Decibels and dBm Quiz
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Questions and Answers

What does a decibel (dB) value indicate when referring to a circuit's gain or attenuation?

  • It is a fixed unit of measurement for power.
  • It represents a comparison between two power values. (correct)
  • It describes the frequency of the signal.
  • It provides the absolute power values.

What is the reference level commonly used in communication when calculating dBm?

  • 10 mW
  • 1 W
  • 0.1 mW
  • 1 mW (correct)

In the formula for dBm, what does the value 0.001 represent?

  • 1 mW expressed in watts. (correct)
  • The output power value.
  • The threshold of hearing.
  • The standard decibel value.

The output of a 1-W amplifier expressed in dBm is calculated as?

<p>40 dBm (D)</p> Signup and view all the answers

What happens to the units when computing a ratio for decibels?

<p>They are canceled. (D)</p> Signup and view all the answers

What does dBm represent when computed with respect to 1 mW?

<p>A relative power value. (C)</p> Signup and view all the answers

What is the meaning of 0 dB in the context of sound measurements?

<p>It represents the least perceptible sound. (D)</p> Signup and view all the answers

When output power, Pout, is expressed in dBm, what is the impact of this value?

<p>It eliminates the need for absolute power values. (B)</p> Signup and view all the answers

What occurs at resonance in a parallel resonant circuit?

<p>The inductive and capacitive reactances are equal. (A)</p> Signup and view all the answers

Why is the impedance of the inductive branch higher than that of the capacitive branch at resonance?

<p>The coil resistance contributes to higher impedance. (B)</p> Signup and view all the answers

What is implied by the statement that the capacitive current is slightly higher than the inductive current?

<p>A net capacitive behavior is present in the circuit. (D)</p> Signup and view all the answers

In a practical parallel resonant circuit, what aspect of the inductor impacts the overall performance?

<p>The resistance within the coil. (B)</p> Signup and view all the answers

What does the figure imply about the phase relationships in a parallel resonant circuit?

<p>Inductive current lags behind capacitive current. (A)</p> Signup and view all the answers

What does formula (1) represent in terms of circuit calculations?

<p>Voltage gain or attenuation (C)</p> Signup and view all the answers

When calculating the decibel gain using formula (2), what is the significance of the logarithm?

<p>It compresses the scale of ratios for easier calculation (B)</p> Signup and view all the answers

What is the correct interpretation if the calculated decibel figure is negative?

<p>There is an attenuation in the circuit (C)</p> Signup and view all the answers

How do you calculate the overall gain or attenuation of a circuit with multiple stages?

<p>By adding the decibel gain and attenuation factors of each circuit (B)</p> Signup and view all the answers

What is the formula to calculate power gain or attenuation?

<p>dB = 10 log(Pout / Pin) (D)</p> Signup and view all the answers

If an amplifier has an input voltage of 3 mV and an output voltage of 5 V, what is the calculated gain in decibels?

<p>64.4 dB (D)</p> Signup and view all the answers

Which of the following scenarios will also result in a negative decibel figure?

<p>When the output power is less than the input power (C)</p> Signup and view all the answers

What does the result of 213.98 dB signify in a given circuit?

<p>Significant power attenuation (D)</p> Signup and view all the answers

What is the primary reason for keeping lead lengths short in RF circuits?

<p>To minimize stray or distributed inductance. (C)</p> Signup and view all the answers

The quality factor Q is defined as the ratio of which two types of power?

<p>Inductive power to resistive power. (D)</p> Signup and view all the answers

If an inductor has a resistance of 45Ω and a calculated quality factor Q of 37.68 at a frequency of 90 MHz, what is the inductive reactance XL?

<p>1695.6Ω (C)</p> Signup and view all the answers

At what frequency are resistors likely to behave as complex RLC circuits?

<p>At high frequencies. (A)</p> Signup and view all the answers

Which situation would not significantly affect the performance of an RF circuit?

<p>Low resistor wattage. (C)</p> Signup and view all the answers

What does stray capacitance between turns in an inductor primarily affect?

<p>Inductive reactance. (D)</p> Signup and view all the answers

In a high-frequency circuit, what can lead to significant performance issues?

<p>Excessive stray capacitance. (C)</p> Signup and view all the answers

Which formula represents the calculation of quality factor Q for an inductor?

<p>Q = $\frac{XL}{R}$ (B)</p> Signup and view all the answers

What happens to the current in a series resonant circuit when the frequency is very low?

<p>The current is very low due to high capacitive reactance. (B)</p> Signup and view all the answers

What is the phase relationship between current and voltage just at resonance in the circuit?

<p>Current is in phase with voltage (0°). (A)</p> Signup and view all the answers

Which statement describes the behavior of the reactances as frequency increases?

<p>Inductive reactance increases while capacitive reactance decreases. (D)</p> Signup and view all the answers

What is the expected voltage drop relationship between a capacitor and inductor below resonance?

<p>Capacitor voltage drop is higher than inductor voltage drop. (D)</p> Signup and view all the answers

What occurs in a resonant circuit when the inductive reactance surpasses the capacitive reactance?

<p>The circuit becomes predominantly inductive, causing the current to lag. (C)</p> Signup and view all the answers

How does the impedance in a series resonant circuit change as the frequency moves from below resonance to above resonance?

<p>Impedance decreases below resonance and increases above it. (D)</p> Signup and view all the answers

What defines the bandwidth (BW) of a series resonant circuit?

<p>The difference between the resonant frequency and the frequencies at which the current falls to 0.707Ipeak. (B)</p> Signup and view all the answers

What is the relationship between current and voltage when the current peak occurs in a resonant circuit?

<p>Current is in phase with voltage (0°). (B)</p> Signup and view all the answers

How does connecting an external resistor affect the quality factor (Q) of a parallel tuned circuit?

<p>It decreases Q and increases bandwidth. (C)</p> Signup and view all the answers

What is the formula used to calculate the equivalent resistance (RP) at resonance for a parallel tuned circuit?

<p>RP = RW * (Q^2 + 1) (A)</p> Signup and view all the answers

Given a circuit with RW = 10 Ω and Q = 30, what is the equivalent resistance (RP) at resonance?

<p>9010 Ω (D)</p> Signup and view all the answers

If the required bandwidth (BW) is 1 MHz and the resonant frequency (fr) is 10 MHz, what is the necessary Q?

<p>5 (A)</p> Signup and view all the answers

What is the new total resistance (RPnew) when setting a parallel tuned circuit bandwidth to 1 MHz with an external resistor?

<p>3000 Ω (D)</p> Signup and view all the answers

How is the external resistor (Rext) determined in a parallel tuned circuit?

<p>Rext = RPnew - RP (D)</p> Signup and view all the answers

What is the defining characteristic of a passive filter?

<p>It uses only passive components. (C)</p> Signup and view all the answers

What primarily distinguishes a filter from other types of circuits?

<p>A filter selectively passes or rejects frequencies. (B)</p> Signup and view all the answers

Flashcards

Decibel Formula for Voltage Gain

dB = 20 log (Vout/Vin). Used to calculate voltage gain or attenuation in decibels.

Decibel Formula for Current Gain

dB = 20 log (Iout/Iin). Used to calculate current gain or attenuation in decibels.

Decibel Formula for Power Gain

dB = 10 log (Pout/Pin). Used to calculate power gain or attenuation in decibels.

Positive Decibel Value

Indicates circuit gain.

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Negative Decibel Value

Indicates circuit attenuation.

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Calculating Overall Gain

Summing the decibel gain and attenuation factors of each circuit stage to determine the overall decibel gain or attenuation of the circuit or system.

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dB Example (Voltage)

Calculating decibel gain with voltage values, e.g., 3 mV input and 5 V output

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dB Example (Power)

Calculating decibel attenuation with power values, e.g., 50 mW input and 2 mW output

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Decibel (dB)

A unit used to express gain or attenuation in a circuit, representing a ratio between output and input power or voltage.

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dBm

A specific type of decibel measurement, where the reference power is 1 milliwatt (mW).

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Decibel Formula (dBm)

dBm = 10 log (Pout/0.001), where Pout is the output power in watts, and 0.001 represents 1 mW in watts.

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Reference Level

A known value (often 1 mW) to which other power values are compared when using dBm.

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Output Power (in a circuit)

The power produced by a circuit or device, often expressed in watts.

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Dimensionless ratio

A ratio that doesn't have units.

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dB relation to sound

0 dB is the least audible sound, 120 dB is the pain threshold.

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Absolute Value (Power)

The actual physical power amount in watts, not a relative comparison.

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Coil Resistance

The resistance of the wire used to create an inductor (coil).

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Stray Capacitance

Unwanted capacitance between turns of an inductor or other components.

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Quality Factor (Q)

Ratio of inductive power to resistive power in an inductor.

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Inductive Reactance

Opposition to changing currents in an inductor at high frequencies.

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Resistor at High Frequencies

behaves like a complex RLC circuit with additional inductance and capacitance.

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Important to Keep Lead Length Short (RF)

In radio frequency (RF) circuits, short leads help minimize stray inductance.

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Q calculation

Calculated as the ratio of inductive reactance to resistance.

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RLC circuit

Combination of resistance (R), inductance (L), and capacitance (C) that affect a circuit's behavior.

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Parallel Resonance

A condition in a parallel RLC circuit where the inductive reactance (XL) equals the capacitive reactance (XC). At resonance, the impedance of the circuit is at a maximum, leading to minimal current flow.

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Inductive Branch Impedance

In a parallel resonant circuit, the impedance of the inductive branch includes the coil's resistance (Rw) along with its inductive reactance (XL).

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Capacitive Current

In a parallel resonant circuit, the current flowing through the capacitor branch is slightly higher than the inductive current.

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Practical Parallel Resonant Circuit

A parallel resonant circuit where the inductor has inherent resistance, making the impedance not perfectly infinite at resonance.

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Phase Relationships

In a parallel resonant circuit, the current through the capacitor (IC) leads the voltage by 90 degrees, and the current through the inductor (IL) lags the voltage by 90 degrees.

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What happens below resonance?

The capacitive reactance (XC) is higher than the inductive reactance (XL). This makes the net reactance capacitive, leading to a leading current in the circuit. The capacitor voltage drop is higher than the inductor voltage drop.

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What happens at resonance?

The inductive reactance (XL) and capacitive reactance (XC) cancel each other out, leaving only the resistance. This results in the current peaking and being in phase with the voltage.

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What happens above resonance?

The inductive reactance (XL) is higher than the capacitive reactance (XC). This makes the net reactance inductive, leading to a lagging current in the circuit. The inductor voltage drop is higher than the capacitor voltage drop.

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Series resonant circuit response

The response of a series resonant circuit is characterized by a peak current at the resonant frequency (fr). Below fr, the current leads the voltage (capacitive), and above fr, the current lags the voltage (inductive).

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What does the frequency response curve look like?

The frequency response curve of a series resonant circuit shows a peak in the current at the resonant frequency (fr). The phase shift is positive (leading) below resonance, negative (lagging) above resonance, and zero at resonance.

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What is bandwidth?

The bandwidth (BW) of a series resonant circuit is the range of frequencies where the current is at least 70.7% of its peak value. It is calculated as the difference between the two frequencies (f1 and f2) where the current is 0.707 times the peak current.

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What happens at very low frequencies?

The capacitive reactance is much greater than the inductive reactance. This results in high impedance and very low current. The current leads the voltage by nearly 90°.

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What happens as frequency increases?

As frequency increases, the capacitive reactance decreases and the inductive reactance increases. The current starts to rise as the reactances approach each other. When they are equal, the impedance is minimal, resulting in peak current.

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Parallel Tuned Circuit Bandwidth

The range of frequencies that a parallel tuned circuit allows to pass through effectively. It's measured in Hertz (Hz).

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Q Factor in Circuits

A measure of the sharpness of a resonant circuit. It quantifies how selective a circuit is to a specific frequency.

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How does Q affect bandwidth?

Higher Q leads to a narrower bandwidth, while lower Q leads to a wider bandwidth.

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How to adjust bandwidth?

You can adjust a parallel tuned circuit's bandwidth by changing the Q factor using an external resistor.

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What's RP?

The equivalent resistance of the parallel circuit at resonance. It determines the initial bandwidth.

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How does external resistor affect RP?

Connecting a resistor in parallel lowers the total resistance (RPnew) of the circuit. This changes the Q factor and therefore the circuit's bandwidth.

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Calculating Rext

The value of the external resistor (Rext) needed to achieve the desired bandwidth is calculated based on RP, RPnew, and the target Q.

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Relationship Between Q, RP, and Bandwidth

The Q factor is directly proportional to the resonant resistance (RP) and inversely proportional to the bandwidth. This means higher RP leads to higher Q and narrower bandwidth.

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Filters

Circuits specifically designed to allow certain frequencies to pass through while blocking others.

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Passive Filters

Filters made using resistors, capacitors, and inductors - no active components like transistors or amplifiers.

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LC Filters

A type of passive filter often used in communication systems, built using inductors (L) and capacitors (C).

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Study Notes

Electronic Fundamentals for Communications

  • Communication electronics requires understanding basic electronic principles, including AC and DC circuits, semiconductor operation, and basic circuit operation (amplifiers, oscillators, power supplies, digital logic circuits).
  • Key concepts include voltage, current, gain, attenuation expressed in decibels, LC tuned circuits, resonance, filters, and Fourier theory.
  • Students should be able to calculate voltage, current, gain, attenuation in decibels for cascaded circuits.
  • Explain the relationship between Q, resonant frequency, and bandwidth.
  • Describe various filter configurations (active and passive) and their differences in selectivity.
  • Explain the benefits and operation of crystal, ceramic, and SAW filters.
  • Calculate bandwidth using Fourier analysis.

Gain, Attenuation, and Decibels

  • Gain is amplification, the ratio of output to input voltage (Vout/Vin).
  • Attenuation is a loss in signal amplitude, the ratio of output to input is less than 1 (Vout/Vin).
  • Decibels (dB) are a logarithmic unit representing gain or loss.
  • Power gain (Ap) is calculated as Pout/Pin, where Pin is input power and Pout is output power.
  • Cascaded circuits' total gain is the product of individual stages' gains.

Tuned Circuits

  • Tuned circuits and filters include inductors, capacitors, and potentially resistors.
  • Reactance (Xc) is opposition to AC current for capacitors.
  • Xl is inductive reactance.
  • Resonant frequency (fr) is the frequency at which inductive and capacitive reactances are equal.
  • Bandwidth (BW) is the frequency range over which the current is high (centered around fr).
  • Q (quality factor) is a measure of selectivity; higher Q means narrower bandwidth (fr/BW).

Filters

  • Filters select frequencies, passing some and attenuating others.
  • Passive filters use passive components (resistors, capacitors, inductors).
  • Active filters use amplifiers and passive components -they can amplify and filter.
  • Types of filters include high-pass, low-pass, bandpass, band-reject/notch.
  • RC filters use resistors and capacitors to achieve filtering effects.
  • LC filters use inductors and capacitors used at higher frequencies.
  • Other types include switched capacitor and SAW filters (based on surface acoustic waves).

Decibels

  • dB is 10 times the base 10 logarithm of the ratio of two power values (Pout/Pin).
  • dB Voltage is calculated by multiplying the dB power result by 20.
  • dBm is a power level relative to a 1 mW reference.

Other

  • Crystal and ceramic filters are used for high selectivity, at higher frequencies with stable performance.
  • SAW filters utilize surface acoustic waves for high-frequency applications.
  • Switched capacitor filters use capacitors, transistors, and amplifiers to make tuned circuits with IC implementations.
  • Fourier analysis breaks down complex waveforms into a sum of sine waves.
  • Rise time is the time it takes a signal to transition from 10% to 90% of its final amplitude.
  • Bandwidth and rise time are related; wider bandwidths allow faster signal rise/fall times.

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Related Documents

Frenzel - Principles of Electronic Communication Systems 4th Ed, Part 2 PDF

Description

Test your knowledge on decibel values, circuit gain, and attenuation with our Decibels and dBm Quiz. This quiz covers essential concepts such as reference levels in communication and the calculation of dBm from amplifier outputs. Challenge yourself and deepen your understanding of these critical measurement units!

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