Topic 1.4: Signals and Noise Flashcardes (CHEM 3100)

What are the two components of every analytical measurement?

Signal and noise

What is a signal?

The component that carries information about the analyte of interest.

What is a noise?

Unwanted extraneous information that obsures measurement of the desired signal.

What analytical figures or merit are strongly influenced by noise?

Precision and detection limits

What is white noise?

Noise that depends on bandwidth (Δf) but is independent of frequency.

How does noise depend on bandwidth?

Noise is proportional to the square root of bandwidth (√Δf).

How can noise be reduced?

By narrowing the frequency bandwidth.

Why does narrowing the frequency bandwidth improve measurements?

It redues noise, improving signal detection and precision.

What are the two major categories of noise?

Chemical noise and instrumental noise.

What is chemical noise?

Noise arising from uncontrollable variables affecting the chemistry of the system.

Give an example of chemical noise.

Changes in humidity affecting sample moisture content.

What are the four types of instrumental noise?

Thermal noise, shot noise, flicker noise, and environmental noise.

What are other names for thermal noise?

Johnson noise or Nyquist noise.

What causes thermal noise?

Random motion of electrons due to thermal agitation.

Is thermal noise white noise?

Yes

How can thermal noise be minimized?

Lower temperature and reduce amplifer bandwidth.

Thermal noise equation?

V_noise(rms)= sqrt(4kTRB)

where:

• k = Blotzmann constant

• T = temperature

• R = resistance

• B = bandwidth

How does temperature affect thermal noise?

Higher temeprature increase thermal noise.

What causes shot noise?

Random movement of discrete charges (electrons or photons) across junctions.

Why is shot noise considered a quantum effect?

It arises from discrete charges carriers.

Is shot noise white noise?

Yes

How can shot noise be reduced?

By narrowing bandwidth

Shot noise equation?

I_noise(rms)= sqrt(2qIdcB)

where:

• q = electron charge

• I_dc = direct current

• B = bandwidth

What is another name for flicker noise?

1/f noise

Whay is it called 1/f noise?

Noise is inversely proportional to frequency

At what frequencies is flicker noise most problematic?

Low frequencies (<100 Hz)

What happens to flicker noise as frequency decrease?

It increases dramatically.

What causes flicker noise?

Imprefections in semiconductor materials that temporarily trap charges.

Which type of noise dominates at very low frequencies?

Flicker noise

What is enviromental noise?

Noise arising from surroundings

Give examples of enviromental noise.

electromagnetic radiation, room lighting, and mechanical vibrations

How is environmental noise minimized?

Good instrument design and setup.

Why is S/N more useful than noise alone?

It describes output quality and detectability of signals.

What happens to S/N as concetration decreases?

It decrease because signal decreases while noise remians relatively constant.

What minimum S/N ratio is required for a meaningful signal?

About 2-3

What S/N ratio is typically assocaited with LOD?

S/N ≈ 3

What S/N ratio is typically associated with LOQ?

S/N ≈ 10

What do we want for S?N?

As large as possible

How is noise often measured experimentally?

Using peak-to-peak voltage (V_p-p)

Relationship between Vrms and Vp-p?

V_rms ≈ 0.35V_p-p

How is S/N calculated?

Single voltage divided by noise voltage.

What are the two major strategies for improving S/N?

Hardware methonds and software methods.

Examples of hardware noise-reduction devices?

Filters, shields, modulators, choppers, and syhchronous detectors.

What type of noise does shielding primarily reduce?

Enviromental noise

How does shielding work?

Conducting material absorbs electromagnetic radiation before it reaches sensitive circuitry.

What type of amplifer is commonly used to improve S/N?

Lock-in amplifer

What noise can amplifiers reduce?

Common-mode noise.

What does a low-pass filter do?

Removes high-frequency noise.

Which noise are reduced by low-pass filters?

Thermal and shot noise.

What does a high-pass filter do?

Removes low-frequency noise.

Which noise is reduced by a high-pass filter?

Flicker (1/f) noise

What is modulation?

Converting a low-frequency of DC signal into a higher-frequency signal.

Why is modulation used?

To reduce the effects of flicker (1/f) noise.

After modulation, what filter is used/

High-pass filter

Why does modulation work?

Flicker noise is much lower at higher frequencies.

What are the two main software methods?

Averaging and digital filtering

What is signal averaging?

Repeated measurements are collected and averaged.

Why does averaging imporve S/N?

Noise is random and tends to cancel, while signal is additive.

What happens to signal after n scans?

Signal increases by n.

What happens to noise after n scans?

Noise increases by √n

By what factor does S/N improve after averaging n scans?

S/N ∝ √n

How many scans are nedded to improve S/N by a factor of 10?

100 scans

10 = √n

n = 100

What mathematical procedure is commonly used in digital filtering?

Fourier transformation

What does Fourier transfomation do?

Converts a time-doamn signal into a frequency-domain signal.

What happens after conversion to the frequency domain?

A digital low-pass filter removes frequencis above the cutoff frequency (f_o).

What retrieves the filtered signal?

Inverse Fourier transformation