I felt like I was watching the movie “Close Encounters of the Third kind” by Steven Spielberg. This movie is about encounters with the extra terrestrial. While close encounters with IT professionals is common today in Bangalore, encounters with ET professionals or professional ETs is a bit rare. Why was I then thinking about aliens and extra terrestrials? We were sitting in the TI cafeteria eating lunch with KRK Rao and he started this story.
“I was once witness to an experiment where the intention was to intercept signals from outer space to see if there is any intelligence in those signals,” KRK said. “In this situation, the input signal is very noisy and elimination of noise is a great challenge.”
I could barely hear him speak in the din of the plates and spoons and the chatter in all directions. My ears were trying very hard to eliminate the noise and catch the signal. I was now ready to appreciate the concept of noise elimination.
Noise Elimination using the UAF
Removing power supply noise is important in all applications, especially in medical appliances. We are beginning to witness numerous applications of electronics in the medical domain – hearing aids, ECG, EEG, glucose meters and pulse oximeters to name a few. In the signal chain of a medical electronic appliance, power supply noise must be eliminated from the input analog signal itself, not the converted digital counterpart. (It is an exercise for you to understand why – see the quiz at the end of this post.)
“The UAF is a great choice for eliminating noise,” said KRK.
Since I was still thinking of close encounters with the extra terrestrial, I wondered if UAF stood for Unidentified Alien Footprints. I am glad I did not ask. For KRK, UAF must mean Undying Analog Fixation, I thought. I was wrong again.
“The Universal Active Filter, discussed as an experiment in the Analog System Lab Kit (ASLK) can be used to eliminate power supply noise,” said KRK, and proceeded to explain the concept of the UAF. You may watch a video of this explanation here. The filter is called “universal,” because the same circuit can perform several filter operations – bandpass, band elimination, high-pass, and low-pass. One of the most important aspects in designing a filter is that it should be tunable– this is because the power supply frequency itself is subject to variation. The “design trick” is to design a band elimination filter with a very narrow band (“notch”) and center it at a higher frequency, say 150 Hz. Then modify the circuit to use the concept of “voltage controlled resistance” and use a control voltage to adjust the notch filter frequency automatically.
A Contest!
It so happens that I was looking for a challenge to throw at students who were taking part in a college-level analog design contest. (We have deployed the college-level contests through the Texas Instruments India University Program, with the goal of exciting the students about analog system design) Not surprisingly, we came up with the following problem statement. (Many thanks to KRK for the problem!)
Figure 1 shows one such analog filter composed of four operational amplifiers.
a) Simulate the filter operation using software such as TINA or PSPICE. Student versions of these softwares are available free. Verify that the circuit behaves like a band-elimination filter. What is the elimination band
b) Construct the circuit using the Analog System Lab Starter Kit (ASLK Starter) or the ASLK Pro. Note that the passive components on the ASLK have a tolerance of + 10%. What is the elimination band that you observed?
c) Modify the circuit of Figure 1 to make it “tunable.” Hint: you will need the analog multipliers as phase detectors on ASLK. Read the Analog System Design Manual– Experiment No.5 to find out how to make a filter voltage-tunable.
a) Tune the filter to have a notch exactly at 50 Hz. Apply 50 Hz square wave signal 100mV amplitude to the input and observe the waveform at the notch output. How will you ascertain that it is tuned exactly to 50 Hz? What additional circuitry is needed to make it automatically get tuned to 50 Hz? Implement the automatic tuning scheme, when 50 Hz is dominantly present in the signal. Verify the working of your automatic tuning circuit, by simultaneously applying 50 Hz with 100 mV peak sine wave and 100 Hz sine-wave with lower amplitude than 100 mV.
The Winners!
Abhijeet Joshi and Prashutosh Gupta, students of IIT Delhi, took on the challenge of removing unwanted power supply noise from an input signal. Here is their solution.
Solution to Part (a) - Universal Active Filter
Abhijeet and Prashutosh simulated the circuit in TINA and implemented it on ASLKv2010 starter kit from Texas Instruments. The simulation results are shown in Figure 2. VG1 is the input square wave signal, whereas VF1 is the output of a band elimination filter. The frequency response of the Band elimination filter shows an elimination band of 7 Hz about the centre frequency of 159Hz. This is the frequency that was achieved using the suggested values of resistors and capacitors; these resistors and capacitors are readily available on the ASLK.
Figure 1: Universal Active Filter
Figure 2: Transient Response of UAF
Solution to Part (c) - Voltage Controlled Filter
To adjust the frequency electronically, the students used voltage controlled resistances; these, in turn, were implemented using the analog multipliers on the ASLK. Refer to Figure 3. V3 is the control voltage, which can be adjusted to vary the effective resistance seen by the stages U1 and U2.
Figure 3: Voltage Controlled Filter
In this implementation, the centre frequency is controlled by using an external control voltage; the centre frequency is directly proportional to control voltage. If R’ is the effective resistance simulated with the help of an analog multiplier and the control voltage is VC, and the angular centre frequency is w0 then, assume C1 = C2 = C, the following equations will help determine the control voltage required for tuning.
Figure 4: Transient Response of VCF
Part (d) - Self-Tuned Filter
The control voltage can be generated using a multiplier as a phase detector which uses the following two inputs – (a) one of the inputs is the input signal and (b) the other terminal is the output of the High pass or low pass filter stage. The multiplier type of phase detector needs a quiescent phase of 90o. Thus using a filter HP or LP response is more suitable than using the output of the Band pass stage. To obtain a constant control voltage, output of multiplier (U7) is low pass filtered using an integrator. The self-tuned filter is shown in Figure 5 and the simulation results are shown in Figure 6. The time to reach steady state can be reduced by using lower capacitance and resistance values than shown in the schematic.
Figure 5: Self Tuned Filter
Figure 6 – Simulation of the Self-Tuned Filter
VF1- Band elimination filter.
VF2- Control Voltage
VF3- Output of Multiplier
VG1- Input square wave signal.
Testing the self-tuned filter
The student team tested the circuit by applying two tones 100m sin(2π50t) and 50m sin(2π100t), and verifying that the filter tunes itself to fundamental i.e. 50Hz tone. The test setup is shown in Figure 7 and the simulation results are shown in Figure 8.
Figure 7: Two Tone Test
Figure 8:Transient Response of Two Tone Test
VF1- band elimination filter.
VF2- Control Voltage
VF3- Output of Multiplier
VG1- 50Hz Input sine wave signal.
VG2- 100Hz Input sine wave signal.
Transient response Bandpass output (VF5) getting locked to 50Hz signal.
Quiz!
(1) Why should the power supply noise be eliminated using an analog filter? In other words, why is it not possible to convert the input to digital domain and then use a digital filter to eliminate the power supply noise?
(2) The power supply frequency is 60 Hz in Europe. What changes are needed in the designs to adapt to the European system?