Micromachined RF has ‘Q' of 10,000

Micromachined RF has ‘Q’ of 10,000Steve Bush  
 
Researchers at the University of Michigan told an audience at ISSCC that they have turned to micromachining to improve one-chip radio receivers.
Radio reception is all about selecting wanted signals from a sea of unwanted RF and noise.
There are two main ways of performing this selection: the ‘analogue’ (superheterodyne) approach, using a narrowband tuned receiver; or the ‘digital’ way, with a wider less critical receiver followed by digital signal processing to pick out the wheat from the chaff.
The analogue approach is difficult to integrate because the maximum Q of an on-chip LC filter is around 10, nowhere near enough for a radio. Instead, the filters have to be made off-chip, wasting space, money, and worst of all, power.
The digital approach removes most of the off-chip components, but requires expensive and power hungry high dynamic range on-chip front-end A/D converters to maintain system signal to noise ratio.
The Michigan team presented an on-chip micromachined resonant structure with a Q of around 10,000 at VHFfrequencies.
These devices are polysilicon cantilever beams, capacitively coupled into surrounding circuitry and consuming minuscule amounts of power.
Two cantilevers, mechanically coupled by a flexible beam, can be used to form a two-pole filter with characteristics sharp enough to cut out adjacent channel and out-of-band interferers.
At 34.5MHz, 1.3 per cent filter bandwidth has been demonstrated. At HF, the situation gets even better, with 0.2 per cent possible. No problems can be foreseen extending the technique up into the gigahertz range.
For multi-channel and frequency-hopping applications, banks of filters are envisaged, with electronic switching between the individual filters.


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