General: Frequency Deviation

What is the frequency deviation for a 12.21 MHz reactance modulated oscillator in a 5 kHz deviation, 146.52MHz FM phone transmitter? [G8B07]

A. 101.75 Hz
B. 416.7 Hz
C. 5 kHz
D. 60 kHz

Whoa, what?  Guess what, dust off that calculator, because here’s where it starts to get mathy.  This question is also terribly worded.

What we’re looking for is the oscillator deviation.  The formula to calculate this is:

Oscillator Deviation = \frac{Transmitter Deviation}{(\frac{Output Frequency}{Oscillator Frequency})}

Simplified, it becomes this:

Oscillator Deviation = \frac{(Transmitter Deviation)(Oscillator Frequency)}{(Output Frequency)}

If we plug in our numbers, like so, don’t forget to use consistent units, so we must turn our 5kHz into .005MHz.

Oscillator Deviation = \frac{(.005)(12.21)}{(146.52)}

Solving…

Oscillator Deviation = .0004167 MHz, or 416.7 Hz, after we move the decimal six (mega!) places.

So our answer is B. 416.7 Hz

Now, mathematically you could think of this as a ratio between the oscillator and transmitter deviations and frequencies.  The equation for that looks like this, and winds up the same when you solve it….

\frac{Oscillator Deviation}{Oscillator Frequency} = \frac{Transmitter Deviation}{Transmitter Output Frequency}

Technician: Electrical Current in the Body

How does current flowing through the body cause a health hazard [T0A02]

A. By heating tissue
B. It disrupts the electrical functions of cells
C. It causes involuntary muscle contractions
D. All of these choices are correct

Lets run down the possible answers, and see what we come up with….

A. By heating tissue.  Your body has a native resistance value to it.  Since you’re basically made of water and other stuff, its actually pretty low, but current flowing in your body will generate heat.  Higher current == higher temperatures.  Not cool.

B. It disrupts the electrical functions of cells.  True. Combined with the heating effect cells can be permanently damaged.  Nerve cells which rely on electrical impulses, are especially vulnerable.

C. It causes involuntary muscle contractions.  Also true.  See answer B.  Those nerve cells can interpret that electricity as instructions, and that can cause involuntary contractions.  This is why you NEVER try to grab somebody who is being electrocuted and cannot let go of what is causing it…  you will likely be unable to let go as well.

Since these are all correct, the correct answer is, of course, D!

Radio Receiver

Amateur Extra: Minimum Discernable Signal

The Amateur Extra torture question of the week is taken from sub-element 4 (amateur practices) section C (receiver performance) [E4C07]

What does the MDS of a receiver represent?

A. The meter display sensitivity
B. The minimum discernible signal
C. The multiplex distortion stability
D. The maximum detectable spectrum

If you stop to think about the possible answers, as they relate to radio receivers, options C and D don’t look very good.  They actually look sort of like gibberish.  Let’s get rid of those.

Option A, Meter Display Sensitivity, sounds like a plausible answer.  But its wrong.  The correct answer is B. The minimum discernible signal, or minimum detectable signal

But what is MDS, anyway?

It’s the lowest power signal your receiver can be received by the input, and then turned into any sort of useful information.  Basically the smallest signal that would be audible (or visible) over background noise.

The actual value of MDS is dependent on a few factors, including bandwidth and temperature.  Theoretically, the lower you can get the temperature of your receiver, the weaker the signal you should be able to detect.

Electrical Components

General: Resistance and Temperature

The General class exam question of the week is from sub-element 6 (circuit components) section A (resistors) [G6A06]

What will happen to the resistance if the temperature of a resistor is increased?

A. It will change depending on the resistor’s reactance coefficient
B. It will stay the same
C. It will change depending on the resistor’s temperature coefficient
D. It will become time dependent

The answer is C. It will change depending on the resistor’s temperature coefficient.  That number basically defines how the material in the resistor reacts to temperature.

A resistor doesn’t have a “reactance coefficient”, as reactance is a property of circuits and elements that respond to AC (Alternating Current.)  It might be measured in Ohms, but the resistor doesn’t particularly care either way.

Staying the same would be ideal, but that is never the case.  And becoming “time dependent” really doesn’t make sense here, unless you’re applying heat, in which case the temperature would continue to increase…. but the resistor’s response to that temperature would still be defined by the temperature coefficient.