RF chip scores goal for Leeds

RF chip scores goal for LeedsRichard Ball
A simple and inexpensive method of mixing audio signals up to millimetre wave frequencies has been developed by researchers at Leeds University.
Central to the system is a new design for a millimetre wave mixer, part of a PhD project carried out by Mike Roberts and sponsored by BT Labs.
“Millimetre mixers have been around for ages, but I developed a new topology,” said Roberts. “A spin-off application was to interface that mixer to a fibre optic network.”
The mixer is designed into a monolithic microwave IC (MMIC). Its active device is a pseudo-morphic high electron mobility transistor (pHEMT).
The circuit self oscillates at 38GHz, reducing cost because no local oscillator is required.
The pHEMT only provides gain up to about 45GHz, so the second harmonic of the circuit at 76GHz was purposely designed to be high in power. “Therefore we are effectively extending the gain of the device,” Roberts said. And the single chip acts as oscillator, mixer and frequency doubler.
The beauty of Roberts’ mixer is the way that signals from a fibre optic cable are mixed onto the carrier.
“We took a baseband signal and modulated it onto a laser diode,” Roberts said. The laser, modulated at 100MHz, shines onto the mixer.
The pHEMT works like a photodiode, up-converting the laser signal. “We get sidebands around 76GHz,” Roberts said.
This can then be transmitted, which due to the high frequency needs a small antenna.
The same mixer can be used to downconvert the transmitted signal in the receiver circuit. Researchers at Leeds are working on this now, Roberts said.
Such a system could be used in the future for short range mobile communications systems. The frequency limits the technique to a range of a few hundred metres, but this is ideal for picocell type applications in built up areas, the home or office.
Small picocell basestation transmitters, linked to the central system via optical fibre, could be built for as little as ?100.


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