## Wednesday, March 11, 2009

### Extra Class Exam Grab Bag, #2, Bob, Session 42

Tonight we will take a little bit deeper look at some of the extra class exam questions. I hope that by the end of this session you will know how to answer these questions without having to memorize them and that you will have gained a little better understanding of some important concepts.

The first couple of questions will be a review of one of Lee's excellent impedance series sessions. Specifically it is Part 9 if you want to go back to review it.

E8A04 (C)
What is the equivalent to the root-mean-square value of an AC voltage?
A. The AC voltage found by taking the square of the average value of the peak AC voltage
B. The DC voltage causing the same amount of heating in a given resistor as the corresponding peak AC voltage
C. The DC voltage causing the same amount of heating in a resistor as the corresponding RMS AC voltage
D. The AC voltage found by taking the square root of the average AC value
~~

This is based on the concept that root-mean-square (RMS) of AC gives you an indication of the power provided by that AC signal. As Lee pointed out in his session. The resistor will heat up until it reaches some equilibrium point. At that point it is dissipating as much heat as it is generating. You can be assured that it is the same amount of average power generated no matter what kind of waveform you pass through it. In the two special cases of a flat DC current and an AC sine wave, you have this special relationship where the root of the mean of the square of the sine wave will yield the DC voltage that would heat the resistor to the same temperature. This means that the power provided to that resistor is the same for the RMS AC and the DC.

Related question:
E8A05 (D)
What would be the most accurate way of measuring the RMS voltage of a complex waveform?
A. By using a grid dip meter
B. By measuring the voltage with a D'Arsonval meter
C. By using an absorption wavemeter
D. By measuring the heating effect in a known resistor
~~

Here are a few related questions on digital signals:
E8A12 (D)
What type of information can be conveyed using digital waveforms?
A. Human speech
B. Video signals
C. Data
D. All of these answers are correct
~~
This should be obvious to all considering that we have phased out analog cell phones so that all human speech is using digital waveforms there and with all the recent news about digital TV there should be no question that video signals can be carried by digital waveforms. Data is what digital waveforms are primarily for.

E8A13 (C)
What is an advantage of using digital signals instead of analog signals to convey the same information?
A. Less complex circuitry is required for digital signal generation and detection
B. Digital signals always occupy a narrower bandwidth
C. Digital signals can be regenerated multiple times without error
D. All of these answers are correct
~~
This one could be a little tricky considering it could appear to be D, all of the above. But A is definitely not always true and in fact is usually not true in my opinion. B is often true but not always and this answer specifically says always. But C, signals can be regenerated multiple times without error is true and is one of the defining differences between digital and analog signals.

E8A14 (A)
Which of these methods is commonly used to convert analog signals to digital signals?
A. Sequential sampling
B. Harmonic regeneration
C. Level shifting
D. Phase reversal
~~
I have to admit that I have only half educated guesses of what harmonic regeneration, level shifting and phase reversal are or how they would be used. What I do know is that they would not be used to convert analog signals to digital. The common way (note the question said common) to convert analog to digital is to measure the amplitude at specific times and transmit that amplitude information. You are taking a sample of the waveform sequentially that is to say in regular steps of time, one after another.

E8A15 (B)
What would the waveform of a digital data stream signal look like on a conventional oscilloscope?
A. A series of sine waves with evenly spaced gaps
B. A series of pulses with varying patterns
C. A running display of alpha-numeric characters
D. None of the above; this type of signal cannot be seen on a conventional oscilloscope
~~
This one also is a bit tricky as you might think that A should be a possible answer. It's possible that a modulated digital signal could look like this but the straight digital signal will look like B, a series of pulses with varying patterns.