Preliminary Report on the Use of Equalization Filters to Derive High CMRR Hongfang (Linda) Xia Advisor: Professor Edward A Clancy

Department of Electrical and Computer Engineering Worcester Polytechnic Institute

1. Background

The conventional EMG signal is electrical activity of muscle, and is detected by the electrodes.

High CMRR can be achieved via the use of flexible array electrodes, but it is expensive and inflexible.

Bipolar

+1

-1

Linear DoubleDifference (LDD)

+1

+1

-2

+1

+1

-4 +1+1

Normal DoubleDifference (NDD)

Also, it can be achieved via the use of software channel equalization.

Ele

ctro

de

Inp

uts

Eq

ualized

Ch

ann

els

e1

Analog Signal Conditioning

Circuit #1A/D

Linear Equalization

Filter #1EMG

1

e2

Analog Signal Conditioning

Circuit #2A/D

Linear Equalization

Filter #2EMG

2

Hardware Software

2. Research Goal

Comparing using the hardware to achieve high CMRR, the software channel equalization is cheaper and more flexible, so the goal here is to construct software equalization filters to correct channel imperfections due to the tolerance of hardware components.

Hardware characteristic • Excite input with chirp form (0-2kHz):

20cos twtwktx

Initial frequency

½ sweep rate

Initialphase

• Measure channel output y(t):• Compare vs. an ideal channel:

jw

jwjwjw

i

jwjwijw

jw

eXeY

eIdealeE

or

eIdealeEeXeY

Equalizer

Issues:• Chirp contains all frequencies.• For high CMRR (e.g., 80dB, bipolar):

- Need accuracy > 1 part in 10,000! - Noise is a big problem!!!! - Need very accuracy algorithm.

• Noise commonly ≈ 0.5% MVC Limits.• Bipolar CMRR to ≈ 45 dB (Very Poor).• Equalization requires low noise!!• Can’t eliminate all hardware noise FILTER• Chirp frequency is time-varying.

3. Solutions:

Solution 1: Modulate with another chirp, then LTI filter.

Solution 2: Time–varying, linear, bandpass FIR filter (LTV).• FIR bandpass filter: Center frequency tracks input chirp

The equalization filter test modulation:

eIN

Analog Signal Conditioning

Circuit #1A/D

Linear Equalization

Filter #1

EMG

1

Analog Signal Conditioning

Circuit #2A/D

Linear Equalization

Filter #2 EMG

2

S eOUT

+

-

CMRR = 20log10(2|eIN| / |eOUT|)

4. The results:For solution 1:

2. Equalizer filter

Zoom view

3. LPF length simulation

INPUT = Chirp only (0-1kHz)

No Equalization

Equalization

Equalization; Frequency Smoothing

For solution 2:

INPUT = Noise only

INPUT = Noise only

INPUT = Chirp only (0-1kHz)

No Equalization

Equalization

Equalization; Frequency Smoothing

x(t)

Mix:Cosine chirp

Low Pass Filter

ExtractMagnitude,PhaseInformation

M(t)

Φ(t)Mix:

Sine chirpLow Pass

Filter

1. 60Hz input, time domain

5. Conclusion and future workConclusion:

Modulating with another chirp and using a low pass LTI

filter can achieve high CMRR (above ~80dB).Future work:

Can cascade LTV filter and modulator.Use a signal with longer duration.…