return loss and impedance measurements on a network analyzer looking into the matching network following the radio RF pins. This gives the OEM the ability to modify the matching network to match the antenna he intends to use. Conducted measurements such as peak RF output power for different amplification levels can be made on a spectrum analyzer or power meter. One can also measure the spurious harmonics across the band of interest. Transmitter Occupied bandwidth and receiver conducted spurious measurements can be made and checked whether they meet regulatory approval specifications.
Reference guideline for PCB layout & schematic
The RF reference module provides a typical application circuit schematic using the radio chip and the PCB layout optimized to meet the module specs. Since the chip companies have invested in their RF layout designers for iterating the boards for best performance, several possible faux pas which the OEM could make in the layout have already been rectified for you. These could include: (a) incorrect placement of components such as placing a crystal under the RF output match thereby introducing sideband noise (b) Broken GND islands degrading receiver performance by several dBs (c) incorrect placement of mechanical elements like screws near the antenna which could easily detune it (d) enclosure blunders which could cause ESD failures.
PCB trace widths on reference modules take into account the requirement of different signals--such as power supply lines and transmit output transmission lines. Additionally the manufacturer typically brings out important IC pins out onto a header or test points to make probing easier. If the OEM intends to increase range by boosting the power output using an external PA, the reference module can provided couple of pins to drive the PA and use an external RX-TX RF switch. Recommended power supply decoupling capacitor values are used in the design. Another big plus of having a reference module available is if the manufacturer is using a PCB trace antenna, you have a working antenna design ready and can skip simulating and testing various other antenna topologies. The reference module gives a good estimate of the number of layers used for the board and board thickness.
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