Category Archives: ultra wideband

Intuitive CMOS transistor modeling

On Oct. 6, 2015, I gave a guest lecture in the lecture series “Structured Electronic Design” (EE4C09) on Intuitive CMOS Transistor Modeling. In there I explain the 5 regions of operation of an MOS transistor (both in weak inversion and in strong inversion, both in triode and saturation, and off), based on the EKV model. For those of you that might still be struggling with understanding how the CMOS transistor works and how it can be employed in first time right, first time best analog and mixed signal circuit design, this lecture is for you.
See the complete lecture, which also treats double loop negative feedback amplifiers, here

Rats go wireless in an analog fashion

Rat carrying wireless systemThis little image on the left shows a rat carrying a wireless system, partly mounted on his head, partly realized as a kind of a backpack. It has been developed by researchers at Harvard University in close collaboration with colleagues at California Institute of Technology and is being used for neurological research on rats in the wild. According to Nature (Febr. 25, online) the entire systems comprises "a tetrode microdrive, for chronic positioning of electrodes in the brain; an integrated circuit for high channel-count neural recordings; and a radio-frequency wireless transmitter. The device takes up to 64 analog voltage signals from neurons in the brain and muliplexes them into one signal that appears in a temporally interleaved fashion, one after the other. Then that signal is transmitted by analog FM radio to a receiver."

The article further reports that good old FM (frequency modulation) transmission has been used as it outperforms digital wireless communication on weight, power drain, throughput and distance.

So why does analog FM outperform its digital counterparts, such as FSK, QPSK, QAM and OFDM? Before answering this question it is important to realize that from a channel-capacity perspective (as defined by Shannon) there is no preference for analog modulation over digital modulation. The answer thus has to follow from practical considerations. Digital modulation implies that the information is transmitted over the wireless radio channel in a digital fashion. As all information in nature, also neural signals are analog in nature and thus, in order to prepare the neural signals for their wireless journey, they have to be converted to the digital domain by analog-to-digital conversion (ADC). This thus requires at least one ADC. Often, depending on the digital modulation type used, channel coding is performed prior to the digital modulation. As a consequence, with these additional blocks, the entire transmitter becomes more complex, which, in turn, entails a larger power consumption and, when battery-operated, a larger battery and thus a larger weight on the head or back of the rat.

Another reason why analog FM may outperform its digital counterparts lies in the frequency spectrum of the transmitted radio signal. FM produces an almost flat frequency spectrum. As a consequence, it is relatively immune to frequency-selective fading, which is good for radio communication over relative long distances. Also, FM transmission does not require a highly linear power amplifier. This is good for its power efficiency and thus for the overall energy efficiency of the transmitter.

One final remark. From the picture it looks like the rat is loaded with a transmitter that has been implemented using discrete components, rather than with a single chip (or integrated circuit, IC). Many of the blocks needed for digital transmission could be implemented with a much smaller form factor and consuming less power when realized on-chip, rendering the antenna the largest component and the largest power consumer. In such a case the choice for either analog or digital could have just as well turned upside down.


Micropower scavenged UWB radios

The windmill is a beautiful artifact and very recognizable in the Dutch landscape. It converts wind energy into rotational motion, and is one of the earliest energy harvesters in history. Today, research on energy harvesting is not only focussing on generating megawatts of electrical power, but also deals with micropower harvesters that can for example be used to power small wireless sensors or RFID tags.

Micopower energy harvesters can convert energy from the environment (vibrations, thermal difference, solar, RF) into electrical energy that can be stored in a capacitor or battery.

This allows wireless sensors to transmit their payload once the harvester has collected enough energy. Thus, in principle, the wireless sensor can have an infinite lifetime. Of course, the amount of energy that can be harvested depends on the application. One of the reasons why micropower scavenged wireless sensors are not yet widely used today is that current radio solutions such as ZigBee or Bluetooth consume (much) more power than the harvester can deliver.

A promising solution for this problem is to use Ultra-WideBand communication. UWB radio makes use of carrierless, short duration pulses to transfer the information. Since the radio can be switched off during the time laps between two pulses, the average power consumption can be low enough so that it can be combined with an energy harvester. That’s why in this group we are working on energy scavenged UWB radios since they seem to be the perfect match. Another distinct advantage is that UWB communication can offer accurate localization within a few cm’s. This means that finding your missing car, laptop or wife will be a piece of cake without ever having to change the radio battery!

Mark Stoopman