CHAPTER 5

Chopping: a technique for noise and offset reduction

5.3. Chopping seen as a modulation technique

Another method for noise and offset reduction is the chopping technique. Chopping is a modulation technique which shifts the spectra of low frequency stationary processes at multiples of chopper frequency out of the band of interest. To understand this, consider a stationary random process x(t) of autocorrelation function Rxx(t) and power spectral density Sxx(f) which is applied to a band limited amplifier A(f) as illustrated in fig.5.4. The modulation signal (chopper signal) m(t) is periodic with a period T and can be expanded in Fourier series:

(5.8)

The Fourier transform of this signal is a sequence of Dirac pulses decaying with the order of the harmonic and having contributions only at odd multiples of the chopping frequency 1/T.

(5.9)

The power spectral density of the output process y(t)=[Ax(t)]m(t) can be found from the following convolution:

 Fig.5.4: Chopper modulation

(5.10)

This gives contributions only at the odd multiples of the chopping frequency 1/T:

(5.11)

Therefore, the output power spectral density is a repeated replica of the input power spectral density at the odd multiples of the chopper frequency rapidly decreasing with the order of the harmonic. That is why foldover effects are not present in the case of chopping. This is the main difference between chopping and sampling methods.

Featured Video
Editorial
 Jeff's MCAD Bloggingby Jeff RowePTC Marches On With New Vuforia AR Platform Release
Jobs
Vice President, Transportation Services for Associated General Contractors of New York State at Albany, NY
Architectural Designer for Champion Home Builders at Troy, MI
GIS Analyst for G2 Partners LLC at San Ramon, CA
GIS Software Developer for UDC at Englewood, CO
Upcoming Events
ASSESS 2017 CONGRESS at Bolger Center Potomac MD - Nov 1 - 3, 2017
FABTECH 2017 at McCormick Place Chicago IL - Nov 6 - 9, 2017
ATX MINNEAPOLIS 2017 at Minneapolis MN - Nov 8 - 9, 2017
2017 China Chongqing International Machine Tool Show (CCIMT) at Chongqing International Expo Center, Yubei Chongqing China - Nov 13 - 16, 2017