FCC Commercial Element 3 Exam Preparation

Comprehensive practice flashcards created from the 2009 FCC Commercial Element 3 Question Pool, covering electronics principles, circuits, digital logic, transmitters, receivers, and navigation systems.

What formula determines the inductive reactance of a coil if frequency ($f$) and inductance ($L$) are known?

$$X_L = 2\text{π} f L$$

What is the out-of-phase power associated with inductors and capacitors called?

Reactive power

What term describes the opposition to the creation of magnetic lines of force in a magnetic circuit?

Reluctance

What metal is usually employed as a sacrificial anode for corrosion control purposes?

Zinc bar

What is the phenomenon of skin effect in electrical conductors?

RF current flows in a thin layer of the conductor, closer to the surface, as frequency increases.

What is the primary purpose of a bypass capacitor?

It removes alternating current by providing a low impedance path to ground.

In Figure 3B1, what is the voltage drop across resistor $R_1$?

$$5\,V$$

In Figure 3B2, what is the voltage drop across resistor $R_1$?

$$9\,V$$

What is the phase relationship between the two signals shown in Figure 3B3?

B is lagging A by $90^\circ$

After two time constants, to what percentage of the supply voltage is a capacitor in an RC circuit charged?

$$86.5\%$$

What turns ratio does a transformer need to match a source impedance of $500\,\Omega$ to a load of $10\,\Omega$?

$$7.1 \text{ to } 1$$

In Figure 3C4, what would be the effect of installing a small variable capacitor in place of the dashed line?

Decrease parasitic oscillations

In Figure 3C5, which capacitor (labeled 1 through 4) is used as a bypass capacitor?

3

What device is usually used to establish a stable reference voltage in a linear voltage regulator?

Zener diode

Which device acts as two SCRs connected back-to-back in opposite directions with a common gate?

TRIAC

What is the principal characteristic feature of a tunnel diode?

Negative resistance region

What is the main advantage of using an op-amp audio filter over a passive LC audio filter?

Op-amps exhibit gain rather than insertion loss.

In the op-amp oscillator circuit shown in Figure 3D6, what occurs if the capacitance of C is suddenly doubled?

Frequency would be lower.

Given the combined DC input voltages in Figure 3D7, what is the output voltage?

$$-5.5\,V$$

Which lamps are lit in the circuit shown in Figure 3D8?

2, 3, 4, 7 and 8

In Figure 3D10, which output waveform corresponds to a square wave input into the integrator?

3

With a pure AC signal input in Figure 3D11, which output waveform appears on an oscilloscope display?

2

What is the valid logic low input voltage range for a TTL device operating at $5.0\,V$?

Zero to $0.8\,\text{volts}$

For the logic input levels shown in Figure 3E12, what are the logic levels at test points A, B, and C?

A is low, B is high and C is high.

For the logic input levels given in Figure 3E13, what are the logic levels at test points A, B, and C?

A is high, B is high and C is high.

How many flip-flops are required to divide a signal frequency by 4?

2

What type of digital multivibrator circuit alternates continuously between two unstable states?

Astable multivibrator

What is the limiting condition for sensitivity in a communications receiver?

The noise floor of the receiver

In Figure 3F15, which block diagram symbol (labeled 1 through 4) represents a local oscillator?

2

Regarding the filter output characteristics shown in Figure 3F16, how are curves A and D categorized?

A is a low pass curve and D is a band stop curve.

What class of amplifier is distinguished by output current flowing for the entire $360^\circ$ of the signal cycle?

Class A

Which matching network provides the greatest harmonic suppression?

Pi-L network

In Figure 3H17, which block diagram symbol (labeled 1 through 4) represents where audio intelligence is inserted?

2

What is the average fully-charged terminal voltage of a lead-acid storage cell?

$$2.06\,\text{volts}$$

What is meant by the term radiation resistance of an antenna?

An equivalent resistance that would dissipate the same amount of power as that radiated from an antenna.

What is the frequency range of Distance Measuring Equipment (DME) in aircraft navigation?

$$962\,\text{MHz} \text{ to } 1213\,\text{MHz}$$

At what position relative to a VOR station are the variable phase and reference phase signals in phase with each other?

$$360^\circ \text{ North, magnetic bearing position}$$

What instrument is used to check the signal quality of a single-sideband radio transmission?

Oscilloscope

What is the channel spacing used for VHF marine radio?

$$25\,\text{kHz}$$

Which primary frequency is used exclusively for international NAVTEX broadcasts?

$$518\,\text{kHz}$$

What is the normal range of pulse widths in shipboard RADAR systems?

$$.05\,\mu s \text{ to } 1.0\,\mu s$$

What is the orbiting altitude of the Iridium satellite communications system?

$$485\,\text{miles}$$

Over how many minutes are Maximum Permissible Exposure (MPE) RF levels averaged for controlled environments?

$$6\,\text{minutes}$$