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A look into field-programmable RF chip

Posted: 02 May 2014     Print Version  Bookmark and Share

Keywords:programmable device  field-programmable radio frequency  FPRF  FPGA  PLL synthesiser 

The filter can boost the signal by 6dB, and is followed by a programmable base band gain stage that can be adjusted to give up to 31dB of gain in 1dB increments. The signal is then mixed to directly give the required modulated RF frequency output. The TX PLL synthesiser multiplies up the input PLL clock by a programmable ratio, and generates a stable frequency with a tight accuracy. The RF gain stage (programmable, of course) provides the final signal boost that is output from the FPRF device. The transmit power level is sufficient for short range communications, say, tens of meters, without any further amplification. Users can use external amplification to increase the range.

Not surprisingly, the receiver path is also highly programmable. The FPRF device offers a choice of three low noise amplifiers (LNAs). A general broadband input stage is designed to handle RF inputs across the spectrum from 300MHz to 3.8GHz. Two further LNAs are optimised for enhanced performance for signals in the range 300MHz to 2.8GHz (Lo LNA) and 1.5 to 3.8GHz (Hi LNA). The block diagram for the receive (RX) path is illustrated in figure 3.

Figure 3: The block diagram for the receive (RX) path.

The mixer in the receive path uses the same PLL clock input as the transmitter, but has a different synthesiser to provide full duplex and direct down conversion. Programmable gain stages and filtering are applied, before the analogue signal is digitised and output as I&Q data streams.

Configuration of all the different elements is performed via a simple SPI interface into the control logic. Each element is programmed by loading a 16bit word that can be static or—for more sophisticated applications—the parameters can be changed on the fly (more on this later).

The configuration is set up using a simple GUI (graphical user interface) that allows control of the features described above, as well as test and loopback modes. This GUI is illustrated in figure 4.

Thus far we've considered a simplified description of the mechanics of the device—but what can you use it for? Well, the chip was designed to be highly flexible, because one of the key applications is for cellular femto and pico cells, as I'll explain. These boxes act as local base stations for cell phones, and link into the internet to provide fast connectivity inside the home or small office. For example, the nearest tower to where I live is on the other side of a hill, so my reception is patchy at best. A femto cell would free me from reliance on signals from the tower.

The challenge in designing an RF chip for these applications is that cellular systems are different around the world—with potentially over 40 different combinations needed on 4G or LTE phones to produce a truly global coverage. So the designers of the FPRF had to make it programmable so it wasn't restricted to a single market.

As soon as you have a chip that encompasses all the cellular frequencies, you also cover many other applications. For example, changing TV transmissions from analogue to digital has freed up a bunch of spectrum for new wireless services. This is called "white space" and companies are working on a huge range of products as diverse as domestic appliances, smart metering, machine-to-machine (M2M) and rural broadband.

Figure 4: GUI for controlling the FPRF device.

One application that Lime uses as a demonstration is a spectrum analyser. The receiver in the chip is swept across the band to listen to all frequencies and the instantaneous output is displayed as the spectral plot. This demonstrates the ability of the FPRF chip to rapidly and dynamically change. Manufacturers of military systems were quick to grasp this, and are busily designing software defined radio (SDR) systems.

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