A taste of teaching


Prof. KRK Rao who is now working with TI had developed a kit which could potentially be a big help for undergraduate lab courses.

Most of the undergraduate labs use breadboards to make the circuits. Breadboards are very unreliable and can lead to a lot of time being spent debugging peripheral issues like loose connections, coupling effects, noise pickup and wrong connections resulting from confusion due to messy wiring and lack of space. Lab time is always precious and the more time spent on such minor details, the less time available for gaining greater insight into the system being built.

There is also the problem with inadequate focus on such fundamental analog elements like PLL and other feedback systems that are handled usually towards the end off the lab courses and that too with too little time devoted to them and their theory of operation. The kit and the accompanying manual describing various experiments which are arranged in such a manner as to allow the student to gain an overview of analog fundamentals and to more importantly realize the power and scope of analog systems can in my opinion be used to good effect by our engineering colleges.

TI through its university relations wing has been trying to get the kit introduced to the various universities in our country so that they can either incorporate it into their curriculum or use the ideas to make changes to their own programs.

The first part of the program involved a familiarization program for the faculty for which I had volunteered to help with a lab session. For the second part which was meant for students I had volunteered to handle a day’s classes and lab session.

It was a very instructive and exciting experience for me. I was meant to take Function Generator and VCO for them. There were 6 students attending the class and all of them seemed eager to get on with the day’s program. I had given a brief class on the Schmitt Trigger and various related concepts like Regenerative Posititve Feedback, hysteresis and voltage controlled resistances the previous week after my friend Joyan was done with his class. I had also given them a couple of problems to work out which were related to the function generator.

When I was attending my entrance coaching classes I always liked it when the teacher gave us problems to work out. The frenzied thinking and solving to get at the answer before anyone else and the enjoyment that comes from sharing the answer with everyone on the board was alone enough to motivate me at times to try harder. I used the same tactic this time too. Letting them solve all the fundamental equations for period of oscillation, duty cycle variations due to various offsets, sensitivity etc. and asking them to come up to the board and write out their solution.

I kept the theory class short and then let them get on with the lab session by 10:45. By 12:15 they had all completed the Function generator and made the necessary readings. After lunch they proceeded to insert the multiplier into the loop to get voltage control of frequency. Then a weird problem struck all of them. The multiplier was behaving very strangely. The output high was not proper. There was some high frequency oscillation at output low. etc etc.. Everyone tried different multipliers and different boards. I even called up Joyan to ask him whether he had faced the problem during the last class.  After a while everyone was starting to lose their patience.

Then I simply went and increased the supply voltage from 10 to 12 volts and voila all the problems disappeared instantly. It was actually surprising considering that they had been using the same boards at 10 volts supply until then. Anyway, I still haven’t managed to find out why the lower supply voltage suddenly stopped being enough for the multiplier. By then we had lost quite a bit of time.

After the VCOs were ready they measured the Kvco and center frequency. Then, I proceeded to give them a short class on PLL operation and also provided them with an overview of how feedback systems can be used to perform inverse operations with the help of a high loop gain.

By 5 they started with the PLL. This was not part of the plan and it was actually a little too ambitious, trying to do everything together the same day. But, I felt that since the VCOs were ready and working it would be perfect if we could quickly put together a PLL. It would have meant optimum use of the lab hours put in up to that point. It proved to be quite a struggle since many of them were not quite clear about the circuit diagram and the correct way of putting it together.

But, they kept on trying. The class was supposed to end by 5:30. But even after 6:45 we were still trying to get the output. I was starting to lose heart by then. Then, one of the guys got the output! It was such a big relief for me as I would have been disappointed if not even one of them had got the output after sitting till 7 O clock.

No matter how many times you see it or how simple it all really is, seeing a PLL capture a signal and struggle to maintain lock at it’s limits is really magical. The phase slowly oscillating into lock as the signal comes into range is a very satisfying sight to watch!

I used the working experiment to demonstrate a few basic principles to them and then let them leave for the day. I really enjoyed the day and it gave me a taste of the joy of teaching!

I made myself familiar with all guys and girls who had come and hope to keep in touch with them in the future too!

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