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A BAW based Transceiver used as Wake-Up ReceiverMarkus Dielacher, Josef Prainsack, Martin Flatscher, Rainer Matischek, Thomas Herndl

Infineon Technologies Austria AG, Graz, Austria

Wolfgang Pribyl

Graz University of Technology, Graz, Austria

Abstract

This work presents a BAW based transceiver which can be used as a wake up receiver. The low power consumption can

be achieved by using a high duty cycle. In contrast to typical quartz based systems, the BAW based approach allows for

a high repetition rate of the duty cycle, which is not possible with conventional systems. The sensitivity of the receiver is

-90 dBm at a current consumption of 8 mA in active mode.

1 Wireless Sensor Networks

Research in wireless sensor networks (WSN) has gained a

lot of attention in recent years. One of the most challeng-

ing topics within this area is power consumption. Wire-

less sensor nodes only have very limited energy available.

They are typically powered by small sized batteries, or

even by energy harvesting devices and have a required

lifetime of several years. Typical applications for WSNs

are smart buildings, environment monitoring, or automo-

tive intra-vehicle applications such as tire pressure moni-

toring (TPMS). Power can be saved by switching the sen-sor nodes off when no data transmission or other activity

is ongoing. This is no problem for sensor nodes which are

only transmitting data and do not act as receivers. In this

way, star topology networks can be built with a base sta-

tion which is always on and listening into the channel if

there is any incoming transmission from a remote sensor

node. Such topologies can be used for example in vehicles

where the base station can be powered by the car battery.

The duty cycle d of the transmitting sensor nodes can beexpressed as the active time tact divided by the total timettot

d =tactttot

=tact

tact + tslp(1)

In this way, the power consumption can be reduced to

any desired value just by making the sleep time tslp longenough in relation to the active time tact. The standby-current in sleep mode is the lower limit and whether the

required tslp is tolerable or not depends on the applicationof interest. By introducing a duty cycle in the transmitting

nodes, it is only possible to build star-topology networks

however. In order to build ad-hoc multi-hop networks it

is necessary to reduce the power consumption of receiv-

ing sensor nodes as well, such that they can monitor the

communication channel without draining the battery.

In [6] three types of rendez-vous schemes are presented:

Purely synchronous. All nodes are synchronized in

time and specific time slots are agreed for commu-

nication. The drawbacks of this scheme are that it

is hard to accomplish in a fully ad-hoc network and

that it causes a high communication overhead which

may result in an increased power consumption.

Purely asynchronous. In this rendez-vous scheme,

each sensor node is equipped with a dedicated wake-

up receiver. This wake-up receiver is designed forextremely low power consumption such that it can

be always on and monitor the communication chan-

nel. The wake-up receiver cannot be used for the

main communication however. It supports only very

low data rates and reported designs suffer from a

rather low sensitivity compared to communication

receivers. The sensitivity of wake-up receivers typi-

cally liesin a range from -50 dBm [10, 7] to -72 dBm

[8] at a power consumption between 50 W [8] and

65 W [9]. The sensitivity can be increased by send-

ing a longer preamble and thus achieve a certain cod-

ing gain, but these measures increase the power re-

quired by the transmitter to send the longer pream-ble. Another way of increasing the range of the

wake-up signal is to increase the output power of the

transmitter for a short wake-up pulse. Whether such

an approach is feasible depends on the application

and where more power is available, at the receiver

or at the transmitter.

Pseudo-asynchronous. Sensor nodes establish com-

munication links on demand. [6] presents two dif-

ferent approaches, one where the communication is

triggered by the transmitter, and another one, where

the communication is triggered by the receiver. This

rendez-vous scheme is based on duty-cycled trans-


2/6

mitters as well as receivers. It is shown in [6] that

this rendez-vous scheme can outperform the purely

asynchronous approach.

The scheme presented in this work is a combination of

the purely asynchronous and the pseudo-asynchronous ap-

proach. Like in the pseudo-asynchronous scheme, the re-ceiver is operated in a duty-cycled mode of operation. Un-

like the protocols in [6] however, where the receiver and

transmitter negotiate a time for transmission the presented

architecture does not rely on bidirectional communication

for forwarding or receiving a data packet. Instead the main

receiver is heavily duty-cycled with very short sleep times

such that it acts as a wake-up receiver. This is possible

because the transceiver presented in section 2 uses bulk

acoustic wave (BAW) resonators to generate the carrier

in transmit mode and the local oscillator (LO) in receive

mode. Compared to conventional quartz based oscillators,

the start-up time of a BAW based oscillator is much shorter.

1.1 Calculating the Duty Cycle

In order to reduce the average power consumption, a duty

cycle d has to be introduced. The active time tact consistsof the start-up time tstart of the receiver and the actual re-ception time trx . The total time ttot of one cycle consits oftstart, trx , and the sleep time tslp.

d =tactttot

=tstart + trx

tstart + trx + tslp(2)

The average power consumption Pavg

can be calculated as

Pavg =Pslptslp + Pstarttstart + Prxtrx

tslp + tstart + trx(3)

If the average power Pavg is limited by the energy source,the duty cycle has to be chosen accordingly. Assuming

that Pslp, Pstart, and Prx have been optimized as much aspossible, there is only one remaining variable, which is the

sleep time. The sleep time which is required for a certain

Pavg can be calculated as

tslp = tstart(Pstart Pavg

Pavg Pslp)

+ trx( Prx Pavg

Pavg Pslp)

.

(4)

The resulting duty cycle can be expressed as

d =(tstart + trx) (Pavg Pslp)

tstart(Pstart Pslp) + trx(Prx Pslp). (5)

Once the required duty cycle is known, tact has to be keptas short as possible in order to allow for a high repetition

rate. Figure 1 presents two different modes of operation.

Both of them have the same duty cycle of d = 1/4, butthe repetition rate of the second one is much higher. In

this way the power consumption of the transmitter can be

reduced because the required preamble length tpre which

the transmitter has to send is exactly one period of the re-

ceivers duty cycle ttot.

tpre = tslp + tact = ttot (6)

receive

sleep

receive

sleep sleep sleep sleep

receive

receive

receive

receive

d = 1/4

d = 1/4

sleep

receive

sleep

power[W]

power[W]

time [s]

time [s]

deff = 1/4

deff = 1/4

Figure 1: Duty cycle with different repetition rates

As shown in Equation 4, the required sleep time depends

on tstart and trx . While trx scales with the number ofbytes to be received and the data rate, tstart is fixed andin order to achieve the highest possible repetition rate, it

has to be ensured, that the first term is only a fraction of

the second one. Otherwise the start-up time dominates the

active time as shown in Figure 2. In this case, the duty

cycle d = 1/4 as in Figure 1, but the effective duty cy-cle deff = 1/24. For the transmitter, this means that thepreamble has to be six times longer. Of course tsleep canbe shorter ifPstart < Prx .

receive

start-up

sleep

d = 1/4deff = 1/24

power[W]

time [s]

Figure 2: Duty cycle dominated by start-up

1.2 Lower Limit of the Start-Up Time

Virtually every high performance receiver requires a very

precise frequency reference to generate a LO signal, which

is then used to downconvert a received signal to lower fre-

quency bands. The start-up time of this high precision os-

cillator limits the start-up time of a receiver. The most

common frequency reference is a quartz crystal. Quartz

crystals are available up to frequencies of around 40 MHz

and with quality (Q) factors around 104. As a rule of


3/6

thumb, the start-up time of an oscillator is the product of

its Q and the period of its center frequency fc.

tstart = Q 1

fc(7)

For a quartz oscillator with a resonant frequency of 10 Mhz

and a Q of 10000, this results in

tstart = 10000 1

10MHz= 1ms. (8)

2 BAW based Transceiver

By using a resonator with a higher resonant frequency, the

start-up time of the receiver can be reduced significantly.

Figure 3 shows the schematic of a transceiver which uses

BAW resonators to generate the carrier in transmit mode

and the LO in receive mode [3]. The BAW based oscilla-

tors directly oscillate at the carrier frequency of 2.1 GHz.In contrast to this, typical quartz based systems use a

voltage controlled oscillator (VCO) which derives its fre-

quency accuracy from the quartz in a phase locked loop

(PLL).

PPF

LO

PPF

I

Q

BAW

BAW

Matching

Network

DAC

ANT

LNA

PA

FSKDemod.

Data/Clk

Recovery

Data FIFO

Filter

NCO

MatchedFilter

Digital

BAW BAW IF Filter

Asyn.

SPIInterface

Filter

Analog

ADCRSSI

clock

Temp.Sensor

Mux Frequency

Divider

DigitalBaseband

TX

Figure 3: Block Diagram of the BAW based Transceiver

2.1 BAW device

The applied BAW resonators are so called mirror-type

BAWs or surface mounted resonators (SMR). They consist

of a piezoelectric layer between two electrodes. When the

piezoelectric layer is excited, an acoustic wave is launched

into the bulk, in contrast to a surface acoustic wave (SAW)

resonator, where the wave propagates along the surface.

In order to prevent the waves from propagating further

into the substrate, there is an acoustic mirror underneath

the resonator which reflects the waves. There exist other

BAW technologies, based on a cavity underneath the res-

onator instead of the acoustic mirror but in terms of robust-

ness the mirror-type BAWs are superior to these so called

membrane-type BAWs [11]. In the presented transceiver,

the BAW devices are not only used for frequency gener-

ation. Two resonators are integrated directly into the low

noise amplifier (LNA) for filtering. A detailed description

of the LNA together with the on-chip matching network

containing also the RX/TX switch is given in [2].

2.2 Oscillator

The applied BAW resonators show a temperature drift of

about -18ppm/C. In order to compensate for this drift, the

temperature is measured and the frequency is adjusted by

means of digitally controlled capacitors in parallel to the

resonator. The allowed temperature ranges from -40C to

+125C. Additionally, the oscillator in the transmit path

can be tuned with a variable DC voltage. The DC volt-

age applied at the bottom electrode of the resonator causes

a change in the stiffness of the piezoelectric material, re-

sulting in a frequency shift. The effect is very linear with

voltage, but as its amplitude is only 40 kHz/V, it is onlyused in the transmit path for modulation.

The Q of a BAW resonator is around 1000. Like for the

quartz based oscillator in equation 8, the start up time of

the BAW based oscillator can be estimated as

tstart = 1000 1

2.1GHz< 0.5s. (9)

2.3 Receiver Topology

The receive path of the transceiver is based on an image

reject architecture. The applied BAW-based LO does not

provide quadrature phases because this would require a

higher current consumption. That is why the LNA is fol-

lowed by a polyphase filter in order to generate the quadra-

ture phases required for the image reject architecture in the

signal path. An image reject architecture is a good choice

if a low intermediate frequency (IF) is desired. In a nor-

mal heterodyne architecture the choice of an appropriate

IF bears a trade-off between image rejection and the rejec-

tion of interfering signals which are located close to the

desired channel [5]. With a lower IF it is easier to sep-

arate the desired signal from neighboring interferers after

downconversion. On the other hand a very low IF means

that the image frequency is very close to the desired signalat RF and cannot be filtered sufficiently before the down-

conversion. The LNA in the presented transceiver includes

BAW resonators which can be used as image reject filters

for a low IF of 10.7 MHz in addition to the image reject

architecture. The receiver architecture is described in de-

tail in [1]. The IF signal is fed into a limiting amplifier,

which outputs a time-continuous binary signal. The binary

signal is then sampled and further processing is done in

the digital domain. Besides this binary signal, the limit-

ing amplifier outputs a received signal strength indicator

(RSSI). This RSSI value is available as an analog voltage

which can also be converted to a digital value by an inter-

nal 10 bit ADC. The sensitivity of the presented transceiver


4/6

is -90 dBm at a data rate of 50 kBit/s and a BER of102.The current consumption in receive mode is 8 mA with a

3.3 V supply.

3 Required Extension to the Pre-

sented Transceiver

With the presented architecture, it is possible to use the

main receiver as wake-up receiver by applying a duty-

cycled mode of operation with a high repetition rate. Not

yet implemented in the presented architecture is a low

power real time clock with the purpose of triggering the

duty cycle. The power consumption of such an oscillator

can be estimated to be low enough such that the power con-

sumption in sleep mode remains below 3 W. This value

includes the real time clock, biasing cells, voltage regula-tors and leakage currents.

The wake-up criterion should be evaluated in two stages.

1. The RSSI value can be evaluated very quickly. If it

is above a certain threshold, a carrier is present. If

this is not the case, then the receiver can go back into

sleep mode immediately. The sensitivity of this first

stage can be configured by choosing the threshold

accordingly.

2. If a carrier has been detected, the receiver remainsactive and polls for a run-in sequence. The sensitiv-

ity of this second stage depends on the data-rate of

the transmitted signal, and is a trade-off with power

consumption. With a lower datarate, the sensitiv-

ity of the receiver increases, but on the other hand,

a lower datarate means that both the receiver and

the transmitter have to be active for a longer pe-

riod, which increases the power consumption (or de-

creases the repetition rate).

4 Optimization of Overall Power

Consumption

In order to compare the BAW based approach to a conven-

tional quartz based system, the power consumption of the

system, including the receiver and the transmitter is eval-

uated. Table 4 contains values for the given parameters.

The values concerning the power consumption of the BAW

based system are taken from [3]. In order to allow for a fair

comparison, a quartz based receiver is assumed which has

the same values for Pstart

, Prx

, trx

and Pslp

. Only tstartis different, and set to 1 ms as calculated in equation 8.

Receiver

Pstart 3 [mW]tstart 0.5 [s] (BAW), 1 [ms] (Quartz)Prx 24 [mW]trx 30 [s]Pslp 3 [W]

TransmitterPtx 18 [mW]Pslp 3 [W]

Table 1: Parameters

Figure 4 shows the systems power consumption assuming

one wake-up event per second, plotted versus the repeti-

tion rate of the receiver. With a higher repetition rate, the

power consumption of the transmitter decreases while the

power consumption of the receiver increases. The power

consumption of the receiver increases because for a given

active time as defined by tstart and trx in Table 4, onlytslp can be scaled to achieve a certain repetition rate. Thusthe duty cycle of the receiver is changed, and a higher rep-

etition rate means a higher power consumption. For the

transmitter, the higher repetition rate means that the pream-

ble can be shorter and the power consumption decreases.

When the two power consumptions are added in order to

evaluate the systems power consumption, an optimum can

be found.

0 100 200 300 400 500200

400

600

800

1000

1200

1400

1600

1800

2000

Repetition Rate [Repetitions / Second]

SystemPowerConsumption[W]

BAW based System

Quartz based System

Figure 4: System power consumption vs repetition rate

For very low repetition rates, the overall power consump-

tion is dominated by the transmitter, while the receiver

dominates the power consumption for higher repetition

rates. The optimum of the overall power consumption can

be found where the two power consumptions are equal.

One can observe, that for the quartz based system, the

power consumption increases much faster with a higher

repetition rate than the power consumption of the BAW

based system. This effect is caused by tstart

which be-

comes the dominating factor compared to trx and tslp.


5/6

5 Measurement and Chip Photo

Figure 5 shows the measured start-up time of the oscillator.

From the moment when the enable signal goes to high, it

takes about 1.5 s until the system is stable. This measure-

ment has been performed with the oscillator in the transmit

path because it can easily be measured at the antenna-pin.The start-up time is a little bit higher than expected from

Equation 9, but still a very good value.

0.00 1.00 2.00 3.00 4.00 5.00 6.00

-0.8

transmitteroutput[V]

oscillatorenable[V]

-0.4

0

0.4

0.8

0

1

2

3

4

0 1 2 3 4 5 6

time [s]

start-up = 1.5 s

Figure 5: Start-Up Measurement

Figure 6 shows the measured 10 bit RSSI value versus the

input power.

0

200

400

600

800

1000

1200

-120 -100 -80 -60 -40 -20 0Input Power [dBm]

RSSI

Figure 6: 10 bit RSSI value vs input power

Figure 7 shows a photo of the transceiver ASIC together

with three BAW dies. Each of the BAW dies contains

eight resonators. The used resonators are connected to the

transceiver via wire-bonds. From two of the BAW dies

only one resonator is connected to the transceiver. These

resonators are used in the oscillators which generate the

carrier signal in the transmit path and the LO signal in the

receive path. From the third BAW die two resonators are

connected to the LNA where they are used for filtering.

The size of the transceiver ASIC is 2 mm by 1.5 mm.

Figure 7: Chip Photo

6 Conclusion

It has been shown that it is possible to use communication

receivers as wake-up receivers. By using appropriate duty

cycles the power consumption can be reduced such that it

can be compared to dedicated wake-up receivers. The pre-

sented approach offers two important advantages:

The only additional circuitry which is required is a

low power real time clock.

The sensitivity of the receiver is not impaired, when

it is used as a duty cycled wake-up receiver. So the

presented approach is capable of outperforming ded-icated wake-up receivers which suffer from a very

low sensitivity.

Furthermore it has been shown that the start-up time of the

LO has a major impact on the achievable repetition rate of

the duty cycle. A high repetition rate is required to allow

for a short preamble in the transmitted signal. In this way

the power consumption of the transmitter and the whole

communication system can be optimzed. That is why the

presented BAW based system with its short start-up time is

superior to conventional quartz based transceivers.

This work has been partly funded by the EC FP7 project

CHOSeN and Austrian FIT-IT project SNOPS.

References

[1] M. Dielacher, M. Flatscher, J. Prainsack, R. Matis-

chek, Th. Herndl, and W. Pribyl. Image rejection in a

receiver frontend by means of baw resonators and an

image-reject architecture. e & i Elektrotechnik und

Informationstechnik, (11):408414, November 2009.

[2] Markus Dielacher, Martin Flatscher, and Wolfgang

Pribyl. A low noise amplifier with on-chip matching

network and integrated bulk acoustic wave resonators


6/6

for high image rejection. In Research in Microelec-

tronics and Electronics, 2009. PRIME 2009. Ph.D.,

pages 172175, July 2009.

[3] M. Flatscher, M. Dielacher, T. Herndl, T. Lentsch,

R. Matischek, J. Prainsack, W. Pribyl, H. Theuss,

and W. Weber. A robust wireless sensor node for in-tire-pressure monitoring. Solid-State Circuits Confer-

ence, 2009. ISSCC 2009. Digest of Technical Papers.

IEEE International, pages 286287, Feb. 2009.

[4] M. Flatscher, M. Dielacher, J. Prainsack, R. Matis-

chek, Th. Herndl, Th. Lentsch, and W. Pribyl. A

bulk acoustic wave(BAW)-based sensor node for au-

tomotive wireless sensor networks. e & i Elektrotech-

nik und Informationstechnik, 125(4):143146, April

2008.

[5] Behzad Razavi. RF Microelectronics (Prentice Hall

Communications Engineering and Emerging Tech-

nologies Series). Prentice Hall PTR, 1997.

[6] E.-Y.A. Lin, J.M. Rabaey, and A. Wolisz. Power-

efficient rendez-vous schemes for dense wireless sen-

sor networks. In Communications, 2004 IEEE Inter-

national Conference on, volume 7, pages 37693776

Vol.7, June 2004.

[7] M.S. Durante and S. Mahlknecht. An ultra low

power wakeup receiver for wireless sensor nodes. In

Sensor Technologies and Applications, 2009. SEN-

SORCOMM 09. Third International Conference on,

pages 167170, June 2009.

[8] N. Pletcher, S. Gambini, and J. Rabaey. A 65 W,

1.9 GHz RF to digital baseband wakeup receiver for

wireless sensor nodes. In Custom Integrated CircuitsConference, 2007. CICC 07. IEEE, pages 539542,

Sept. 2007.

[9] N.M. Pletcher, S. Gambini, and J.M. Rabaey. A 2

GHz 52 W wake-up receiver with -72 dBm sensi-

tivity using uncertain-IF architecture. In Solid-State

Circuits Conference, 2008. ISSCC 2008. Digest of

Technical Papers. IEEE International, pages 524

633, Feb. 2008.

[10] S. von der Mark and G. Boeck. Ultra low power

wakeup detector for sensor networks. In Microwave

and Optoelectronics Conference, 2007. IMOC 2007.

SBMO/IEEE MTT-S International, pages 865868,

29 2007-Nov. 1 2007.

[11] R. Aigner, High performance RF-filters suitable for

above ic integration: film bulk-acoustic- resonators

(FBAR) on silicon, in Custom Integrated Circuits

Conference, 2003. Proceedings of the IEEE 2003,

Sept. 2003, pp. 141146.

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