Intern Project Report Chlorophyll a/b-chlorophyll a sensor for the Biophysical Oceanographic Sensor Array Mary Ma Mentor: Zbigniew Kolber August 21 st, 2003
Introduction Photosynthetic organisms found in the ocean are an important part of ocean ecology. These organisms include both plants and photosynthetic bacteria. Photosynthetic organisms all contain some type of chlorophyll or bacterio-chlorophyll which are pigments necessary to capture light energy. These pigments fluoresce following absorption of light under certain conditions which give an indication of the status of photosynthesis. My project involves building a fluorescence sensor capable of measuring the amount of fluorescence emitted from the photosynthetic organisms. Fluorescence sensors are commercially available but they are large size and consume lots of power. The goal of my project is to build a sensor that is small size and has low power consumption. It will be used as part of a system to measure the concentrations of phytoplankton and photosynthetic bacteria in the ocean based on the fluorescence signals at 685 nm and 880 nm. Overview of the Design: Noise Analysis For our purposes, light can be considered in terms of photons, packets of energy with zero rest mass. The higher the light level, the more of these energy packets are available. As the level of light, and therefore the photon rate, decreases, it becomes more susceptible to interference from random events and noise from many sources. Figure 1 shows the many sources of interference which can make the recovery of a low light level tedious or difficult. The typical 1/f curve consists of a variety of unusual sources not usually taken into consideration. The frequency of 1/f noise approaches zero, or DC. Power/bandwidth signal 1/f noise White noise Frequency (hz) Figure 1: 1/f noise and signal
Structure and Methods 1. Photo sensor Photo diodes and photo multipliers are the most popular photo sensors. Since photo diodes have much smaller size than photo multipliers, my project was designed to use a photodiode as the fluorescence detector. Photo diodes also have higher dark noise than photo multipliers. 2. Lock-in amplifier One of the most popular methods to reduce noise and recover a low level detected light signal is to modulate the signal. As shown in Figure 2, the signal is moved in frequency from the noisy 1/f area at zero frequency to a less noisy area at some arbitrary frequency f. m Figure 2 : By moving the signal away from noisy low frequency regions, a low level Detected light signal can be recovered. This type of system locks the center frequency of the narrowband amplifier to the modulation frequency. The heart of lock-in amplifier is best described by considering Figure 3. The signal of interest is fed in parallel to inverting and non-inverting unity gain amplifiers. The output of these two amplifiers is selected by the switch position as determined by the polarity of a reference signal ( f r ). If the signal we are looking at is v in = V o sin( ω t ) and we multiply it by a square wave which has a Fourier series of 4 1 1 v sq = (sin( ωt) + sin(3ωt ) + sin(5ωt ) +...), we get π 3 5 2V o 2 2 2 vout = (1 cos(2ωt ) cos(4ωt ) cos(6ωt )...), the output has a DC π 3 15 35 level proportional to the input voltage.
Figure 3: With the phase-sensitive amplifier technique, the center frequency of the narrowband amplifier is locked to the modulation frequency. 3. Structure of the sensor system The system generates a stable light source. The light from a stable light source is passed through a sample and reaches a detector (photo diode). The resulting electrical signal from the detector is amplified and processed by a lock-in amplifier. The output of the low pass filter gives an indication of the amount of light transmitted by the sample. Figure 4 is a block diagram of the sensor system.
12v Voltage to current converter Narrowband filter Photo diode Generator Current to voltage converter Phase shifter Output of sensor Amp. Narrowband filter Phase sensitive detector Lowpass filter Figure 4: Block diagram of sensor system 4. Circuit design 1) Generator and Narrowband filter Since ceramic resonators are easy to operate and give stable waveforms, this project uses a ceramic resonator ( X921-ND), its center frequency is 445kHz. Ceramic filters have high Q quality and narrowband character. The ceramic filter (TD2332-ND) is the narrow filter used in this sensor system. This circuit is shown in Figure 5. The inverter used is a Schmitt Inverter (74VHC14) fabricated with silicon gate CMOS technology. The inputs have hysteresis between the positive-going and negative-going input thresholds, which are capable of transforming slowly changing input signals into sharply defined ones. So even though the input signal is not a standard CMOS logic level, it still works well. 2) Voltage to current converter The output of the narrowband ceramic filter is a sinusoidal wave. Its DC offset is equal to zero. Considering the power consumption, my project uses single power supply for this voltage to current converter. The chip used (LT1630) has rail-to-rail input and output op amps with a 30MHz gain-bandwidth product and 10v/us slew rate. It is good for a single power supply. Because of the single
5v power supply, the DC offset voltage of input should be 2.5v or 2v. It is not necessary to worry about the ac output range because this circuit output is current not voltage. Figure 5: Schematics of generator and narrowband filter The output of the op amp drives a JFET which provides current for an LED (bright blue light). Because the JFET has voltage-control-current character, it is easy to control the current value of the drain. The feedback loop is important for this circuit design. Figure 6 below shows the voltage to current converter circuit. Capacitors C1 and C3 are important for the feedback because they can increase the ac feedback speed to keep the inputs of op amp balanced. Capacitors C5 and C6 are used to filter the output noise. The purpose of the LED is to create a light source for the sensor.
Figure 6: Schematics of voltage to current converter 3) Phase shifter Phase shifter circuit design can be achieved in several ways. One method uses an op amp to achieve a low output impedance and buffer the phase-shifted signal. The type of circuit is shown in Figure 7. Figure 7: Phase shifter using op amp But this type of circuit easily oscillates because it has a positive feedback loop in some frequency conditions. To avoid oscillations, my project changed the design to a simple RC shifter. The RC shifter can cause a phase shift for a sinusoidal wave. To get a square wave, a comparator is added to convert the sinusoidal wave into a square wave at the output of the RC phase shifter. The output of the comparator may not produce a good square wave, but using an inverter can sharpen the square wave. The RC-phase shifter circuit is shown in Figure 8. In Figure 8, C1 and R2 combined are the RC-phase shifter. Changing the R2 value can change the phase value of the output. Because the phase change causes the voltage of impedance to be modified, adding a capacitor (C3) avoids influence from the dc voltage on the rest of the circuit. The entire phase shifter includes the RC-phase and a comparator (LM393) which converts the sinusoidal wave to a square wave.
4) Phase sensitive detector A phase sensitive detector is the most important and difficult part of this sensor system. Several ways can be used to achieve the function. One popular method is shown in Figure 3. Simply using an analog switch and an op amp can realize the function. This method requires that one considers the requirements for the dc component of signal. If the input signal of the phase sensitive detector changes the dc component value, the op amp has to change the dc offset to meet the signal requirement. If we add a capacitor to eliminate the dc component, that may solve the offset problem. But in our case, the signal detected by the photo diode is a wave modulated by the dc signal. If we use a capacitor to eliminate the dc components, it means that we eliminate the useful signal. Here we used a clever method which is to use switch capacitors to avoid the need to consider the dc offset effect on the op amp. Figure 8: Schematics of RC-phase shifter The switch capacitor circuit is shown in Figure 9. The circuit design is also based on the idea which is shown in Figure 3. The chip used for the switch capacitor (LTC1043) is a monolithic, charge-balanced, dual switched capacitor instrumentation building block. A pair of switches alternately connects an external capacitor to an input signal and connects the charged capacitor across an output port. My project uses the reference square wave as the control clock to control the switch. There are two input paths into the differential amp (LTC1050): one passes through the switch on the positive phase of the control clock and is connected into the positive input of the differential amp; the other
passes through the switch on the negative phase of the control clock and is connected into the negative input of the differential amp. The differential amp design requires a very precise match between the components. In theory the design appears simple, but in practical experiments it is very hard to realize. First, the input signal may be not symmetrical, so it brings some unbalanced elements (etc. dc component) into the differential circuit. Adjusting the resistors of the differential amp helps to make the circuits balanced. Second, the component precision is also considered during the circuit experiments. Furthermore, the op amp must work on the double power supply and have good symmetrical character in the internal circuits. Figure 9: Schematics of the phase sensitive detector Results 1) The generator output wave (455KHz) is shown in Figure 10. This generator is used for the control clock of the system and drive the voltage to current converter. 2) The voltage to current converter wave for the LED is shown in Figure 11. The circuit successfully generated a light source for the sensor. 3) The phase shifter input (sinusoidal) and output (square) waves are shown in Figure 12. This demonstrates that the phase shifter is capable of adjusting the phase of the input wave to match the phase requirement. 4) The result from the phase sensitive detector (not shown) demonstrates the charge or discharge wave for the capacitors. The slope of the wave is high, the tr value is 23 ns, but the bandwidth of the differential op amp LTC1050 is 30MHz. So the
result of the differential amp becomes a sinusoidal wave. Since the amp eliminates the high frequency component, it doesn t affect our low frequency signal. Figure 10: Wave of generator output
Figure 11: The current wave for the LED
Figure 12: The output and input waves for the phase shifter. The square wave is the output and sinusoidal wave is the input. Summary of experience During my project many experiments failed, which resulted in my gaining experience in problem-solving. For example, I spent lots of time fixing oscillations. A variable resistor makes the design easier, but it usually becomes the source of oscillations. Board wires are another source of oscillations. Long wires were used on the board for my project. When I cut them for reuse, the wires caused some oscillation trouble for the circuits. The dc path is disconnected, but the ac path still passes the wires because the cut points act as small capacitors on the board. Therefore, I had to use another board which didn t have long wires. Another problem is that the switch capacitors chip is apparently damaged and must be replaced. Reference The art of electronics Paul Horowitz and Winfield Hill