1.1 What is Noise?any ‘unwanted” part of the analytical signalalways some noise in a signal

1.2 Signal-to-noise ratio (S/N) for a set of data (replicate measurements)

for a temporal-varying signal

For meaningful measurements, S/N 3,

RSDsx

NS 1

S

Sx

NS

Ss

55

2.1 White Noise – amplitude invariant with respect to frequency Thermal Noise -voltage fluctuation due to random electron motions in the resistive elements

k: Boltzmann’s constantT: absolute temperatureR: resistancef: frequency bandwidth,

fkTRrms 4

r

f31

Hzfs

f

r

r

33 01.0

31

Shot Noise-current fluctuations due to random motion of electrons cross a junction (e.g., PN interface, space between anode/cathode)

I: average currente: charge of electron

fIeirms 2

2.2 Flicker Noise – amplitude varies with 1/f, drift in instruments

2.3 Environmental Noise

- different forms of noise that arise from the surroundings

- some occurs at known discrete frequencies

- some unpredictable, and difficult to correct (e.g., TV stations, computers, motors,etc)

2.4 Composite Noise Spectrum

Fig. 5-3 (p.113)

2.4 Composite Noise Spectrum

White Noise reduce f, temp, resistance, and I Flicker Noise make measurements at frequencies >100kHz Shielding & Grounding absorbing electromagnetic noise

But signal often at or near dc (low freq) often directly proportional to resistance often directly proportional to current often measured with transducers having very large f (fast response, PMT

f >107Hz)

3.1 Reducing f (white noise)3.1.1 Analog filtering: low-pass RC circuit

Fig. 5-5 (p.115)

High-frequency components rejected, and f reduced

A slow varying dc signal containing high frequencies with bandwidth extending over wide range

fRGCC 21

3.1.2 Digital filtering: Fourier transform/smooth

-It is easy to smooth/filter signal as well as noise. Make sure that the result is not distorted

- trade-off between resolution and noise. Need high point density to prevent losing information.

control in the frequency domain by manipulating pass function

Fig. 5-12 (p.121)

3.2Increasing f (flicker noise) We need to move f to >100kHz… How?

- Modulate: encode analytical signal at a high frequency, where 1/f noise is negligible- Amplify the signal at the modulation frequency, while reduce the noise.- demodulate the signal

Lock-in Amplifier

Chopper

Fig. 5-8 (p.117)3. Demodulate

2. Amplify modulated signal1. Modulate

3.3Signal Averaging

Total intensity of signal: increase linearly with the number (n) of replicate signals

Noise: increase as (n)1/2

S/N increase as (n)1/2

n

SSN

n

ixi

ii

1

2)(

ii

i

n NSn

NnnS

NS

n

iiin nSSS

1

ii

n

iinn NnnN

1

22

3.3.1 An Example for Signal Averaging

3.3.2 Signal Averaging For a Spectrum

Get S/N increased with n½

Need good synchronization for replicate scan

Fig. 5-10 (p.119)

3.3.3 Boxcar Averaging A approach for smoothing irregularities

A single –channel signal averagerselect a single delay time

integrated signal over selected gate time

average signal for n-replicate

repeat at new delay time

S/N increases with (averaging time)1/2

Fig. 5-11 (p.119)

3.3.3 Boxcar Averaging

select a single delay time

integrated signal over selected gate time

average signal for n-replicate

repeat at new delay time

Fig. 5-11 (p.119)