General RF Electronics
Answers to RF Cafe Quiz #4

All RF Cafe quizzes would make perfect fodder for employment interviews for technicians or engineers - particularly those who are fresh out of school or are relatively new to the work world. Come to think of it, they would make equally excellent study material for the same persons who are going to be interviewed for a job.

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Note: Many answers contain passages quoted in whole or in part from the text.

Return to RF Cafe Quiz #4

1. Which of the following can cause frequency intermodulation products in a system?

c) Both a) and b)

Intermodulation products are generated whenever currents of different frequencies flow in a nonlinear junction. Dissimilar metals and impurities at connector interfaces and in cable dielectric materials form nonlinear junctions. Ferromagnetic materials in isolators and circulators enter into a nonlinear region when in saturation. At low powers, the products all have powers below thermal noise, but when used in high power transmission systems, these Passive Intermodulation (PIM) products can and often do fall within the receiver band. Read more here.

2. What is the melting temperature of standard
    60/40, tin/lead solder?
c) 186°C (386°F)

Knowing the melting temperature of the solder you use all the time is useful information. Look here for a list of other solder alloy melting temperatures.

3. What is the frequency band for the 900 MHz GSM cellular band?
a) Tx: 880-915 MHz / Rx: 925-960 MHz

Click here for a matrix of wireless communications bands and protocols.

4. What does GSM stand for?

c) Global System for Mobile Communications

Click here for a matrix of wireless communications bands and protocols.

* GSM originally stood for Groupe Spécial Mobile - thanks to Eric for this info.

5. What does POTS stand for (in communications)?

a) Plain Old Telephone System

Maybe it's too simple to be true, but it is.

6. Which of these pairs of materials in the triboelectric series have the greatest charge transfer potential?

c) Glass & Hard Rubber

The triboelectric series is a list of materials that, when rubbed together, transfer charge from one to the other. The farther apart the two materials are on the series, the greater the charge that is transferred. See here for a table of common materials.

7. Along which side of rectangular waveguide is an "E" bend made?

b) Shorter side

A common mnemonic employed to help remember is that an E-Bend is bent in the Easy direction (along the short side). This is the direction of the E-field in the TE10 mode. Conversely, An H-Bend is bent in the Hard direction (along the long side). This is the direction of the H-field in the TE10 mode.

8. What is the lowest modulation index at which an FM carrier is suppressed?

a) 2.40

Depending on the modulation index (m) chosen, the carrier and certain sideband frequencies may actually be suppressed. Zero crossings of the Bessel functions, Jn(b), occur where the corresponding sideband, n, disappears for a given modulation index, b. The carrier is the 0th sideband, so n=0. The next time the carrier disappears is for m=5.49. Here is more information.

9. How much current is required through the human body to cause an onset to muscular paralysis during electrocution?

b) 21 mA

Most non-EE's think voltage causes electrocution, but we know better (don't we?). A higher voltage causes a higher current to flow, but ultimately it is the current that cause body tissues to react. Click here for more current levels.

10. At what frequency is electromagnetic energy maximally absorbed due to oxygen in the atmosphere?

c) 63 GHz

Both water and oxygen absorb electromagnetic energy in the atmosphere. Oxygen has its first peak at around 22 GHz, but a much larger peak occurs at 63 GHz due to oxygen. Secure communications between satellites in orbit use the 63 GHz band to prevent interception by terrestrial receivers. Clever - eh? Check out this absorption chart.