A MICRO ULTRA LOW POWER RF RADIO FOR NEURAL

SIGNAL RECORDING

Rui Zhong, Richard Liu

Cullen College of Engineering,University of Houston, 4800 Calhoun Road, Houston, Texas, United States

Haleh Fotowat, Fabrizio Gabbiani

Department of Neuroscience, Balor College of Medicine, Houston,Tx, United States

Keywords: Micro Radio, Neural Signal Recording, RF Radio, Ultra Low Power.

Abstract: This paper reports the development of a small, light-weight and mini-battery operated low power radio

transmitter for neural signal recording. This system allows recording of neural signal down to the level of 50

μV. A micromachined electrode is used to obtain these signals. The neural signal is then amplified 10,000

times and transmitted to the receiver at 433 MHz. The manufactured radio system is 7 mm by 6 mm in size

and 0.5 gram in weight, with a power consumption of 750 μW. Radio functionality is tested and verified on

locusts.

1 INTRODUCTION

Wireless recording of neural activity from the

central and peripheral nervous systems has long

been pursued by physiologists as a way to

understand the operation of individual neurons, to

decipher the relationship between the environment

stimuli, nerve system response and the activities

generated by the nerve signal (Akin, 1998). One of

such research area is visually guided escape

behaviors, which are critical to the survival of many

animals.

In this design, we are required to detect the

neural signals from main nerves of a locust.

Therefore a small size, light weight and low power,

cell battery based RF wireless telemetry system is

needed. Unfortunately, the power consumption of

available commercial radio chips is too large to be

sustained by a small cell battery. In this design, we

simplified the radio structure as much as possible to

reduce power consumption. Firstly, a RF BJT

oscillator is used to generate the carrier, instead of a

Frequency Synthesizer. Secondly, AM modulation

scheme is used instead of the typical FSK

modulation in commercial radio chips. Thirdly, the

whole system is analog. At last, a 30 AWG copper

wire with the length of 40mm is used as antenna

with the radio transmitter working at 420 MHz.

After several experiments, it is verified that this

design is small enough to be carried by a locust and

light enough to allow the bug to jump as usual. The

received signal provides strength and time

information of the neural activities.

2 RADIO SYSTEM DESIGN

2.1 System Overview

The radio transmitter is divided into two functional

parts: amplifier stage and transmitter stage. Detected

neural signals are typically in the range of 50 to 150

micro volts. In order to process this signal, a high

gain amplifier stage is used to amplify it 10,000

times. The amplified neural spikes are then sent to

the transmitter stage for transmitting. Direct

coupling is used between these two stages. The

whole system runs on a 3 Volts cell battery and the

power consumption is only 750 μW in total. The

complete system schematic is shown in Figure 1

236

Zhong R., Liu R., Fotowat H. and Gabbiani F. (2008).

A MICRO ULTRA LOW POWER RF RADIO FOR NEURAL SIGNAL RECORDING.

In Proceedings of the First International Conference on Biomedical Electronics and Devices, pages 236-239

DOI: 10.5220/0001055702360239

 SciTePress

Figure 1: System Overview.

2.2 Transmitter Stage

For most neural experiments, researchers are mainly

interested in the when this signal is generated and

how many neural spikes are observed, other than

their accurate amplitude value. Thus, simplifying the

transmitter design has little influence on the research

itself, but it can greatly reduce the power

consumption, and reduce the size and weight of the

system.

In our design, the transmitter is basically a 433

MHz BJT Colpitts oscillator. Colpitts Oscillator is

well known for its simplicity and robustness. Figure

2 is the structure of our transmitter stage, which is a

common-base (CB) Colpitts oscillator. In Figure 2

we can see that C1, C2 and L1 forms the positive

feed back loop, and R1, L2 and R

provide the DC

bias for oscillation. C3 is the capacitor for common

base structure.

Because of its wide application area, Colpitts

oscillator is well studied. The oscillation frequency

of this circuit can be calculated by equation (1).

⎟

⎠

⎞

⎜

⎝

⎛

⋅

(1)

In this Colpitts oscillator application, the

operation frequency is about 433 MHz. Therefore,

several issues need to be taken care of. These issues

will be discussed in the following subsections.

2.2.1 Selection of Inductor L1

Three issues are important when deciding the L1

inductor value.

First, according to equation (1), the value of L1

is directly related to the oscillation frequency. For

433 MHz band operation, it is usually in the nano-

Henry range.

Figure 2: Transmitter Structure.

Second, the value of inductor L1 is also related

to the quality factor of Oscillation. The relation is

expressed in Equation (2).

∑

∝

(2)

Where

⋅

∑

(3)

And

is the serial resistance of L1.

In the oscillation loop, larger quality factor Q

leads to more stable and accurate oscillation and

lower power dissipation. From equation (2), for the

same

∑

C , greater L1 and smaller

will contribute

to a higher system quality factor. This indicates that

we should choose a high Q inductor, whose

A MICRO ULTRA LOW POWER RF RADIO FOR NEURAL SIGNAL RECORDING

237

small. And the L1 to

∑

C ratio should be large

enough.

Finally, up limit does exist for the value of L1.

Increase L1 too much will decrease its self-resonant

frequency and force the corresponding

∑

C decrease

to the BJT’s parasitic capacitance region, which may

cause unstable oscillation (Razavi, 1998).

The selection of L1 value is a compromise

between these issues discussed above.

2.2.2 Selection of C1 and C2

According to the analysis in section 2.2.2, to achieve

a higher quality factor, C1 and C2 should be as small

as possible. On the other hand, however, C1 and C2

can not be too small. Because C1 is in parallel with

the junction capacitance C

and C2 is in parallel

with the junction capacitance C

, Rp is the

equivalent resistance. Its relation to the loop is

shown in equation (4). The equivalent oscillation

loop is shown in Figure 3.

Figure 3: Equivalent Oscillation Loop.

Figure 3 tells us if C1 and C2 are too small, Cce

and Cbe will dominate the oscillation frequency.

Typically, C1 and C2 should be greater than Cce and

Cbe. Therefore,

∑

C should lies inn the range of 1 pf

to 4 pf.

Also, known

∑

C , the value of C1 to C2 directly

depends on their ratio, C1/C2. Again, compromise

exists when selecting this ratio. Smaller C1/C2 ratio

means larger loop quality factor but smaller positive

feedback gain, while larger C1/C2 ratio gives greater

feedback gain but lower quality factor (Razavi,

1998). In practice, experiment shows that 3.3pf and

18pf capacitor pair gives stable oscillation and

makes it easy to start the oscillation. Now, we can

modify equation (1) to obtain practical equation for

oscillation frequency f

)/1( +

(4)

⎟

⎠

⎞

⎜

⎝

⎛

⋅

(5)

Where

⋅

(6)

And

⋅

(7)

2.2.3 Selection of Rf and R1

is a DC negative feedback resistor, which helps

stabilize the transistor current gain. R1 is used to set

up the collector I

. However, R

and R1 are also the

main power consumption components in this circuit.

The power they consume can be estimated by

equation (7) and (8).

fCRf

RIP ∗=

(7)

RIP

∗= (8)

Since I

is usually large, R

should be as small as

possible. And because I

is typical in uA range, thus

is usual less than 50 uW. In practical design, for

the purpose of low power consumption, we

simultaneously decrease R

and increase R1, the

final power consumption of the oscillation is only

750 uW, compared to the typical 10 mW power

consumption for a commercial transmitter, this is a

great advantage.

2.3 Amplifier Stage

Figure 6 is the structure for amplifier stage. It

consists of two JFET operational amplifiers. Each

stage runs on a single power supply and has a gain

of 100. To save power, an OPA with only 40 uA

current consumption at 2V is used. Important design

rules about amplifier stage are exploded below.

2.3.1 Power Line Decoupling

In practical application, the battery has non-zero

internal impedance, which induces AC voltage on

the power line.

If not carefully decoupled, the AC voltage on the

power becomes an input to amplifier stage. This will

cause the amplifier stage self-oscillate. Also, smaller

R20 and R19 (as shown in Fig 4) value may be

necessary to further ensure no self-oscillation will

occur.

BIODEVICES 2008 - International Conference on Biomedical Electronics and Devices

238

Figure 4: Amplifier Stage Structure.

2.3.2 Direct Coupling

In this design, we can use the DC bias component of

the OPA as the power supply voltage V

for the

next transmitter stage. Therefore, a direct coupling is

used between amplifier stage and transmitter stage.

When no neural signal input appears, the output

voltage of the amplifier should be half of the battery

voltage. That is the V

power supply for the next

transmitter stage.

When neural signal is connected to the amplifier

input, the amplified signal of interest will directly

couples to transmitter, that is add to V

3 EXPERIMENT RESULTS

Figure 5 shows the radio installed on a locust. A pair

of micromachined electrode is used to collect neural

signal from the locust. Figure 6 demonstrates the

neural spikes collected and observed at the receiver

side oscilloscope.

Figure 5: Locust with micro radio installed.

Figure 6: a) neural spikes measured on the transmitter

side b) neural spikes observed on the receiver

oscilloscope.

4 CONCLUSIONS

This paper discusses a design scheme of a single

BJT transistor oscillator based RF Radio for neural

signal recording. Comparing to other radio existing,

this design has two important advantages. First, it is

an ultra low power radio, with a power consumption

of only 750 uW, only 7.5% of the available

commercial transceiver chips. This feature makes it

possible to be powered by a tiny watch battery.

Second, this radio is small and light, it is only 7 mm

by 6 mm in size and 0.5 gram in weight. These two

advantages make this radio especially suitable for

wireless recording tasks performed on small

animals, insect, such as locusts and pigeons. The

communication range of this radio is up to 20 feet.

REFERENCES

Akin, T., 1998. A Wireless Implantable Multichannel

Digital Neural Recording System for a

Micromachined Sieve Electrode, Solid-State Circuit,

VOL 33, No 1. IEEE Journal of.

Razavi, B., 1998. RF Microelectronics, Prentice-Hall, Inc.,

New York, 2nd edition.

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