Method and device for detecting a vital sign carrying signal using a phase-locked loop

Abstract

A method of detecting a vital sign comprising at least one of a heart rate and a respiratory rate of a subject is provided. In one aspect, the method includes transmitting a radio frequency signal towards the subject; and receiving a reflected signal from the subject, wherein the transmitted signal is reflected by the subject and Doppler-shifted due to at least one of the heart rate and the respiratory rate to form the reflected signal. The method also includes mixing the reflected signal with a first reference signal; and providing a vital sign carrying signal based on the mixing to a first input of a phase or frequency comparator. The method further includes generating an adjustable second reference signal and providing the reference signal to a second input of the phase or frequency comparator; and generating an output signal, by the phase or frequency comparator. The method includes varying at least one of a phase and a frequency of the adjustable second reference signal based on the output signal to track a phase or frequency of the vital sign carrying signal.

Claims

1. A device for detecting a vital sign comprising at least one of a heart rate and a respiratory rate of a subject, comprising: a transmitter arranged to wirelessly transmit a radio frequency signal towards the subject, wherein the radio frequency signal transmitted towards the subject is a fixed-frequency signal; a receiver arranged to receive a reflected signal from the subject, wherein the transmitted signal is reflected by the subject and Doppler-shifted due to at least one of the heart rate and the respiratory rate to form the reflected signal; a mixer for mixing the reflected signal with a first reference signal so as to provide an intermediate frequency signal; and a signal processing circuitry comprising a phase or frequency comparator and a reference signal generator and that is configured to implement a phase-locked loop (PLL) and to receive the intermediate frequency signal, wherein the phase or frequency comparator is configured to receive a vital sign carrying signal based on the mixing of the reflected signal with the first reference signal on a first input and receive an adjustable second reference signal from the reference signal generator on a second input of the phase or frequency comparator, the phase or frequency comparator further configured to generate an output signal based on the vital sign carrying signal and the second reference signal, and wherein the reference signal generator is configured to vary at least one of a phase and a frequency of the adjustable second reference signal based on the output signal of the phase or frequency comparator to track a phase or frequency of the vital sign carrying signal and wherein the transmitter and receiver are configured to operate as a Doppler radar.

2. The device according to claim 1, wherein the signal processing circuitry is implemented in a digital signal processor.

3. The device according to claim 2, further comprising a vital sign estimator configured to receive an output from the digital signal processor and determine at least one of a heart rate or a respiratory rate based on the output from the digital signal processor.

4. The device according to claim 1, wherein the PLL further comprises an integrator and functions as a phase demodulator.

5. The device according to claim 3, wherein the vital sign estimator is further configured to estimate a magnitude of a tissue displacement due to at least one of heart rate and respiratory rate by determining an amplitude of a frequency component of the output from the digital signal processor.

6. The device according to claim 3, wherein the vital sign estimator is configured to perform a frequency analysis.

7. The device according to claim 1, wherein the PLL is configured to function as a demodulator.

8. The device according to claim 1, wherein a frequency of the first reference signal is different from a frequency of the transmitted signal.

9. The device according to claim 1, wherein the intermediate frequency signal output by mixing of the reflected signal with the first reference signal is converted into the digital domain.

10. The device according to claim 9, wherein the phase or frequency comparator and the reference signal generator operate in the digital domain, and wherein the reference signal generator comprises a numerically controlled oscillator.

11. The device according to claim 1, wherein the received reflected signal is divided into an inphase and a quadrature component.

12. The device according to claim 11, wherein the vital sign carrying signal is a complex form combination of the inphase and the quadrature components.

13. The device according to claim 1, wherein the transmitted signal is generated by a first phase-locked loop and the first reference signal is generated by a second phase-locked loop, wherein the first phase-locked loop and the second phase-locked loop use the same reference clock.

14. The device according to claim 1, wherein the transmitted signal is generated by mixing a signal from a first oscillator and a second oscillator, and wherein the first reference signal is generated by the second oscillator.

15. The device according to claim 1, further comprising a vital sign estimator to determine at least one of a heart rate and a respiratory rate by performing a frequency analysis of a signal based on the output signal of the phase or frequency comparator.

16. The device according to claim 1, wherein a frequency of the vital sign carrying signal is tracked.

17. The device according to claim 1, further comprising an integrator so that a phase of the vital sign carrying signal is tracked and the output signal from the phase or frequency comparator is integrated.

18. The device according to claim 17, wherein the integrator estimates a magnitude of tissue motion based on heart or respiratory action of the subject by determining an amplitude of a frequency component of the integrated output.

19. A method of detecting a vital sign comprising at least one of a heart rate and a respiratory rate of a subject, the method comprising: transmitting a radio frequency signal towards the subject; receiving a reflected signal from the subject, wherein the transmitted signal is reflected by the subject and Doppler-shifted due to at least one of the heart rate and the respiratory rate to form the reflected signal; mixing the reflected signal with a first reference signal; providing a vital sign carrying signal based on the mixing of the reflected signal with the first reference signal to a first input of a phase or frequency comparator; generating an adjustable second reference signal by a reference signal generator and providing the adjustable second reference signal to a second input of the phase or frequency comparator; generating an output signal by the phase or frequency comparator based on the vital sign carrying signal and the adjustable second reference signal; and varying by the reference signal generator at least one of a phase and a frequency of the adjustable second reference signal based on the output signal of the phase or frequency comparator to track a phase or frequency of the vital sign carrying signal, the method using the device according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above, as well as additional objects, features and advantages of the disclosed technology, will be better understood through the following illustrative and non-limiting embodiments of the disclosed technology, with reference to the appended drawings. In the drawings like reference numerals will be used for like elements unless stated otherwise.

(2) FIG. 1 is a schematic block diagram of a first device which may be used for determining a heart rate and/or a respiratory rate of a subject.

(3) FIG. 2 is a schematic block diagram of a second device which may be used for determining a heart rate and/or a respiratory rate of a subject.

(4) FIG. 3 is a schematic block diagram of a third device which may be used for determining a heart rate and/or a respiratory rate of a subject.

(5) FIG. 4 is a schematic block diagram of a fourth device which may be used for determining a heart rate and/or a respiratory rate of a subject.

(6) FIG. 5 is a schematic block diagram of a fifth device which may be used for determining a heart rate and/or a respiratory rate of a subject.

(7) FIG. 6 is a schematic block diagram of a sixth device which may be used for determining a heart rate and/or a respiratory rate of a subject.

(8) FIG. 7 is a schematic block diagram of a seventh device which may be used for determining a heart rate and/or a respiratory rate of a subject.

(9) FIG. 8 is a schematic block diagram of an eighth device which may be used for determining a heart rate and/or a respiratory rate of a subject.

(10) FIG. 9 is a flowchart of a method according to an embodiment.

DETAILED DESCRIPTION OF CERTAIN ILLUSTRATIVE EMBODIMENTS

(11) FIG. 1 is a schematic block diagram of a first device 100 which may be used for detecting and determining a heart rate and/or a respiratory rate of a subject. The subject may be a human, however the disclosed technology is equally applicable to other mammal or animal subjects. The subject may in the following also be referred to as the target.

(12) The device 100 includes a transmitter 110 arranged to transmit a radio frequency signal T(t) towards the subject. The signal T(t) is generated by a signal generator 136 which will be described in detail below. The signal T(t) is transmitted towards the subject via a transmitter antenna 112. The transmitter 110 may optionally include an amplifier for amplifying the signal generated by the signal generator 136 prior to transmission by the transmitter antenna 112. The transmitter 110 may, for example, be arranged to generate the signal T(t) with a frequency in the range of 300 MHz to 300 GHz.

(13) The device 100 further includes a receiver 120 arranged to receive a radio frequency signal R(t) resulting from a reflection of the transmitted signal T(t) by the subject. The reflected signal R(t) may be received via a receiver antenna 122. The receiver 120 may optionally include an amplifier 124 for amplifying the received signal R(t) prior to demodulation thereof.

(14) Each one of the transmitter antenna 112 and the receiver antenna 122 may for instance be arranged as a patch antenna, a beamforming antenna or a horn antenna.

(15) For the purpose of detecting the heart rate and/or respiratory rate the transmitter 110 is advantageously oriented such that the transmitted signal T(t) is directed towards a chest region of the subject. Correspondingly, the receiver 120 is advantageously oriented such as to receive the reflected signal R(t) from the chest region of the subject.

(16) The heartbeat and the respiration of the subject cause a respective periodic motion or displacement of the tissue in the chest region of the subject. Assuming that the subject is facing in the direction of the transmitter 110 and the receiver 120, the tissue in the chest region of the subject will, due to the heartbeat and respiration, exhibit a time-varying displacement along the direction of propagation of the transmitted signal T(t) and the reflected signal R(t). Upon reflection of the transmitted signal T(t) the displacement will result in a time-varying Doppler-shift when the reflected signal R(t) is formed. In other words, the heartbeat and the respiration of the subject will result in a modulation (which may be expressed as a time-varying frequency or phase shift) of the reflected signal R(t).

(17) The device 100 further comprises a first reference signal generator 134, which will be described in detail below. The first reference signal generator 134 may provide a first reference signal, which may hereinafter also be called a local oscillator (LO) signal.

(18) The reflected signal R(t) and the local oscillator signal LO(t) may be provided to a mixer 132, which may down-convert the reflected signal R(t) to an intermediate frequency f.sub.IF, which is the difference between a transmitted frequency f.sub.t and a reference frequency f.sub.r of the local oscillator signal. The mixing of the reflected signal R(t) and the local oscillator signal LO(t) may thus provide an output signal to a signal processing circuitry 140 for analyzing the reflected signal.

(19) The mixer may be, for instance, a diode mixer, a diode ring mixer, a switching mixer, a Gilbert cell mixer or some other type of frequency-conversion mixer. The mixer may be a balanced or double-balanced mixer.

(20) In order to demodulate or extract the phase and/or frequency modulation of the reflected signal R(t), caused by the displacement of the tissue, the device 100 employs a signal processing circuitry 140 which implements a phase-locked loop (PLL). The PLL may operate in a phase-demodulator or frequency-demodulator configuration.

(21) The first device 100 implements the PLL in digital domain and the signal processing circuitry 140 may thus be provided as a digital signal processor (DSP). The DSP 140 may comprise a lowpass filter 142 for passing only the down-converted baseband signal B(t) from the mixer 132. The DSP 140 may further comprise an analog-to-digital converter (ADC) 144 for converting the analog signal to digital domain forming a vital sign carrying signal B[n]. Alternatively, the lowpass filter 142 and the ADC 144 may be external to the DSP 140 and the DSP 140 may receive the vital sign carrying signal B[n] at an input of the DSP 140.

(22) The PLL includes a phase or frequency comparator 146. The phase or frequency comparator 146 includes a first input and a second input. The phase or frequency comparator 146 may receive the vital sign carrying signal B[n] on the first input and an adjustable second reference signal F[n] on the second input. The phase or frequency comparator 146 is arranged to provide an output signal V.sub.e[n] which is indicative of a phase difference between the vital sign carrying signal B[n] and the adjustable reference signal F[n].

(23) The PLL further includes a loop filter 148, which may receive the output signal from the phase or frequency comparator 146 and may be arranged to extract only the difference of the signals on the first and second inputs to the phase or frequency comparator 146 (and hence remove any other combinations of these signals).

(24) The PLL may further include an integrator 150, which may receive the lowpass filtered signal. If an integrator 150 is used, the phase of the vital sign carrying signal will be tracked and the PLL will be operated in a phase-demodulator configuration. A phase-demodulator configuration allows determining a displacement of the target, as phase and displacement are related by a constant which depends on the transmitted frequency and sensitivity of a generator of the adjustable second reference signal. Thus, the use of an integrator 150 in the PLL may allow estimating a magnitude of tissue motion based on heart or respiratory action of the target based on the determined phase.

(25) The PLL further includes a numerically controlled oscillator (NCO) 152. The NCO 152 implements a reference signal generator and provides an adjustable second reference signal F[n] as output, which is provided on the second input of the phase or frequency comparator 146. The NCO 152 thus receives a control voltage V.sub.c[n] and produces a feedback signal F[n] accordingly.

(26) However, the integrator 150 of the PLL may also be omitted, whereby the lowpass filtered signal may be directly passed to the NCO 152. This implies that the frequency of the vital sign carrying signal will be tracked instead and the PLL will be operated in a frequency-demodulator configuration.

(27) As will be understood by the person skilled in the art, the DSP 140 may be implemented in a number of different manners. The DSP 140 may be implemented in hardware, or as any combination of software and hardware. The DSP 140 may for instance be implemented as software being executed on a general-purpose computer, as firmware arranged, for example, in an embedded system, or as a specifically designed processing unit, such as an Application-Specific Integrated Circuit (ASIC) or a Field-Programmable Gate Array (FPGA), In a particular embodiment, the DSP 140 may be implemented as software providing algorithms for performing operations of the phase or frequency comparator 146, the loop filter 148, the integrator 150 and the NCO 152.

(28) The implementation of the DSP 140 in software provides a high flexibility of the device 100, as the software may be easily changed. For instance, a gain of the phase or frequency comparator 146 may be changed to alter characteristics of the device 100 and adapt the device 100 to a changed set-up of detecting a vital sign.

(29) For the purpose of detection of heart rate or respiration rate an upper limit of the frequency range of main interest may be 10-20 Hz. Accordingly, the loop filter 148 may be adapted to suppress frequencies above 10-20 Hz (i.e., by providing a cut off frequency falling in the range 10-20 Hz). However, for the purpose of obtaining a loop characteristic of the PLL such that the PLL may reliably track the frequency/phase shift of R(t) due to tissue displacement caused by the heartbeats and/or respiration, the loop filter 148 may be adapted to suppress frequencies above a threshold frequency in the range of one or a few kHz to one or two MHz (i.e., by providing a cut off frequency falling in the range 1 kHz to 2 MHz).

(30) It should be noted that the phase or frequency comparator 146 may produce new signals at the sum and difference of the phases (frequencies) present at the first and second inputs. Since the two inputs are at the same frequency when the loop is locked, there is one output at twice the input frequency and one output proportional to a cosine of the phase difference. The loop filter 148 may also be arranged to remove the doubled frequency component.

(31) However, if the vital sign carrying signal is provided at an intermediate frequency f.sub.IF=0 (f.sub.t=f.sub.r), it may not be possible to separate the sum and differences of the phases and therefore distortions to the vital sign detection may be generated. Thus, the transmitted frequency f.sub.t and a reference frequency f.sub.r should differ in order to enable detection of the vital signs. The transmitted frequency f.sub.t and a reference frequency f.sub.r may be chosen such that the output signals (sum and difference of the reflected signal and the first reference signal) from the mixer 132 may be easily separated. Also, the transmitted frequency f.sub.t and a reference frequency f.sub.r may be chosen such that the difference is sufficiently high to avoid mixer flicker noise.

(32) As shown in FIG. 1, the device 100 may comprise an oscillator 116, which may provide a reference clock signal. The reference clock signal may be provided as input to the signal generator 136 and may drive the forming of the radio frequency signal T(t) by the transmitter 110. The signal generator 136 may be implemented as a phase-locked loop, which receives the reference clock signal and input of a transmit frequency f.sub.t and generates a continuous wave radio frequency signal having the transmit frequency f.sub.t.

(33) The reference clock signal from the oscillator 116 may also be provided as input to the first reference signal generator 134. Similar to the signal generator 136, the first reference signal generator 134 may be implemented as a phase-locked loop, which receives the reference clock signal and input of a reference frequency f.sub.r and generates a continuous wave radio frequency signal LO(t) having the reference frequency f.sub.r.

(34) The reflected signal R(t) is based on the transmitted signal T(t). Since the transmitted signal T(t) and the first reference signal LO(t) are formed on basis of input from the same oscillator 116, the reflected signal R(t) and the first reference signal LO(t) are partially correlated and residual phase noise may therefore insignificantly influence the vital sign carrying signal B[n].

(35) In use of the device 100 for detecting heart rate and/or respiratory rate of a subject, a radio frequency signal T(t) is transmitted by the transmitter 110 towards the chest region of the subject. The transmitted signal T(t) is reflected by tissue of the chest region of the subject. The reflected signal R(t) is received by the receiver 120. As described above, the reflected signal R(t) will be modulated by the time-varying displacement of the tissue emitting the reflected signal R(t).

(36) The total tissue displacement x(t) due to heartbeat and respiration of the subject may be expressed as:
x(t)=x.sub.l(t)+x.sub.h(t)=X.sub.l sin(2f.sub.lt)+X.sub.h sin(2f.sub.ht)(Equation 1)
where x.sub.l(t) and x.sub.h(t) indicate respectively the mechanical displacements produced by the respiration and the heart. As shown in Equation 1, x.sub.l(t) and x.sub.h(t) may be approximated as periodic functions, where X.sub.l and X.sub.h are the maximum mechanical displacements caused by the expansion and contraction of the lungs and the heart and f.sub.l and f.sub.h are the vital signs frequencies which represent information that may be desired to be determined. X.sub.l and X.sub.h may for instance on average be about 0.5-10 mm and 0.05-0.1 mm, respectively, for an adult. Depending on the subject and on the health condition, f.sub.l and f.sub.h generally are within 0.1-3 Hz. These ranges however only represent non-limiting examples and the system 100 is usable for detection of heart rate and/or respiratory rate in even broader ranges of tissue displacement amplitudes and frequencies. It should also be noted that the above approximation is only provided as an example to facilitate understanding of the principles of the disclosed technology and the disclosed technology is not dependent on a particular choice of approximation.

(37) The reflected signal R(t) is (subsequent to the mixing with the first reference signal) provided to the first input of the phase or frequency comparator 146 where B[n] is compared to the feedback signal F[n] provided to the second input of the phase or frequency comparator 146. As described above, the feedback signal F[n] tracks the phase of the downconverted reflected signal R(t). Therefore, the output signal V.sub.e[n] of the phase or frequency comparator 146 becomes proportional to the modulations induced by the time-varying displacement of the tissue.

(38) The vital sign carrying signal B[n] can be expressed as:

(39) $\begin{array}{cc}B\left[n\right]=C\left(t_n\right)\cos \left[2f_{\mathrm{IF}}{t}_n+\frac{4x\left(t_n\right)}{}+\frac{4d_0}{}++\left(t_n\right)\right]& \left(\mathrm{Equation}2\right)\end{array}$
where n is the n.sup.th sample acquired at a sampling instant t.sub.n=n/f.sub.s, where f.sub.s is a sampling frequency, C(t.sub.n) is the voltage amplitude modulated by the target motion, d.sub.0 is a mean distance between the antennas 112, 122 and the target, A is the wavelength of f.sub.t, takes into account phase shift at the target surface and the phase offsets between radio blocks and is normally fixed, while (t.sub.n) is residual phase noise which may be insignificant and neglected, for example since LO(t) and R(t) are partially correlated, as explained above.

(40) When the lock condition of the PLL is satisfied, the feedback signal can be expressed as:
F[n]=sin(2f.sub.IFt.sub.n+.sub.tune[n])(Equation 3)
where .sub.tune[n]=K.sub.NCO*V.sub.c[n] is the phase change necessary to track the phase modulation produced by the target and K.sub.NCO is sensitivity of the NCO 152 (rad/V). When the lock condition of the PLL is achieved, the phase of the feedback signal F[n] equals the phase of the vital sign carrying signal B[n], which may be expressed as:

(41) $\begin{array}{cc}B\left[n\right]=2f_{\mathrm{IF}}{t}_n+\frac{4x\left(t_n\right)}{}+\frac{4d_0}{}+=F\left[n\right]=2f_{\mathrm{IF}}{t}_n+{}_{\mathrm{tune}}\left[n\right].& \left(\mathrm{Equation}4\right)\end{array}$

(42) Thus, .sub.tune[n] is a copy of the phase of the vital sign carrying signal B[n] and the time-varying displacement of tissue may be extracted accurately from the output signal V.sub.e[n] of the phase or frequency comparator 146. The output signal V.sub.e[n] may be filtered by the loop filter 148 to form a filtered signal V.sub.LPF[n] wherein the doubled frequency component is removed so as to further facilitate extraction of the time-varying displacement of tissue.

(43) A derivative of the phase may be expressed as:

(44) $\begin{array}{cc}\frac{{}_{\mathrm{tune}}\left[n\right]-{}_{\mathrm{tune}}\left[n-1\right]}{\frac{2}{fs}}=\frac{2}{}\frac{x\left(t_n\right)-x\left(t_{n-1}\right)}{\frac{1}{fs}}=f_{\mathrm{tune}}\left[n\right].& \left(\mathrm{Equation}5\right)\end{array}$

(45) The expression in Equation 5 shows that f.sub.tune[n] is proportional to a frequency of B[n] by which speed information of the target may be estimated.

(46) The characterization of the phase change signal .sub.tune[n] in Equation 4 is valid on a condition that the position of the subject is fixed in relation to the device 100. However, a main advantage of the device 100 is that it may be used for detecting the heart rate and/or respiration rate even in a non-idealized scenario wherein the distance between the subject/target (also chest wall) and the system 100 is not fixed. This may be understood by considering the effect of a step change of the distance between the subject and the device 100. A step change of the distance will result in a step change of the phase difference between the vital sign carrying signal B[n] and the second reference signal F[n]. The PLL of the DSP 140 will respond to the step change of the phase difference by changing the phase of the first reference signal F[n] to track the phase difference. After a transient (the duration of which is determined by the dynamics of the PLL) the PLL of the DSP 140 will reacquire a lock wherein the reference signal F[n] will catch up/fall back with the phase of the reflected signal R(t).

(47) The static target distance d.sub.0 will produce a DC level on top of which there is a modulation based on vital signs. This implies that in a steady state, the phase modulation based on vital signs may be extracted as being centered around the fixed phase shift. If the target changes position, the PLL can track this change and the vital signs will be centered to a new DC level due to the new target position.

(48) As may be understood, this discussion is equally applicable to other sources for static and semi-static phase offsets, such as radio block delay. The PLL of the DSP 140 will hence force the receiver 120 to operate at its optimum point, which corresponds to the point where a phase difference between the vital sign carrying signal B[n] and the feedback signal F[n] is relatively small, wherein the signal level of the low-frequency components of the output signal V.sub.e[n] will be close to zero.

(49) In the event that the chest region of the subject undergoes a periodic movement along the direction of propagation of the transmitted signal T(t) and the reflected signal R(t), it follows from the above that the signal .sub.tune[n] may be expressed as:

(50) $\begin{array}{cc}{}_{\mathrm{tune}}\left[n\right]=\frac{4x_h\left(t_n\right)}{}+\frac{4x_l\left(t_n\right)}{}+\frac{4d\left(t_n\right)}{}+& \left(\mathrm{Equation}6\right)\end{array}$
where the additional term d(t.sub.n) represents a variation of the target distance about the mean distance d.sub.0, wherein the variation is not due to the vital signs. This variation may be periodic or may be any arbitrary movement, which need not be centered to a specific position. Arbitrary movements may be handled as a transient as discussed above in relation to shifting of the target distance d.sub.0. A period variation may also be handled and may be expressed as:
d(t)=X.sub.s sin(2f.sub.st)(Equation 7)
where X.sub.s represents the maximum amplitude of the periodic variation of the subject distance and f.sub.s represents the frequency of the variation. Since the signal .sub.tune[n] is free from any cross terms between the heart rate, the respiration rate and the periodically varying target distance, the respective frequencies of the vital signs may be readily distinguished and extracted even in the presence of periodic subject movements. Provided a frequency of the periodic subject movements falls outside the typical range of frequencies of the heart rate and the respiration rate, the signal contribution due to periodic subject movements may even be removed from .sub.tune[n] by filtering based on a priori knowledge of standard ranges of the heart rate and the respiration rate. The filtering may be arranged to pass frequencies in each of the standard ranges of the heart rate and the respiration rate to also enable removing a frequency of a periodic variation therebetween.

(51) The output signal V.sub.e[n] of the phase or frequency comparator 146 represents the frequency variations (i.e., the frequency modulation) of the vital sign carrying signal B[n] resulting from the tissue displacement due to the heart beat and respiration (and other periodic chest region movement), if any. The corresponding phase variations (i.e., the phase modulation) may be directly extracted from the phase change signal .sub.tune[n] generated by the NCO 152. The phase variations are also represented by the integrated signal V.sub.c[n], which is proportional to the phase variations through K.sub.NCO, and hence phase variations may also be extracted from the integrated signal. This may be used in analog implementations of the PLL, as discussed below, where the phase change signal .sub.tune[n] is not directly available to be extracted.

(52) The phase information allows demodulating phase of the vital sign carrying signal so that instantaneous displacement of the target may be determined. The phase information may be available in the phase change signal .sub.tune[n] of the DSP 140, even if the PLL is set to be frequency-demodulating. However, in analog implementations of the PLL, as discussed below, if no integrator 150 is included in the PLL, the vital sign carrying signal may be frequency demodulated allowing estimating only a frequency of the heart or respiratory action.

(53) The device 100 allows extracting linearly the phase modulation produced by the target. Thus, a magnitude of tissue motion based on heart or respiratory action and/or a heart rate or respiratory rate may be determined based on the signal processing performed by the device 100. The device 100 enables the extraction of the phase modulation while avoiding the null point and small angle approximation issues.

(54) This may also be confirmed by considering the situation when the lock condition of the PLL is satisfied. Then the phase difference .sub.e[n] (or the frequency difference) between B[n] and F[n] is such that output of the phase or frequency comparator 146 is essentially zero. This means that the feedback signal F[n] forces the phase or frequency comparator 146 to operate in a linear region, where it is possible to consider sin(.sub.e).sub.e.

(55) For the purpose of detecting the magnitude of tissue motion, the heart rate and/or the respiratory rate, the device 100 may further include a vital sign estimator 160. The vital sign estimator 160 may be arranged to receive the phase change signal .sub.tune[n], the filtered output signal V.sub.LPF[n] and/or the integrated signal V.sub.c[n] as input. The vital sign estimator 160 may be implemented as a separate processing unit receiving input from the DSP 140 or may be integrated in a common processing unit that may perform the operations of the DSP 140 and the vital sign estimator 160.

(56) The vital sign estimator 160 may be arranged to determine or estimate the heart rate and/or the respiratory rate by performing a frequency analysis of the signal V.sub.LPF[n] or V.sub.c[n]. The frequency analysis may include determining a frequency of at least one frequency component of the signal V.sub.LPF[n] or V.sub.c[n] within a given frequency interval. The frequency component(s) may be respective frequency components of the signal V.sub.LPF[n] or V.sub.c[n] which fall within the given frequency interval and which have a respective amplitude which exceeds a threshold level. The frequency interval may correspond to an expected frequency range of the vital sign(s) to be determined, i.e., the heart rate and/or the respiratory rate. The frequency interval may for example be 0.1-3 Hz. The threshold level may be set such that the influence of noise is minimized without reducing the sensitivity of the measurement too much. The vital sign estimator 160 may output the determined frequency/frequencies as an estimate of the heart rate and/or respiratory rate. The vital sign estimator 160 may identify the component of the two components having the lowest frequency as the respiration rate and the other component as the heart rate. The output may for example be presented on a display connected to the device 100 or stored in a storage device for further analysis and post-processing. The vital sign estimator 160 may further be arranged to estimate a magnitude of a tissue displacement due to at least one of the heart rate and the respiratory rate by determining an amplitude of a frequency component of the integrated output.

(57) It should also be realized that the vital sign estimator 160 may be arranged to analyze the signals in other manners. For instance, the analysis may be performed in time domain by using filters to separate the frequencies f.sub.l and f.sub.h and then determine the time domain signals to estimate the vital signs.

(58) In a more basic implementation the vital sign estimator 160 may simply be adapted to detect whether a heart rate and/or a respiratory rate is present, for example by determining if the frequency interval includes any component(s) of an amplitude exceeding a (respective) threshold level. The vital sign estimator 160 may accordingly output a signal indicating whether such components were detected or not.

(59) Referring now to FIGS. 2-8, several different embodiments of a device for detecting a vital sign will be described. The devices of these embodiments have many features in common with the device 100 shown in FIG. 1, and in the following description, mainly differing features will be described.

(60) In FIG. 2, an analog device 200 is shown. Thus, the device 200 is very similar to the device 100 shown in FIG. 1. However, instead of using a DSP 140, an analog signal processing circuitry 240 is used for implementing a phase-demodulating circuitry that receives the down-converted reflected signal R(t) from the mixer 132.

(61) The analog signal processing circuitry 240 comprises analog components for implementing a PLL. Thus, the analog signal processing circuitry 240 comprises a lowpass filter 242 passing the downconverted vital sign carrying signal B(t) to a phase or frequency comparator 246. The phase or frequency comparator 246 may be implemented as a mixer. However other types of phase or frequency comparators 246 may also be used such as a phase-frequency detector, a charge-pump phase detector or an exclusive-OR type of phase comparator.

(62) The signal processing circuitry 240 further comprises a lowpass filter 248, an integrator 250 and an adjustable reference signal generator 252. The adjustable reference signal generator 252 may be formed as a PLL, which is connected to the oscillator 116 (so that residual phase noise may still be neglected). The PLL 252 receives a control voltage V.sub.c(t) from the integrator 250 and produces a phase-modulated feedback signal F(t) accordingly, which is then provided at the second input of the phase or frequency comparator 246.

(63) The lowpass filtered signal of the output signal V.sub.LPF(t) and the integrated signal V.sub.c(t) may be output respectively from the signal processing circuitry 240 in order to allow extracting vital signs from the output signals. A vital sign estimator 260 may then be implemented in a processing unit, whereby the signals may first be analog-to-digital converted, or the vital sign estimator 260 may be implemented in analog circuitry for determining vital signs.

(64) In FIG. 3, a similar analog device 300 is shown, wherein the signal processing circuitry 340 is very similar to the device 200 in FIG. 2. However, the device 300 is arranged to generate signals in a different manner compared to the devices 100, 200 discussed above.

(65) The device 300 comprises a first local oscillator 316 and a second local oscillator 326. The transmitter 320 comprises a mixer 318, which receives a signal from the first local oscillator 316 on a first input and a signal from the second local oscillator 326 on a second input. The mixer 318 may then produce a signal which comprises combinations of the frequencies of the signals from the first and second local oscillators 316, 326. The signal from the mixer 318 may be further passed through a bandpass filter 319 for selecting a desired frequency output by the mixer 318. The bandpass filter 319 could also be a lowpass filter or a highpass filter. Thus, a single-tone continuous-wave radio frequency signal T(t) may be passed to the transmitter antenna 112.

(66) Further, the first reference signal generator may be implemented by the second local oscillator 326. The second local oscillator 326 may thus provide the local oscillator signal LO(t) which may be provided to the mixer 132 for down-converting the reflected signal R(t). Since the second local oscillator 326 is used both in generating the local oscillator signal LO(t) and in generating the transmitted signal T(t), a simple set-up may be used.

(67) The signal processing circuitry 340 of the device 300 corresponds to the signal processing circuitry 240 of the device 200, except that the adjustable reference signal generator 352 now receives a signal from the first local oscillator 316 as input for generating the phase-modulated feedback signal F(t). Thus, the first local oscillator 316 may be re-used and provide a signal which may be used both in generating the feedback signal F(t) and in generating the transmitted signal T(t).

(68) It should be realized that the manner of generating the transmitted signal T(t) and the first reference signal LO(t) shown in FIG. 3 may also be employed in the device 100 shown in FIG. 1.

(69) In FIG. 4, another analog device 400 is shown. The device 400 corresponds to the device 200 of FIG. 2, except that the signal processing circuitry 440 is arranged for tracking a frequency of the vital sign carrying signal B(t) instead of tracking the phase. Thus, there is no integrator arranged in the signal processing circuitry 440 and the adjustable reference signal generator 452 is arranged to provide a frequency-modulated feedback signal F(t). An integrator 450 may be arranged at an output of the signal processing circuitry 440 for integrating the output signal and providing corresponding phase variations to the frequency variations of the output signal V.sub.e(t).

(70) Similarly, in FIG. 5, yet another analog device 500 is shown. The device 500 corresponds to the device 300 of FIG. 3, except that the signal processing circuitry 540 is arranged for tracking a frequency of the vital sign carrying signal B(t) instead of tracking the phase. Thus, similar to the device 400, there is no integrator in the signal processing circuitry 540 and the adjustable reference signal generator 552 is arranged to provide a frequency-modulated feedback signal F(t). An integrator 550 may be arranged at an output of the signal processing circuitry 540.

(71) Referring now to FIGS. 6-8, quadrature architectures of the device are shown. The quadrature architectures provide a similar approach to demodulating information as shown in the devices of FIGS. 1-5. Thus, in comparison to prior art vital sign detection using quadrature architecture, the device does not need to use phase unwrapping algorithms or a differentiate and crossmultiply algorithm.

(72) With the quadrature architecture, the reflected signal can be downconverted in baseband (f.sub.IF=0 Hz), as it is possible to separate the sum and difference of the phases because the inphase and quadrature channels are combined in a complex form.

(73) Thus, in FIG. 6, a device 600 is shown, wherein a local oscillator 616 is used for generating the transmitted signal T(t). The local oscillator 616 is also used for generating a first reference signal.

(74) The received signal is divided into two channels for forming an inphase and a quadrature signal. Then, a first mixer 632a mixes the received signal with the first reference signal from the local oscillator 616 so as to form an inphase signal I. A second mixer 632b mixes the received signal with a first reference signal based on a 90 shifting of the signal from the local oscillator 616 so as to form a quadrature signal Q.

(75) The device 600 may further comprise a signal processing circuitry 640, which may be implemented in digital domain. The signal processing circuitry may comprise a combiner 644, which receives the inphase signal I and quadrature signal Q after mixing and forms a complex form digital signal I+jQ. The digital signal processing circuitry 640 may further comprise a digital implementation of a PLL corresponding to the digital signal processing circuitry 140 of the device 100.

(76) As shown in FIG. 7, a quadrature architecture device 700 may alternatively be arranged to generate the transmitted signal T(t) and the first reference signal in a similar way as used by the device 300. Thus, the device 700 may comprise a first oscillator 716 and a second oscillator 726. The first oscillator 716 may provide input to the NCO 752 of the signal processing circuitry 740 for generating the feedback signal F[n].

(77) As shown in FIG. 8, a quadrature architecture device 800 may according to a further alternative be arranged to generate the transmitted signal T(t) and the first reference signal in a similar way as used by the device 100. In the device 800, a vital sign carrying signal formed by mixing the reflected signal with the first reference signal is divided into two channels in digital domain. Then, the vital sign carrying signal may be mixed with a signal from a local oscillator having the intermediate frequency f.sub.IF for forming an inphase signal I. Further, the vital sign carrying signal may be mixed with a signal from the local oscillator being shifted 90 so as to form a quadrature signal Q. The inphase signal I and the quadrature signal Q may then be combined to form a complex form digital signal I+jQ. The digital signal processing circuitry 840 may further comprise a digital implementation of a PLL corresponding to the digital signal processing circuitry 140 of the device 100.

(78) Referring now to FIG. 9, a method for detecting a vital sign will be summarized.

(79) The method comprises transmitting 902 a radio frequency signal towards the subject. The method further comprises receiving 904 a reflected signal from the subject, wherein the transmitted signal is reflected by the subject and Doppler-shifted due to at least one of the heart rate and the respiratory rate of the subject to form the reflected signal.

(80) The method further comprises mixing 906 the reflected signal with a first reference signal. The mixed signal may be further processed, for example lowpass filtered and/or analog-to-digital converted, to generate a vital sign carrying signal. The vital sign carrying signal is provided 908 to a first input of a phase or frequency comparator.

(81) The method further comprises generating 910 an adjustable second reference signal by a reference signal generator and providing the reference signal to a second input of the phase or frequency comparator. Thus, the phase or frequency comparator may generate 912 an output signal based on the vital sign carrying signal and the second reference signal. The output signal may be further processed, for example, by lowpass filtering and/or integration, before being provided back to the reference signal generator in a loop.

(82) The reference signal generator may further vary 914 at least one of a phase and a frequency of the adjustable second reference signal based on the output signal of the phase or frequency comparator to track a phase or frequency of the vital sign carrying signal.

(83) Thus, a phase and/or frequency of the vital sign carrying signal may be extracted and the output signal of the phase or frequency comparator may be output to a vital sign estimator for determining a vital sign of the subject.

(84) In the above the disclosed technology has mainly been described with reference to a limited number of examples. However, as is readily appreciated by a person skilled in the art, other examples than the ones disclosed above are equally possible within the scope of the disclosed technology.

(85) For example, the various electronic elements of the devices 100-800 may be implemented in one or more integrated circuits. Furthermore, the devices 100-800 employ a separate transmitter antenna 112 and a separate receiver antenna 122. However, it is equally possible to instead arrange the transmitter 110 and the receiver 120 to transmit/receive via a common antenna. The transmitter 110 and the receiver 120 may be connected to a common antenna via a circulator or coupler arranged to direct transmitted signals T(t) from the transmitter 110 (for example from the transmission output of the signal generator 136 or from the output of the amplifier if present) to the common antenna and to direct reflected signals R(t) from the common antenna to the receiver 120 (for example to an input of the mixer 132 or to an input of the amplifier 124 if present). Hence the same antenna may be used for both transmission of the signal T(t) and for reception of the reflected signal R(t).