The ultra-low-power radio receiver will measure extremely low frequency (ELF, 30 Hz to 3 kHz) and 

effects of plasma waves which permeate high latitude polar regions, examples of which are discrete

chorus emissions and wideband auroral hiss emissions discussed briefly in Section C. Interferometric

measurements must be conducted simultaneously at multiple electromagnetically quiet high latitude sites,

Currently available ELF/VLF receiver designs (see Figures 2 and 3) do not allow such measurements,

primarily due to their physical size (requiring expensive shelters) and high power consumption, which

prohibits sufficiently long duration (e.g., a few months or ideally year-round) observations.

While the interferometric measurement is one specific scientific objective that would be uniquely

facilitated by the new instrumentation, the ultra-low-power ELF/VLF receiver to

develop will also bring about major simplifications (and thus cost savings) in logistics and resource

demands for ELF/VLF measurements currently in progress at manned and unmanned polar sites, in

Antarctica as well as in Alaska and other northern polar regions. Once it is developed, the highly

complex and bulky receiver systems currently deployed at these sites can all be replaced with the 

ultra-low-power ELF/VLF receiver with no loss whatsoever in resolution, bandwidth, coverage or other

capabilities, and in fact with substantially improved flexibility of operation and data acquisition modes.

Since the new receiver will be integrated onto a single chip, fabrication of duplicate copies will require

minimal effort. Thus, we fully expect that many other users will use this new receiver to conduct ELF/VLF

observations at sites worldwide, using the flexible DSP capability of the unit to configure its operation

to their particular needs.

electronics [e.g., Machado, 1996]. The line of research pursued at Stanford involves tunable-threshold,

low voltage CMOS integrated circuits, which consume ~50 to 100 times less power than conventional

is a 2-GHz single-chip radio receiver RF front-end recently fabricated [Shana’a et al., 1999] in the Ultra

provide a total of ~50 W average power, supplied by a thermoelectric generator powered by ten tanks

of propane (each 30 inches in diameter and 53 inches in height and weighing ~700 lbs), flown in every

The power budget allocation for the ELF/VLF receiver at an AGO site is ~9 W, of which only ~6 to

this requirement to ~10 mW, representing nearly a thousand fold savings in power for the ELF/VLF

system, and facilitating a ~20% reduction in the power requirements for the entire AGO. This means that

housing to be buried under the sensors (presumably at some distance of ~500 ft or more from the AGO

Power consumption by passive instruments such as an ELF/VLF receiver at a manned polar station

constitutes a very small fraction of the total power available, much of which goes to the provision of

the working environment and various other support for the station personnel. Thus, on the surface, the

ultra-low-power feature of the proposed receiver would not appear bring about a significant improvement

in logistics from the point of view power consumption. However, a very serious and progressively

deteriorating problem at such sites is the increasing level of electromagnetic interference in the vicinity

of the station, coming about from the rapid expansions of computer usage and other infrastructure at the

sites. As examples, video and computer monitors have scan rates within the VLF range and thus produce

radiative interference, and increased power usage and infrastructure at the stations leads to increased

levels of power line ‘hum’, which often dominates the ELF spectrum.

ft from the station, the maximum length being determined by the need to maintain analog signal integrity

(dispersion, distortion, attenuation, and dynamic range) during long distance transmission over a coaxial

cable. At the present time, the ELF/VLF antennas at both South Pole and Palmer are at maximum distances

from the station, and still suffer very significantly from station-generated electromagnetic interference.

At South Pole, the signal reception on one of the antennas (oriented such that it receives maximum noise

from the station) is now extremely noisy to the point of being nearly unusable.

This emerging practical problem can be alleviated with the use of the ultra-low-power

ELF/VLF receiver can either be powered by one or more batteries (see Section D) or by a low voltage

line level cable from the AGO, with the data brought back to the AGO either by a serial data cable or an

optical fiber.

The substantially improved signal-to-noise ratio realized by such use of the ultra-low-power

ELF/VLF receiver at a manned polar station comes about with absolutely no loss of capability or functionality,

since the digital samples of the broadband waveform from both magneic antennas would be

continuously available at the station to process or record in ways as required by the particular scientific

objectives. These digital signals can be received and processed on a PC at the station, or the required

processing can be implemented within the receiver using its DSP capabilities, in which case reduced

data can be transmitted to the station for recording. The latter is not likely at a manned site, since ample

resources for data recording are usually available and the tendency is to record raw (unselected) data.

Other obvious benefits of the use of the ultra-low-power ELF/VLF receiver at a manned site

include the substantial reduction in rack space needed for ELF/VLF equipment at the station (what is

shown in Figure 3 is essentially reduced to a single PC computer to receive digital data from the unit

equipped a CD or DVD writer), substantially increased robustness/reliability, ease of duplication, and

components.

The ultra-low-power and highly compact properties of the ELF/VLF receiver is likely to revolutionize

accessible once a year (with ground transportation, by ship, by small aircraft, or other means) even if no

logistics resources (power, shelter, etc.) are available at that location. The unit will be enclosed

in a hermetically sealed dewar (See Figure 9) and will operate autonomously for an entire year when

flash memory and is downloaded during annual visits for battery replacement. Depending on the application

and the cost of flash memory chips at the time several to tens of GBytes of data can be collected

in a given year. In comparison, an entire year’s worth of ELF/VLF data from an AGO site (Figure 2) is

acquisition, and with the use of on-board DSP processing for event-based acquisition.

One obvious application of autonomous deployment of the ultra-low-power ELF/VLF receiver

system is ELF/VLF interferometry, which is described in Section C. This application requires the

deployment of multiple autonomous units, the data from which is acquired in annual visits. The spacing

and configuration of sites depends on particular applications, a few of which are briefly described in the

next section.

The scientific justification of the instrument development is underscored by the fact that

ELF/VLF observations are currently conducted at a host of polar sites, some of which are shown in

Figure 4. The scientific objectives of these observations vary across the full spectrum of ionosospheric

and magnetospheric physics, ranging from investigations lightning-induced precipitation of energetic

radiation belt electrons at low latitudes (e.g., Palmer), to the studies of the microphysics of wave-particle

interactions at middle to subauroral latitudes (e.g., at Halley Bay (HA), A80, and A77), to investigations

of substorms, auroral activity and associated waves and ionospheric effects (at SP, A84, P2, and P3), and

to studies of polar cap phenomena and waves on open field lines at high latitudes (P1,P4, and P6). The

ultra-low-power ELF/VLF receiver will bring about major simplifications and new capabilities

in all of these studies.

We now provide brief background on one particular type of plasma wave, namely discrete ELF/VLF

chorus emissions. ELF/VLF chorus, named for its charactericstic sequence of repeating, usually rising

and often overlapping coherent tones, ranks as the most intense of all naturally generated plasma wave

emissions observed in the Earth’s magnetosphere, occurring regularly in association with disturbed

magnetospheric conditions and seen frequently in association with microburst electron precipitation

[e.g., Rosenberg et al., 1981] and other auroral activity. The onset time of growth of chorus has in past

work been associated with arrival of magnetic disturbances [Gail and Inan, 1990]. However, the nature

of such association has in general been complex and variable from event to event. More recently, a much

more direct association of chorus onset and turn-off with sudden fluctuations in solar wind dynamic

pressure and southward turnings of the IMF have been observed, both in situ [Lauben et al., 1998]

and on the ground [Salvati et al., 2000]. The ground-based observation is particularly exciting, since

the relationship between the solar wind and this dominant plasma wave emission can be continuously

monitored. Understanding the circumstances of that are conducive to the generation of these intense

emissions is important especially in view of the dominant nature of chorus in terms of its intensity and

its known effects as the driver of energetic electron precipitation, pulsating aurorae, and maybe even the

morningside diffuse aurora [Inan et al., 1992].

One important challenge in ground-based observations of waves of magnetospheric origin is the determination

of the location and shape of ionospheric regions which are illuminated from above which then

waveguide as depicted in Figure 6a. In general, the source region cannot be determined from measurements

at one location but can be determined with simultaneous measurements at multiple sites. Especially

relative phase and amplitude can be measured, so that a true determination of the exit region and its extent

can be realized. Ideally, multiple sites should be spaced at distances less than the lower ionospheric height

The development and implementation of the single-chip ultra-low-power ELF/VLF receiver

concept has only recently become feasible as a result of work carried out by the Ultra Low Power (ULP)

radioscience group of the STAR Laboratory, under the direction of Professor L. Tyler and Dr. I. Linscott.

This group is completely independent from the Very Low Frequency (VLF) research group headed by

Professor U.S. Inan, which is involved in a wide range of ELF/VLF observations in the high latitude polar

regions. The main scientific thrust of the ULP group has been planetary radioscience studies, specifically

primarily in preparation for an upcoming NASA spacecraft mission to Pluto.

Technological developments pursued for the purpose of this planetary mission, and specifically the

work carried out in the last three years, has serendipitiously facilitated the realization of an ultra-lowpower

ELF/VLF receiver for use in the polar regions of the Earth. The ingredients that bring about this

capability are the technical expertise in RF microelectronics, access to high performance integrated circuit

fabrication technology, and proven technical competence for design and construction of RF integrated

circuits that function as single-chip 2GHzradios. Our program will build upon these ingredients,

to meet the technical challenges in producing an ultra-low-power ELF/VLF radio receiver on a single

chip and to integrate the receiver with a high performance signal processor that can extract selected bands,

cross correlate signals from multiple antennas, and detect critical structure of the frequency dependence

of a VLF signal. In the following, we first provide a brief background of the past Stanford ULP work

and then describe the technical approach and methodology.

The STAR Laboratory ULP group was organized to bring together related interest and work with the

intent of affording opportunity for a synergism that would spark the innovation of a microminiaturized

receiver for uplink radioscience. The efforts within this group in addition to (i) the single chip radio

project, include (ii) development of ultra low power CMOS circuits, (iii) work on the optimization of

needed on-chip spiral inductors, (iv) theoretical investigation into the stability of small oscillators, and

(v) methods for building microminiaturized stable oscillators. Recent achievements in each of these five

areas are summarized below.

The poor performance of single chip radio receivers with traditional heterodyne downconversion is due

of the signal-to-noise ratio at the input versusthat at the output) of the Low Noise Amplifier (LNA),

and of the low power Gilbert cell mixers, typically 14 dB, that could be integrated on-chip. The more

familiar double-balanced mixers, although having a lower noise figure, use much more power and are

thus seen as unsatisfactory. A major milestone achieved during the past year is the development of a

insightful simultaneous optimization of the minimum possible noise figure for a low noise amplifier

(LNA), in MOS technology, while matching the input impedance of the LNA to an external antenna.

This discovery was based on the realization that noise figure of the LNA would be minimal by choosing

input impedance was governed only by the device size and by making the devices large enough the input

impedance could be exactly matched to 50 ohms, the output impedance of an antenna. Surprisingly,

these design features were apparently not understood within the semiconductor industry. This discovery

indicated that an on-chip LNA could be designed with a noise figure as low as 1 dB, while being matched

to 50 ohms. This method for simultaneous noise optimization and impedance matching was presented

in a paper to the 1999 Solid States Circuits Conference in San Francisco. Within the same year, it was

speculated that the same design methodology could be applied to the Gilbert cell mixer and may result in

as much as an 8 dB reduction in the noise figure of that mixer. This approach led to the implementation of

traditional heterodyne down conversion in a single chip fully integrated 2 GHz receiver. Several months

of design and simulation followed, leading to an LNA design with estimated performance of +14 dB gain,

matched to 50 ohms, and a 1.2 dB (or 50 Kelvin) noise figure. The LNA was integrated with the Gilbert

cell mixer. The estimated noise figure of the mixer as 8 dB, and in combination with the LNA resulted

in an estimated total noise figure for the receiver of just 1.3 dB. This new design made it possible for a

single chip radio to be designed that meets the needs of planetary radioscience missions and is shown in

Figure 7a.

The micro radio receiver front end prototype was tested by first converting to a sub-micron BiCMOS

layout and then fabricating the chip in a premier, 0.25 micron BiCMOS process through a cooperative

agreement negotiated with MAXIM Inc., a manufacturer of RF microelectronics located in the Bay Area.

The fabricated micro-radio prototype chip was delivered in mid-July 1999, and subsequent tests have

demonstrated that the chip is fully functional and meets or exceeds the performance estimated from its

design. A photograph of the chip is shown in Figure 7b. In particular, the LNA of the chip has a gain

of 14 dB, and is matched at 50 ohms input impedance over the frequency range from 1.6 GHz to 1.8

GHz, with a noise figure of 1.2 dB at 1.6 GHz rising only to 1.35 dB at 1.8 GHz. This noise figure is a

world record for LNA performance in this technology and is a remarkable validation of the innovative

insight of our PhD student O. Shana’a for low noise optimization and of the chip design methodology.

After the front end prototype was completed and while the chip was in fabrication, the second stage for a

microreceiver (the IF stage) was designed. The completed design was also laid out in BiCMOS as well

and is currently in fabrication. The experience and expertise acquired in the microreceiver project will

be directly applied to the design and develeopment of a VLF micro receiver with particular emphasis on

incorporating the proven design methodologies to obtain a very low power (i.e, 10 mW), and a very low

noise figure (i.e, 2 dB), ELF/VLF receiver.

Spiral planar inductors are an effective means of obtaining on-chip inductance for RF microelelctronic

designs. Spiral inductors have become very popular for this purpose. The Q of spiral inductors are low,

due to the resistivity of the traces and parasitic capacitances. While some design tools are available that

estimate spiral inductor performance, finding optimum choices for factors like number of turns and width

of traces has been limited to trial and error. An electromagnetic model for spiral inductors has recently

in the post LNA programmable filters which will be required in the ELF/VLF receiver.

A radio receiver requires a stable frequency. A typical frequency reference uses an ovenized crystal,

and together with its control and buffer electronics, the oscillator is relatively large and power hungry.

As we shrink the radio receiver to a single chip, the oscillator stands out as one of the last components

of a single chip radio that needs to be miniaturized. For this purpsoe, we have built close ties with

the Ultra Stable Oscillator Development Group at John Hopkins APL, where excellent capabilities are

available and recent progress in the miniaturization has been made. This association has stimulated our

interest in investigating alternatives for stable oscillators. Thus we have been looking into the behavior of

oscillators to understand the nature and limitations on stability. Two PhD students in the ULP group, Ed

Post and Mitch Oslick have collaborated to discover the role of waveform symmetry in the upconversion

the oscillator and thus its utility as a frequency reference. The students have found that components

resultant waveform fluctuates the frequency producing the lower bound on the Allen deviation. These

Our desire to miniaturize the oscillator has led to investigations into alternatives to ovenized crystal oscillators

as frequency references. We have been interested in arrays of coupled oscillators as a potential for

an oscillator that could be implemented on a single chip integrated circuit. Accordingly, arrays of Van der

Pol oscillators have been modeled and a software simulator written to study the collective properties, such

as modes, of the array. Weakly coupled oscillators exhibit a tendency to transition from a disorganized,

uncorrelated state of individual oscillators to a state of collective, phase locked oscillation much the same

model demonstrates this effect. We are now investigating the relationship between the frequency

distribution width of the individual oscillators, and coupling strengths, to the Luopinov coefficient which

improving stability of a frequency and timing reference in a low power ELF/VLF receiver and will be

evaluated in the design.

A typical ELF/VLF radio receiver first amplifies and filters the signal sensed by the antenna. In view

of the frequency range of interest, being 30 Hz to 30 kHz, no downconverting is needed, although this

can be implemented in cases where only a narrow band is targeted for observation. The baseband (either

original or downconverted signal) is directly sent to the A/D intefrace. An oscillator with good stability

is needed for the frequency references in the receiver as well as the clock reference in the digital interface.

The conditioned signal in the receiver is bandlimited by antialiasing filters with selectable lowpass cutoff

frequency and then sampled by the digital interface. Sampled sequences of the VLF waveform are

transformed by a digital signal processing subsystem into compact, high precision, large dynamic range

representations that are then stored in the instrument in high capacity memories. The configuration of

the receiver, as well as the digital interface, signal processing subsystem and memory store are managed

by an intelligent controller. The controller allows the instrument to be programmable and thus capable

of performing a wide range of ELF/VLF observational scenarios (broadband, narrowband, continuous,

synoptic) to match the scientific requirements of particular applications.

ELF/VLF receiver front end designs have been optimized based on many years of experience, with the

ELF/VLF receiver preampllifier systems currently in operation are based on this type of a design, and

sizes with the same input impedance are used depending on the sensitivity desired, ranging from 1.7x1.7

m square loops to triangular antennas with 30-ft baselength (known as the IGY antenna). A 4.9x4.9 m

below the atmospheric noise level in the terrestrial environment and is thus sufficient for most applications.

We will develop a miniaturized integrated ELF/VLF radio receiver that is low power, low

mass, and frequency stable, and consisting of (i) a single chip VLF receiver with an on-chip input LNA,

(ii) a stable frequency reference, and (iii) a microprocessor based control with DSP subsystem. The

connectivity of these four elements is illustrated schematically in Figure 8, below.

resistance. Generally, matching the antenna impedance to the input impedance of a low noise amplifier

(LNA) often means sacrificing sensitivity because of the difficulty of simultaneously optimizing both the

matching impedance and the noise figure of the amplifier. For example, the minimum noise figure of

a good LNA might be 1 dB, with the LNA at room temperature and matched to an impedance that has

the lowest noise figure. When matched instead to the impedance of an antenna, the LNA noise figure

typically increases to 3 dB. However, discussed in the previous section, the Stanford ULP group has had

recent success at RF frequencies in simultaneously optimizing the LNA for both matching the antenna

impedance and minimizing the noise figure for a single chip receiver. In this case, antenna impedance

depends on device size while noise figure was discovered to depend on device current density. Since

device size and device current density are independant characteristics in an integrated circuit, they can be

simultaneously optimized. When this optimization was performed, a noise figure of 1.1 dB was obtained

for an LNA in a BiCMOs integrated circuit that was closely matched to the antenna impedance, with the

design shown in Figure 7.

At VLF frequencies, good receivers have a noise figure close to 3 dB, while the LNA in those receivers

have a noise figure closer to 1 dB. A similar opportunity is to reduce the receiver noise

figure by optimizing the noise figure and the impedance match. We will to integrate the VLF receiver

front end components in a single CMOS chip. By integrating the receiver components, the LNA will

be implemented in CMOS. Although CMOS devices tend to have higher noise figures than, for example

devices in bipolar technology, the trend to small device size offsets this difference to where CMOS

performance that is at least as good as the current discrete component implementations.

We further will investigate the attractive prospect that the receiver noise figure can be improved

through the process of simultaneous optimization of the impedance match and noise figure minimization.

New elements appear in adapting the method from RF to the VLF and promise insight into the behavior

of low frequency amplifiers. For example, the low, almost purely reactive impedance of the VLF antenna,

and white noise distribution at higher radio frequencies. We will incorporate these differences in a

fresh approach that simultaneously optimizes the impedance matching while minimizing the noise figure

of the receiver.

The VLF receiver incorporates a miniature GPS receiver as the source of a stable time and

frequency reference. The power and size of GPS receivers has been shrinking rapidly, however a large

time resolution needed in an ELF/VLF receiver (note that the maximum bandwidth is 100 kHz

twice per day, to synchronize a thermally isolated crystal resonator which is then the source for a timing

generator and frequency reference. A thermally isolated, good quality crystal resonator has an Allen

drift and variation could easily be compensated to the precision needed by the ELF/VLF receiver. GPS

The technology in FLASH memory has been leapfrogging its capacities with no sign of slowing down.

Present, commercially available memories have up to 64 Mbyte capacities, (see e.g. 16Mx8, Samsung

K9F2808U0M, 32Mx8 Toshiba TC58256FT, and 64Mx8 Toshiba TC58512FT) and a handful of these

chips would have the capacity to store 1 Gbyte of data. The great advantage of FLASH memory is

retention at zero power. The power used by these memories in writing to them is dependent on data

substantially higher data rates some compaction, or data compression would be necesssary. As part of

our development we will evaluate methods of compression and data abstraction using the DSP

capabilities of the receiver to optimize the power allocation between the signal processing and memory

transactions.

When used at AGOs or manned stations, the data from the ultra-low-power receiver can be brought

back to the station over a serial data line or an optical fiber as discussed in Sections B.1 and B.2. Our

fiber optic link are commercially available (e.g., Telebyte Model 373) and that the relatively low data

within a 10 mW power budget. As we investigate these possibilities, we expect to significantly benefit

from discussions with Professor Leonid Kazovsky of Star Lab, an expert in optical fiber technology.

The announcement of comercially available very low power DSPs, (see e.g. TI TMS320LC545, and

TMS320C55x core), with power vs performancee in the range of 0.25 mW/MHz down to 0.05 mW/MHz,

opens the door to using DSP in a miniaturized VLF receiver operating at 10 mW. In addition, work in

the ULP group in STAR Lab has produced an ultra-low-power FFT processor with the lowest power

SPIFFIE is a 1024-point single-chip FFT processor designed for low power operation, operating at a

mW. This performance represents an energy efficiency of over 75 times better than any previous FFT

processor. By evaluating both commercially availiable DSP units and the continued development of ULP

DSP systems a digital signal processing capability can be included in the ELF/VLF receiver

within the power/performance goals.

The ultra low power consumption of the ELF/VLF receiver provides a particular challenge

when deployed for use in cold polar region environments (e.g., the Antarctic plateau) where the outside

range, without a more sophisticated thermal enclosure.

In this connection, we have consulted Professor R. Moffat of the Mechanical Engineering department

of Stanford, who is a world reknown expert in heat transfer, insulation and electronics cooling. A

preliminary design of a thermal insulation dewar by Prof. Moffat is shown in Figure 9 and

briefly described below.

The system consists of two small, rectangular, stainless steel capsules, one inside the other. The inner

one containing the ELF/VLF receiver and nonvolatile (flash) memory boards. Both are hermetically

sealed. Lead wires (6 are shown) stick out of one end of the inner capsule through gas tight seals. This

is important because they have to hold a fairly good vacuum in the space around this capsule. Any

volatile products that outgas from the receiver or its extension wires (or their insulation) will rapidly

raise the pressure in the supposed vacuum space. An increase in pressure will increase the heat loss to

is suspended inside the outer capsule by its own lead wires, aided by some sort of a spider like, low

conductivity structure that is slipped into the system. It should not be pressed tightly in, since that would

the inner container, and also fitted loosely around the inside of the outer pressure vessel. A third layer

of foil should be "built in" to the spider-like positioning and supporting structure. These three layers of

gold will reduce the radiant heat loss to a very low value. If the pressure inside the pressure vessel can be

inside the inner container is not important, hovever it must not leak. To acommodate a small amount of

outgassing a ’getter’ will be mounted in the vacuum space.

The dewar design will be further optimized during the development effort, which Professor

Moffat will participate as a Co-Investigator. Several dewar designs will be tested in the thermal insulation

laboratory facilities of Prof. Moffat’s research group, with an eye on the desire to keep the cost of the

dewar low, so that the total cost of an ultra-low-power ELF/VLF receiver together with antennas, battery,

can be ideally powered by batteries, even when used at an AGO or manned site. For this purpose, we

are to investigate the potential use of batteries produced by Hawker, Inc., which are currently used

in Antarctica. Our preliminary investigation indicates that their Cyclon series of the Hawker batteries

specifications, in the Cyclon series, is 8 A-hr, at 2 V, in an approximately 5 cm diameter, 14 cm high,

rigid plastic container that weighs 840 gm. To run a 10 mW, VLF receiver at 2V, for a year, would require

16 to 20 J-cells in parallel, assuming the batteries were buried in the snow at an ambient temberatureof

inside the battery cluster to provide an a bit of extra thermal protection and simple access.