CHEM 524 -- Course Outline (Part 9)

VI. Signal Processing (Text: Chapter 4.5)

A. Signal conditioning -- Detectors convert light to "electrons", now need to process

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

• Current source, I ‡ V, could just use a resistor, R_L Where V_o = i_d R_L
• Alternate: current-to-voltage convertor, op amp with R_f feedback: V_o = - i_d R_f
• Voltage invertor, op amp ratio feedback to input resistor, flip sign: V_o = - V_in(R_f/R_in)
• AC couple (capacitor in series) to block DC, such as bias voltage
• PMT -- current follow (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 -- summer adds two sources

1. Lock-in Amplifier

• -- detects components of signal that have same frequency and in-phase as reference signal (from modulator) and outputs a DC voltage proportional to rms of signal
• --- synchronous demodulation (repeating signal) -- phase has information
• --- filters 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) gives most dynamic range and wide selection of filtering

1. Box car averager -- signal detected and integrated only during a gate

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

2. Multichannel averager-- series of time windows, put pulse into "bin" matching time

-- average over many pulses, distribution in bins gives profile

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

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