Steve Holland's Blog

November 2013 Previous month Next month

Since many are finishing exams and can't fathom a week and a half without classes, here is a little tidbit you may find interesting:


This is a very interesting article about a tiny FM modulator:


The article calls it a transmitter, but really a transmitter implies at least a power amplifier and filtering, if not also the antenna -- which is a key distinction because this tiny modulator will not broadcast without an antenna, and antennas don't scale down in size gracefully the way circuits do!


What I like most about this article is the intuitive, mechanical picture it creates of how frequency modulation (FM) is produced.  To create frequency modulation, often a voltage-controlled-oscillator (VCO) is designed that outputs a tone at a certain carrier frequency (e.g. WMSE operates at 91.7MHz), but the catch is that this output frequency is proportional to the amplitude of a signal (voltage) applied to it's input (this is called direct modulation-other schemes do exist).  As a result, as an input signal voltage moves positive, the frequency moves up by some amount, and if the input voltage is negative, the frequency moves below the carrier frequency, in direct proportion.   So for example if you applied a sine wave to the input of a VCO, and tracked the output frequency of this oscillator over time, the instantaneous frequency would trace out a sine wave!  Typically this frequency tuning is achieved using resonant circuits that change their resonant frequency according to this input voltage;  it is typically done all electronically, from very simple circuits to very complex circuits.


This article uses a tiny piece of graphene to act as the resonant device -- in this case mechanically!  Now, there are many examples of existing technologies that use either mechanical or material properties to establish or change an oscillator frequency, but still this is neat.


Much like a quartz crystal, the graphene material has a resonant frequency that can be extracted by placing it in a circuit (here a feedback circuit). The researchers found that much like the string of a guitar, if the material is stretched, the mechanical resonant frequency increases, and if it is relaxed, the resonant frequency decreases.  Now picture anchoring the ends of a strip of this graphene material, and placing it near an electode.  If a voltage is applied to the electrode, the graphene is attracted to the electode due to electromagnetic forces, and "bows" towards the electrode, much like stretching a guitar string.  A-ha!  Now here is the key idea:  applying a voltage to the electrode stretches out the graphene material in direct proportion to the applied voltage,  and the resonant frequency of the graphene is directly proportional to the amount of stretching, hence the graphene's mechanical resonant frequency is directly proportional to the applied voltage!


For those interested, there is a bit more detail in the Columbia news release here:  Columbia Engineers Make World’s Smallest FM Radio Transmitter | The Fu Foundation School of Engineering & Applied Scienc…, and for those REALLY interested, here is a link to the Nature journal article: