CHEM 524 -- Course Outline (Part 9)

PDF version, maybe, but do not get optimistic

VI. Signal Processing (Text: Chapter 4.5)

A. Signal conditioning -- Detectors convert light to "electrons", now need to process signal into something we can interpret and store

1. Preamplifiers -- goal to get signal level above interfering sources of noise

• Current source, need an I to V convertor, could just use a resistor, R_L Where V_o = i_d R_L
• OP AMP – while these are made of discrete components, most of our thinking will use operational amplifiers as building blocks
• Idea is to have two inputs +/- at same potential but
• These have very high input impedance
• Output current is delivered to achieve balance in inputs through feedback loop
• Alternate I to V: current-to-voltage convertor, op amp with R_f feedback: V_o = - i_d R_f
• Voltage amplifiers can invert of not, and can sum two or more inputs
• Voltage invertor, op amp ratio feedback to input resistor, flip sign: V_o = - V_in(R_f/R_in)
• Non-inverting design, V_o = V_in(R₁ + R₂/R₁)
• Summing amp, if balance resistors, no gain, V_o = -R(V₁/R + V₂/R)
• AC couple (place capacitor in series) used to block DC, such as bias voltage, and avoids overloading the amplifier if high gain needed for small AC component
• PMT – use a current follower (as above) or
• pulse count -- just amplify those pulses > threshold and count

2. Filter -- low/high pass f_c = 1/2pRC

-- band pass f_bp = 1/2p(LC)^1/2

3. Other amplifiers –

buffer or voltage follower, matches impedance, protects source –

Differential amp has output amplifying difference of two inputs. Big thing is common mode rejection, stray signals on each are rejected.

Discriminator—only amplifies signals/pulses above a threshold, rejects low level noise

B. Voltage measurement

1. DVM and multimeters—high input impedance, variable functions, slow

2. Oscilloscopes—variable time response, gain vary also—get wave form / transient

C. Demodulators—address time variation of signal

1. Lock-in Amplifier

· detects components of input signal that have same frequency and

o are in-phase with a reference signal (e.g. from a modulator) and

o outputs a DC voltage proportional to rms of signal (applies gain to that signal)

o synchronous demodulation (repeating signal mixed with reference) -- phase has information

o filters are important part of commercial designs

§ couple signal in (optional band pass, adds dynamic range) and

§ DC out (low pass--expressed as time constant)

· New models have digital signal processing (DSP) which gives enhanced dynamic range and a wide selection of filtering

2. Pulsed techniques

a. Sample and hold

1. think of a switch that allows charge to accumulate on a capacitor,

2. you control the switch opening and can measure the charge

b. Box car averager –basically a moveable sample and hold—

1. signal detected and integrated only during a gate

2. control of delay and width of gate permits profile measurement of shape for repeated pulses

c. Multichannel averager—

1. series of time windows, put pulse into "bin" matching time

2. average over many pulses, distribution in bins gives profile

d. Transient recorder/digital oscilloscope -- digitize wave form each event by writing out signal on a charge sensitive device and reading back with digitizer

D. Computer data acquisition -- ubiquitous -- spectrometer control and data collection

a. A/D converter -- create digital form – various methods (clocks, discriminators, etc.)

i. time for conversion (rate of data input) and digital precision are tradeoff

ii. 12-bit microsec feasible, 16-bit slower, higher are special

b. Storage of data - average repeated experiments - correct errors – archive on disc or other media

c. Display data—graphics with interactive features very important

Homework--

Links

Lock-in Applet from Univ. Konstanz--worth doing

http://www.lockin.de/

Brown Univ. lab write-up on lock-in use and theory

http://www.chem.brown.edu/chem116/labs/exp8.html

Another lock-in simulator, must download

http://www.inform.umd.edu/EdRes/Topic/Chemistry/ChemConference/Software/Spreadsheets/WWW/lockin.html

Lockin companies

Princeton Applied Research/ PAR/ EG&G/ Ametek now covers EG&G lines it seems, plus some tech notes and explanatory pages: http://www.signalrecovery.com/index.html

lockins:   http://www.signalrecovery.com/lockinde.htm

Boxcars:  http://www.signalrecovery.com/SigAvDet.htm

Stanford Research Systems --http://www.thinksrs.com/products/sci.htm

Lockin—830 http://www.thinksrs.com/products/SR810830.htm

Oscilloscopes

Techtronix -http://www.tektronix.com/

http://www.tek.com/Measurement/cgibin/framed.pl?Document=/Measurement/scopes/home.html?wt=257&link=/Measurement/scopes/home.html&FrameSet=oscilloscopes

Hewlett Packard/ Agilent Technologies, old HP instruments

http://www.home.agilent.com/USeng/nav/-11145.0/ia.html

DVM, etc.

Fluke—multimeters etc,

http://www.fluke.com/

Many Others--

Digital companies—miriad!