WO2012156988A2

WO2012156988A2 - Low power analog fm transceiver for bio-telemetry applications

Info

Publication number: WO2012156988A2
Application number: PCT/IN2012/000331
Authority: WO
Inventors: Shojaei Baghini MARYAM; Kumar Gowdhaman SANTHOSH; Anavangot VINEETH; Mukherjee JAYANTA
Original assignee: Indian Institute Of Technology, Bombay
Priority date: 2011-05-13
Filing date: 2012-05-04
Publication date: 2012-11-22
Also published as: WO2012156988A3

Abstract

According to the invention, there is provided a low power analog FM transceiver for bio- telemetry applications which comprises an FM transmitter for receiving an input signal for wireless transmission and an FM receiver configured for receiving the frequency modulated signal transmitted from the FM transmitter. The FM transmitter having a frequency chopper for increasing bandwidth of the input signal according to a given modulation index, a low pass filter; a frequency modulator for performing frequency modulation of the filtered signal; and a non-linear high efficiency power-amplifier for amplifying the frequency modulated signal. The FM receiver comprises a low noise amplifier for amplifying the received frequency modulated signal to generate an amplified signal having a pre-estimated SNR lesser than a desired SNR of output signal of the FM receiver; a linear frequency demodulator for performing frequency demodulation of the signal of the pre-estimated SNR to generate a frequency demodulated signal of the desired SNR; and another frequency chopper for decreasing bandwidth of the frequency demodulated signal to obtain the output signal of bandwidth similar to bandwidth of the input signal.

Description

TITLE OF THE INVENTION

Low power analog FM transceiver for Bio-Telemetry Applications

FIELD OF THE INVENTION

This invention relates to a low power analog FM transceiver. More specifically, the present invention relates to an FM transceiver for biotelemetry applications.

BACKGROUND OF THE INVENTION

Biomedical signals have low information speed, low data rate and are transmitted over short range in Biotelemetry applications. Due to low date rate and speed, and short range transmission, error rate is low and power requirements of a biomedical signal transceiver are also low.

Conventionally, ZigBee standard has been widely used for telemetry of biomedical signals, as it targets low power and low data rate short range data transmission. However, the Zigbee standard employs digital modulation techniques, which requires digital baseband hardware and analog-to-digital converter (ADC). The baseband hardware and ADC consume significant amount of power and space in the Zigbee transceiver.

Further, the Zigbee transceiver employs QPSK modulation techniques for transmission and reception of signals. The QPSK modulated signal does not have a constant envelope. Therefore, a high efficiency non-linear RF power amplifier cannot be used at transmission side of a Zigbee transceiver. This limitation is a major bottleneck in reducing power dissipation of a Zigbee transceiver.

Alternatively, analog FM modulation techniques can be used for telemetry of biomedical signals, as they don't require digital base band hardware and a non-linear power amplifier can be used at transmission side because the FM modulation has constant envelope characteristics. However, there are chances of thermal noise corrupting the FM signal, resulting in output signal of low SNR and sensitivity.

Hence, there is a need for a transceiver for biotelemetry applications which has power dissipation lower than that of Zigbee transceiver but has SNR and sensitivity comparable to Zigbee standard.

DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

Various embodiments of the present invention provide a low power analog FM transceiver for bio-telemetry applications. The FM transceiver comprises an FM transmitter configured to receive an input baseband signal for wireless transmission. The FM transmitter have a frequency chopper configured to increase bandwidth of the input signal according to a given modulation index, a low pass filter configured to filter high frequency harmonics of the chopped signal, a frequency modulator configured to perform frequency modulation of the filtered signal to generate a frequency modulated signal of the given modulation index, the frequency modulated signal having a frequency linearly proportional to voltage of the filtered signal, and a non-linear high efficiency power-amplifier configured to amplify the frequency modulated signal for wireless transmission. The FM transceiver further comprises a FM receiver configured to receive the frequency modulated signal transmitted from the FM transmitter. The FM receiver having a low noise amplifier configured to amplify the received frequency modulated signal to generate an amplified signal having a pre-estimated SNR lesser than a desired SNR of output signal of the FM receiver, a linear frequency demodulator configured to perform frequency demodulation of the signal of the pre-estimated SNR to generate a frequency demodulated signal of the desired SNR, and another frequency chopper configured to decrease bandwidth of the frequency demodulated signal to obtain the output signal of bandwidth similar to bandwidth of the input signal.

Preferably, the pre-estimated SNR is minimum SNR required at input of the FM demodulator at the given modulation index to obtain the desired SNR at output of the FM demodulator. Preferably, the low noise amplifier has a pre-defined noise figure determined based on the pre- estimated SNR.

Preferably, the input signal is a low frequency bio-medical signal having a frequency band of (0.3-100Hz).

Preferably, the frequency modulator is a linear voltage-controlled oscillator with a predetermined centre frequency of order of MHz.

Preferably, the FM transmitter further comprises a local oscillator and a mixer to up-convert the frequency modulated signal to a signal of frequency of order of GHz.

Preferably, the FM receiver further comprises a local oscillator and mixer to down convert signal amplified by the low noise amplifier to a signal of IF frequency of order of MHz. Preferably, the FM receiver further comprises a low pass filter for filtering high frequency harmonics from signal chopped by the another frequency chopper.

Preferably, the frequency demodulator is a PLL based demodulator.

Preferably, the given modulation index is determined based on the desired SNR and desired power consumption of the low noise amplifier. ^'

These and other aspects, features and advantages of the invention will be better understood with reference to the following detailed description, accompanying drawings and appended claims, in which,

FIG. l illustrates an FM transceiver in accordance with an embodiment of the present invention; FIG.2 illustrates SNRj_n vs SNR_oUt characteristics of FM demodulation;

FIG.3 illustrates architecture of an FM transmitter in accordance with an embodiment of the present invention; FIG.4 illustrates architecture of an FM receiver in accordance with an embodiment of the present invention; and

FIG.5 is a table illustrating specifications of FM transceiver in accordance with various embodiments of the present invention.

Referring now to drawings, and more particularly to Fig.l , wherein various embodiments of the invention can be performed. Fig. l illustrates a wireless FM transceiver 100. The FM transceiver 100 is a low power fully analog FM transceiver capable of telemetry of biomedical/bio-potential signals. The FM transceiver 100 comprises an FM transmitter 102 and an FM receiver 104.

The FM transmitter 102 is configured to receive an input signal, perform frequency modulation of the input signal using an FM modulator and send the modulated signal for transmission to the FM receiver 104 through a wireless medium. The FM receiver 104 is configured to receive the FM modulated signal and perform its demodulation using an FM demodulator to provide an output signal which is quite similar to the input signal. The overall power dissipation and size of the transceiver 100 is lower than that of a Zigbee transceiver but still SNR and sensitivity is comparable to that of Zigbee standard.

A very useful and interesting relationship between SNR of the input and output signals of the FM demodulator has been used to develop and design various components and specifications of the FM transceiver 100. The relationship is illustrated by the following equation:

where β is modulation index of the frequency modulated signal outputted by the frequency modulator, SNR_in is SNR of the input signal of the FM demodulator and SNR_0Ut is SNR of the output signal of the FM demodulator.

The modulation index β of a frequency modulated signal is illustrated by the following equation: β = ¾^ (2)

where Af_max is peak frequency-deviation of the frequency modulated signal and B_m is bandwidth of the modulating signal, i.e. input to the FM modulator.

The relationship between SNR_in and SNR_out of the FM demodulator is further illustrated with reference to Fig.2 at four different values of modulation index β (10, 20, 50 and 100). From the figure, it is apparent that it is possible to have SNR_0Ut higher than SNR_in if SNR_in \^'s above some threshold value. The threshold value is referred to as SNR_min. This threshold value increases with modulation index. In other words, an FM demodulator has an inherent capability of increasing the SNR of a signal during demodulation process if input SNR is above a threshold value. At a modulation index of 10, for a desired SNR_0Ut of 47dB, SNR_min (minimum SNR required at the input of FM demodulator) is 25dB, whereas, at a modulation index of 20, SNR_min is 30dB for SNR_out of 58dB, and at a modulation index of 60, SNR_min is 30dB for SNR_0Ut of 55dB.

Having SNR_in lesser than SNR_0Ut enables low power design of components disposed before the FM demodulator without compromising on the SNR of the final output signal of the FM demodulator.

Keeping in view of specifications of existing Zigbee compliant transceivers for biotelemetry applications, the desired SNR of the output signal of low power transceiver 100 is around 50dB and the desired sensitivity is around -84dBm. In order to obtain an SNR_min ranging between 25-30dB, for SNR_0Ut of 50dB, the modulation index must range between 10- 60.

The biomedical/bio-potential signals have a very low frequency band (0.3-100Hz). If the signals of such low bandwidth are directly applied to the FM modulator, the resultant modulation index of the modulated signal would be very high, and at a very high modulation index, it is not possible to have SNR_0Ut higher than SNR_in at low value of SNR_in.

Therefore, the FM transmitter 102 is designed and developed in such a manner that the resultant modulation index of the frequency modulated signal is not very high and for a desired SNR_0Ut of around 50dB, SNR_min is around 25-30dB.

Referring to Fig. 3, the FM transmitter 102 comprises a chopper 2, a low pass filter 4, a frequency modulator 6, a local oscillator 8, a mixer 10, a power amplifier 12 and a transmitting antenna 14.

The chopper 2 is configured to receive low frequency input signal 1 of frequency of order of Hz and chop them by a chopping frequency of order of kHz to generate chopped signal 3 of increased bandwidth of order of kHz. The chopper 2 facilitates increasing the bandwidth of sthe input signal 1 and up-converting the input signal 1 to a higher frequency in accordance with a given modulation index. Thus, the chopper 2 plays a crucial role in manipulating bandwidth of the input signal 1 so as to obtain a signal of optimum modulation index at output of the frequency modulator 6, and hence SNR_mi_n.

In an exemplary embodiment, the input signals 1 are low -frequency bio-medical signals having a frequency band of (0.3-100Hz), and the chopper 2 have a chopping frequency ranging between 2.5 kHz-5 kHz.

The low pass filter 4 is configured to filter the high frequency harmonics of the chopped signal 3 and generate filtered signal 5. In an exemplary embodiment, when the chopping frequency is 5kHz and the input signal have a frequency band of (0.3-100Hz), then the filtered signal 5 has a frequency band of 5kHz-5.1kHz. Similarly, when the chopping frequency is 4kHz, the filtered signal 5 has a frequency band of 4kHz-4.1kHz.

The frequency modulator 6 is configured to perform frequency modulation of the signal 5, which is the output from the low pass filter 4 to generate a frequency modulated signal 7 of the given modulation index. The frequency modulated signal 7 has a frequency linearly proportional to voltage of the input signal 5. The signal 5 may also be referred to as modulating signal as it is subjected to frequency modulation by the modulator 6. Preferably, the frequency modulator 6 is a linear voltage-controlled oscillator with a pre-defined center frequency of order of MHz.

In an exemplary embodiment, when the frequency modulator 6 has a center frequency as

2MHz, and it receives modulating signal 5 of frequency band 5kHz-5.1kHz, it generates modulated signal 7 of frequency band 2MHz-2.5MHz with modulation index as 10 and transmission bandwidth B_T as 100kHz, where the transmission bandwidth B_T of a frequency modulated signal is illustrated by the following equation:

Β_τ = 2 β + l)B_m (3)

The local oscillator 8 and the mixer 10 are configured to up-convert the modulated signal 7 to a signal 9 of frequency of order of Ghz. The power amplifier 12 is a high efficiency nonlinear amplifier configured to amplify up-converted signal 9 for wireless transmission through antenna 14. Preferably, the power amplifier 12 is a Class E power amplifier which provides highest power efficiency and can be used herein for amplification of the frequency modulated signal 9 because the frequency modulated signal 9 has constant envelope characteristics. When designed for a given modulation index of 10, the FM transmitter 102 dissipates 7mW power at 4dBm output power.

Referring to Fig.4, the FM receiver 104 comprises a receiving antenna 20, a low noise amplifier 22, a local oscillator 24, a mixer 26, a band pass filter 28, a frequency demodulator 30, a chopper 32, and a low pass filter 34.

The receiving antenna 20 is configured to receive the signal transmitted from the FM transmitter 102.

The Low Noise Amplifier (LNA) 22 is configured to amplify the signals sensed by the antenna 20 for generating an amplified signal 25 of a pre-estimated SNR lesser than a desired SNR of output of the FM receiver 104. The pre-estimated SNR is minimum SNR (SNR_min) required at input of the FM demodulator 30 at the given modulation index to obtain the desired SNR at output of the FM demodulator 30.

The pre-estimated SNR is crucial in determining a Noise Figure (NF) of the LNA 22.

The Noise Figure .(NF) of the LNA 22 is a ratio that indicates how much noise power the LNA 22 will contribute to the total noise of the receiver 104. The Noise Figure (NF) is represented by the following equation: Noise Floor (F) = -\ 14dB + NF + 101og ?_r (4) where Noise Floor (F) is a measure of the signal created from sum of all noise sources and unwanted signals within the receiver 104.

The Noise Floor (F) is related to the sensitivity of the receiver 104 and SNR_min by the following equation:

Sensitivity = F + SNR_min . (5)

Based on equations (4) and (5), the lower the SNR_mtnis, the higher the Noise Figure (NF) is.

The power consumption of the LNA 22 decreases with increase in the Noise Figure

(NF). Therefore, higher the Noise Figure (NF) is, lower the power consumption of the LNA 22.

However, the LNA 22 with a higher Noise Figure (NF) produces a signal of lower SNR. When the LNA 22 has a flexibility of outputting a signal of SNR lesser than the desired SNR of the demodulator 30, then it can be designed for a higher Noise Figure (NF), and thus of low power.

Therefore, it is possible to design low power LNA 22 without compromising on the SNR of the output signal of the FM transceiver 100.

The local oscillator 24, the mixer 26 and the band pass filter 28 are configured to down- convert the amplified signal 25 to a signal 27 of IF frequency of 2MHz. Low IF allows to alleviate flicker noise from the mixer 26.

The frequency demodulator 30 is configured to perform frequency demodulation of the signal 27 to generate signal 31 having voltage linearly proportional to frequency of the input signal 27. Any suitable frequency demodulator which facilitates linear conversion of frequency to voltage can be considered within the scope of this invention. Examples of frequency demodulator 30 may include, PLL-based demodulator, frequency to voltage converters, and the like.

The chopper 32 is configured to decrease the bandwidth of the demodulated signal 31 to original frequency band (0.3-100Hz). In other words, the chopper 32 is configured to down- convert the demodulated signal 31 to a signal of baseband frequency.

The low pass filter 34 is configured to filter the high frequency harmonics of the signal outputted by the chopper 32 and generate an output signal 35 similar to input signal 1.

Fig.5 illustrates specifications of the FM transceiver 100 for three different values of chopping frequency of the chopper 2. The message maximum frequency is referred to as maximum frequency of the signal outputted from the chopper 2. It can be seen from the table that the message maximum frequency varies with chopping frequency. For example, an input signal of frequency band (0.3-100)Hz, when subjected to chopping frequency of 4kHz generates a message of maximum frequency 4.1kHz, further, when the input signal is subjected to chopping frequency of 5kHz, a message of maximum frequency 5.1kHz is generated. Similarly, the modulation index β varies with chopping frequency and so the transmission bandwidth.

It can be seen from the table that in case-Ill, with chopping frequency of 5kHz, the modulation index is 10, desired SNR at FM demodulator output 31 is 47dB while minimum required SNR at demodulator input 27 is 25dB. This enables relaxation of Noise Figure of the LNA 22 to around 15dB. Since Noise Figure is not stringent, the LNA 22 can be designed at low power. Further, the sensitivity of receiver 104 is -84dBm and required IIP3 of receiver front-end is -26.5dBm. These specifications are very close to Zigbee specifications.

The proposed FM transceiver is a fully analog circuit. Therefore, ADC Block is not required at the transmitter end. Further, since the FM modulation has constant envelope, a high- efficiency non-linear power amplifier can be used at transmitter end for wireless transmission of the signal. Hence power consumption of the transmitter is quite less with respect to a Zigbee transceiver. Further, introduction of a chopper stage before the FM modulator facilitates achieving optimum modulation index and thereby making use of inherent advantage of FM to design low power LNA for a given SNR_out .

An FM transceiver architecture has been designed in 180nm UMC CMOS technology and the specifications of FM transceiver have been developed. With the specifications of case III, simulation results show that output signal of 47dB SNR is achieved. The low power FM transceiver has signal-to-noise ratio comparable with Zigbee standard. The transmitter dissipates 7mW power at 4dBm output power. Further, the power dissipation in the LNA 22 and the mixer 26 is 3.6mW.

Although the invention has been described with reference to a specific embodiment, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiment, as well as alternate embodiments of the invention, will become apparent to persons skilled in the art upon reference to the description of the invention. It is therefore contemplated that such modifications can be made without departing from the scope of the invention as defined in the appended claims.

Claims

CLAIMS :

1. A low power analog FM transceiver for bio-telemetry applications comprising:

an FM transmitter configured to receive an input signal for wireless transmission, the FM transmitter having:

a frequency chopper configured to increase bandwidth of the input signal according to a given modulation index;

a low pass filter configured to filter high frequency harmonics of the chopped signal;

a frequency modulator configured to perform frequency modulation of the filtered signal to generate a frequency modulated signal of the given modulation index, the frequency modulated signal having a frequency linearly proportional to voltage of the filtered signal; and

a non-linear high efficiency power-amplifier configured to amplify the frequency modulated signal for wireless transmission, and

an FM receiver configured to receive the frequency modulated signal transmitted from the FM transmitter, the FM receiver having:

a low noise amplifier configured to amplify the received frequency modulated signal to generate an amplified signal having a pre-estimated SNR lesser than a desired SNR of output signal of the FM receiver;

a linear frequency demodulator configured to perform frequency demodulation of the signal of the pre-estimated SNR to generate a frequency demodulated signal of the desired SNR; and another frequency chopper configured to decrease bandwidth of the frequency demodulated signal to obtain the output signal of bandwidth similar to bandwidth of the input signal.

2. The low power analog FM transceiver as claimed in claim 1, wherein the pre- estimated SNR is minimum SNR required at input of the FM demodulator at the given modulation index to obtain the desired SNR at output of the FM demodulator.

3. The low power analog FM transceiver as claimed in claim 1, wherein the low noise amplifier has a pre-defined noise figure based on the pre-estimated SNR.

4. The low power analog FM transceiver as claimed in claim 1, wherein the input signal is a low frequency bio-medical signal having a frequency band of (0.3-100Hz).

5. The low power analog FM transceiver as claimed in claim 1 , wherein the frequency modulator is a linear voltage-controlled oscillator with a pre-determined centre frequency of order of MHz.

6. The low power analog FM transceiver as claimed in claim 1, where in the FM transmitter further comprises a local oscillator and a mixer to up-convert the frequency modulated signal to a signal of frequency of order of GHz.

7. The low power analog FM transceiver as claimed in claim 1, where in the FM receiver further comprises a local oscillator and mixer to down convert signal amplified by the low noise amplifier to a signal of IF frequency of order of MHz.

8. The low power analog FM transceiver as claimed in claim 1, where in the FM receiver further comprises a low pass filter for filtering high frequency harmonics from signal chopped by the another frequency chopper.

9. The low power analog FM transceiver as claimed in claim 1, wherein the frequency demodulator is a PLL based demodulator.

10. The low power analog FM transceiver as claimed in claim 1, wherein the given modulation index is determined based on the desired SNR and desired power consumption of the low noise amplifier.