System Technician
How many symbols are available at the output of a quadrature phase shift keying (QPSK) modulator?
4 symbols, each containing 2 bits of data
How many doped semiconductor layers are in a junction field effect transistor (J-FET) compared to a bipolar junction transistor?
A J-FET is comprised of only two layers of doped semiconductor material in contrast to that of three doped layers in a BJT.
What type of signal is generated, sent, and received without a ground reference?
A balanced signal is generated, sent, and received without a ground reference.
What is done to an operational amplifier (op amp) to create a low-pass filter circuit?
A capacitor is placed in parallel with the feedback resistor that is connected to the op amp inverting input (-).
What op operational amplifier (op amp) can be configured to have only two output states, high and low?
A comparator op amp circuit has only two output states, high and low. Which output state the comparator takes on depends on the relative voltages on its two inputs.
What characterizes a narrowband frequency modulation (FM) carrier?
A narrowband FM carrier is characterized by a modulation index of less than one, modulation frequencies that range from 300-3,000 Hz, and frequency deviation that ranges from 10-30 kHz.
What geometric shape is a sine wave based upon?
A right-degree triangle.
From what is a sine wave created?
A sine wave is created from the values of sine through a full counterclockwise rotation at 0, 45, 90, 135,180, 225, 270, 315, and 360 degrees plotted on a graph.
In an N-channel enhancement mode metal oxide semiconductor field effect transistor (MOSFET), what effectively turns the P substrate between the two wells into an induced N-channel?
A suitable positive voltage placed between the gate and the source.
Upon what principle do frequency modulation (FM) demodulator detector circuit designs operate?
All FM demodulator detector circuit designs operate on the principle of a voltage being created when the signal frequency moves from the center frequency.
What are the three elements of a carrier wave that can be manipulated?
Amplitude, frequency, and phase.
In a hybrid fiber/coax (HFC) network what type of crude amplitude modulation (AM) detector causes common path distortion (CPD)?
An envelope detector.
What type of operational amplifier (op amp) circuit can be configured to accept multiple signals at its inverting input?
An op amp can be configured as a summing amplifier that accepts multiple signals at its inverting input.
What information remains after the intermediate frequency (IF) signal is detected, or demodulated?
Analog or digital baseband information.
What type of amplitude modulation (AM) is used for analog television transmission?
Analog television transmission use vestigial sideband (VSB) to conserve bandwidth.
What property of a capacitor enables it to be used with an operational amplifier (op amp) to create a filter circuit?
At very low frequencies, capacitive reactance is high.
Of the three basic transistor designs, why is the common emitter the most common configuration?
Because it is a practical amplifier design, the common emitter is the most common transistor design configuration.
When the base of a bipolar junction transistor (BJT) is forward biased, why does most of the charge from the emitter wind up replenishing depleted charges in the collector region?
Because the base region is very thin, when the base is forward biased, only a little of the charge that crosses from the emitter becomes base current. Most of the charge from the emitter winds up replenishing depleted charges in the collector region.
Why must modulation of a wave's phase be relative to a reference phase?
Because the phase dimension of a carrier wave is at an angle, to convey information, modification of the phase must be relative to a reference phase.
How is hysteresis added to an operational amplifier (op amp) comparator circuit to ensure the output is always either high or low?
By adding positive feedback from the op amp output to the non-inverting input.
In an operational amplifier (op amp) summing amplifier circuit, how can unequal weights be given to the signals being summed together?
By using different value input resistors for each signal.
How much do you know about carrier waves?
Changing the speed that the radius completes its cycle, or frequency, varies the length of the sine waves within a carrier. Changing the angle of the radius during and between cycles modifies the amplitude of the carrier wave. Phase refers to the difference in timing between two sine waves of the same frequency.
What class of amplifier conducts current during the entire input waveform and is inherently linear?
Class A amplifiers conduct output current over the entire input waveform and are inherently linear.
Which class of amplifier utilizes a push-pull amplifier configuration in which each transistor conducts current during opposite cycles of the input signal?
Class B amplifier.
class amplifier
Class B is typically used as an output amplifier. Class D approaches but never achieves 100% efficiency. Class C has an output that is very high in harmonics that must be attenuated by a filter network.
Which class of amplifier has an output that is very high in harmonics and must be attenuated by a filter network?
Class C amplifier.
Which class of amplifier efficiency approaches, but never achieves, 100%?
Class D amplifier.
In a radio receiver, what is almost universally used to conduct the energy between the antenna and the receiver tuner?
Coaxial cable.
Which logic combines N-channel and P-channel metal oxide semiconductor field effect transistors (MOSFET) onto a single substrate?
Complementary metal oxide semiconductor (CMOS)
What enables the collector current in an operational amplifier's differential amplifier to be independent of the two input voltages, VI+ and VI-?
DC bias circuitry in the differential amplifier emitter circuit.
What is used in some receivers to remove all attenuation before the amplifier until the received signal level reaches a designed threshold?
Delayed automatic gain control (AGC).
What stage of the RF receiver recovers the baseband information from the modulated IF signal?
Demodulation, the process of recovering the baseband information from the modulated IF signal, is done in the demodulator stage, also called the detector stage, of a receiver.
Which logic family was popular until transistor-to-transistor logic (TTL) started taking over in the late 1960s?
Diode-transistor logic (DTL) preceded TTL and was popular until TTL started taking over in the late 1960s.
What is used in set-top boxes (STB) to strike a balance between image rejection, the bandwidth of the intermediate frequency (IF) filter, and the frequency at which demodulation is done?
Double IF conversion.
What logic family of bipolar transistor logic circuits is used extensively where the fastest speeds are required at the expense of power dissipation?
Emitter coupled logic (ECL).
What is another name for the bipolar junction transistor (BJT) common collector configuration?
Emitter follower.
What are the two types of detection that are used to demodulate amplitude modulation (AM) waveforms?
Envelope and synchronous detection are used to demodulate AM waveforms.
In an AM waveform envelope, for a modulation index of 0.5, the modulation signal is the same amplitude as the carrier.
False
What does the modulation index in amplitude modulation (AM) express?
For AM, the levels of modulation are expressed in the modulation index; the ratio of the modulating signal level to the unmodulated carrier.
What voltage can be expected on both input terminals of an operational amplifier (op amp) for linear operation?
For the amplifier to operate in its linear region, the voltage on both inputs, VI- and VI+, must be the same.
Double sideband suppressed carrier
Gives greater range for the same transmitted power or allows lower power transmission, preserving the battery in portable radios.
Op Amp Comparator
Hysteresis is added to an operational amplifier (op amp) comparator circuit to ensure the output is always either high or low by adding positive feedback from the op amp output to the non-inverting input.
What are the ideal characteristics of an operational amplifier (op amp)?
Ideally, an op amp has infinite input impedance, zero output impedance, and infinite gain.
Amplifier Properties
Impedance is the total opposition to alternating current (AC) flow in a circuit, is expressed and measured in Ohms, and is the sum of the resistance and reactance. The gain of an amplifier refers to the ratio of input-to-output level, which can be power or voltage. The likely cause of non-linear distortion in the broadband cable network is an amplifier.
What activity consumes most of the power in complementary metal oxide semiconductor (CMOS) circuits?
In CMOS circuits, most of the power is consumed charging and discharging the unavoidable capacitances in each junction.
What provides the orthogonal reference in orthogonal frequency division multiplexing (OFDM)?
In OFDM, the peak power of each subcarrier aligns with the minimum power of the adjacent subcarrier, which makes each subcarrier orthogonal to their adjacent subcarrier.
How much do you know about the AM waveform?
In a hybrid fiber/coax (HFC) network, a corroded connector or seizure screw forms a crude diode that acts as an envelope detector. When demodulating an AM waveform, high distortion and insensitivity to low signals can be overcome using synchronous detection.
What is a voltage offset in the context of an op amp?
In an op amp, a voltage offset occurs when there is a difference in voltage on the two input terminals.
Why can Carson's Rule be used to determine the bandwidth of a frequency modulation (FM) carrier?
In most cases, the amplitude of the sidebands furthest from the carrier frequency are so low that they can be ignored.
In the circular reference with the right triangle, what represents the amplitude or power in the sine wave?
In the circular reference with the right triangle, the triangle's vertical line length changing while the radius length remains constant during a full cycle represents the amplitude or power in the sine wave.
In what application would a bipolar junction transistor (BJT) common base configuration be used?
In what application would a bipolar junction transistor (BJT) common base configuration be used?
How does complementary metal oxide semiconductor (CMOS) technology combat capacitance in a circuit?
It combines N-channel and P-channel metal oxide semiconductor field effect transistors (MOSFET) onto a single substrate.
What is the characteristic of a voltage follower for which the circuit is named?
It is called a voltage follower because the output voltage is an exact replica in amplitude and phase of the input voltage.
What information does the circuit designer need to know to calculate the signal voltage and current range on the output of a bipolar junction transistor (BJT)?
Knowing the resistance of the circuit following the amplifier stage (the load resistance), the circuit designer can calculate the signal voltage and current range on the output of the BJT.
In an operational amplifier (op amp), what is a common practice to minimize voltage offsets at the inputs to an inverter?
Make the net DC resistance the same on both inputs to the op amp.
What characterizes N-doped silicon that is used in bipolar junction transistors (BJT)?
N-doped silicon has an excess of free electrons compared to P-doped silicon.
Calculating the signal voltage and current range on the output of a bipolar junction transistor (BJT) involve repeated applications of what?
Ohm's law.
N-doped material - Excess electrons
P-doped material - Holes, or shortage of electrons
What is a consequence of high-speed charging and discharging of the capacitance in a complementary metal oxide semiconductor (CMOS)?
Power is dissipated as heat.
Vestigial sideband
Produced by using filters to eliminate most of the lower sideband so that only a "vestige" of the sideband remains.
What distinguishes quadrature phase shift keying (QPSK) from quadrature amplitude modulation (QAM)?
QPSK only uses phase modulation, whereas QAM combines phase and amplitude modulation (AM) to provide higher data throughput than QPSK. Thus, QPSK is designated as a different modulation scheme, separate from the various QAM designations.
What is reactance?
Reactance, is the opposition to current flow in a circuit when capacitance and inductance are introduced, causing a phase shift between the voltage and current.
What type of bandpass filters offer the precision, stability, and bandwidth most suitable for TV and set-top box (STB) intermediate frequency (IF) sections?
Surface acoustic wave (SAW) filters.
Under what principle do frequency modulation (FM) detector circuit designs operate?
That voltage is created when the signal frequency moves from the center frequency.
What is a characteristic of the filters in the intermediate frequency (IF) section of a receiver?
The IF filter is either very flat on the top (the passband) or has some desired shape, and the sides (skirts) of the IF filter are usually very sharp (steep).
What is created during the intermediate frequency (IF) mixing process that could interfere with tuning?
The IF mixing process creates an image signal that is always 88 MHz (twice the IF) above the received channel frequency and can interfere with tuning.
In a basic N-channel junction field effect transistor (J-FET), what when negative bias is applied to the gate?
The J-FET depletion region is expanded, decreasing the current that is transported between the source and the drain.
What two significant signals go into the mixer to create the intermediate frequency (IF)?
The RF signal frequency at the output of the front-end filter and the local oscillator (LO) signal.
What are the peaks and minima of the amplitude modulation (AM) waveform envelope related to?
The amplitude of the modulation signal.
What is the relationship of the bandwidth of an amplitude modulation (AM) carrier and its modulation signal?
The bandwidth of an AM carrier is twice the bandwidth of its modulation signal.
What circuit is in the input stage of almost all operational amplifiers (op amps)?
The differential amplifier is in the input stage of just about all op amps.
What is the main characteristic of a frequency modulation (FM) waveform?
The frequency of the FM waveform deviates in proportion to the modulation sine wave while the amplitude remains constant.
What part of a receiver has several functions that include limiting the number of signals that reach the amplifier and mixer?
The front-end filter.
What is a frequently overlooked source for ingress, signal leakage, or intermittent operation when connecting a coaxial cable to a transmitter or receiver?
The ground connection is a frequently overlooked source for ingress, signal leakage, or intermittent operation when connecting a coaxial cable to a transmitter or receiver.
What distinguishes an operational amplifier (op amp) configured as a voltage follower with gain from an inverter circuit?
The input signal in the voltage follower with gain goes to the non-inverting input (+).
Applying Carson's Rule to calculate the bandwidth of a narrowband frequency modulation (FM) signal, which factor below must be known?
The maximum modulation signal frequency.
Why could be the mixer stage in a receiver be a Class C amplifier?
The mixer stage in a receiver could be a Class C amplifier, which has an output that is very high in harmonics.
Why is the modulation information generally called baseband information or a baseband signal?
The modulation information is usually in its basic native electrical format, which is why it is often called baseband information or a baseband signal.
What do the number of filter poles generally correspond to?
The number of inductors plus capacitances that are in a filter circuit.
How many sidebands of significant amplitude are created by the frequency modulation (FM) process?
The number of sidebands of significant amplitude that are created by the FM process is dependent on the FM modulation index.
Which of the following are features of the differential amplifier?
The op amp's differential amplifier must be comprised of well-matched transistors as a key requirement for an op amp to come close to meeting its ideal characteristics. The differential amplifier is a combination of two Class A common emitter amplifiers put together in a particularly useful way. The non-inverted input is the common jargon for the input that is in-phase with the output.
QPSK
The output of the two mixers are multiplexed into one signal and two bits of data are transported simultaneously in the same clock cycle.
QAM
The outputs of the two AM modulators are multiplexed into one signal and four bits of data or more are transported simultaneously in the same clock cycle.
What prevents the subcarriers in orthogonal frequency division multiplexing (OFDM) from interfering with each other?
The peak power of each subcarrier aligns with the minimum power of the adjacent subcarrier.
In an operational amplifier (op amp), what is open-loop gain?
The pre-determined gain of the op amp without feedback.
What determines the voltage gain of an operational amplifier (op amp) inverter?
The value of the input and feedback resistors.
Which of the following is true about intermediate frequency (IF) amplification? Choose all that apply.
To maintain consistent amplitude levels out of the receiver, automatic gain control (AGC) range is typically applied in the IF stage. In most designs, a filter immediately follows the mixer, but some designs, such as those that use a SAW filter, require an amplifier before the filter. Delayed automatic gain control (AGC) is used in some receivers to avoid distortion in the mixer when the signal being received is too strong.
Where and how is the automatic gain control (AGC) voltage developed to maintain the intermediate frequency (IF) amplifier at consistent amplitude levels?
To maintain the IF amplifier at consistent amplitude levels, the AGC voltage is developed in the detector (demodulator) section by sampling the IF signal level and comparing it with the desired level.
Why do metal oxide semiconductor field effect transistors (MOSFET) have internal zener diodes from the gate to the source or substrate?
To prevent damage from static electricity.
Applying Carson's rule, the maximum modulation signal frequency is the most significant factor in determining the bandwidth of a narrowband frequency modulation (FM) carrier.
True
In a conventional amplitude modulation (AM) transmission, how much power is in the sidebands and how much is in the carrier?
Two-thirds of the power is in the carrier with one-third in the sidebands.
In what application, would you suspect a silicon controlled rectifier (SCR) could be causing radio frequency interference (RFI) in a customer premises?
Unfiltered switching of an SCR in a light dimmer could be the source of RFI in the customer premises.
For a 1 volt input and R1 value of 47 Ohms, match the R2 value to the gain.
Unity gain - 47 Ohms Gain of 10 - 470 Ohms Gain of 100 - 4,700 Ohms Gain of 1,000 - 47,000 Ohms
Single sideband
Used in marine, aviation, military, and amateur radio applications for long range voice communications.
How many data bits are in each symbol at the output of a quadrature phase shift keying (QPSK) modulator?
When the output from the QPSK modulator is plotted on a polar coordinate graph, each quadrant contains one symbol with 2 bits of data.
Even though it may be omitted from the block diagram of a receiver, why should the power supply be the first thing that is checked when troubleshooting a balky circuit?
When troubleshooting a balky circuit, the first thing to always check is the power supply because troubleshooting an improperly powered circuit is futile.
An op amp circuit that does the same thing with only resistors and capacitors is an attractive alternative to inductors that can get big and expensive at low frequencies.
When using an operational amplifier (op amp) circuit to filter signals, what component from a passive filter circuit is replaced?
What precautions must be observed when working with circuits that contain metal oxide semiconductor field effect transistors (MOSFET)?
When working on circuits that contain MOSFETs, a grounded anti-static mat is recommended, and anyone working with the circuit should also be grounded to the anti-static mat.
Why is it undesirable to operate an op amp with open-loop gain?
While it may sound desirable to operate an op amp with open-loop gain, this condition is unstable with temperature and the voltage gain can vary from device-to-device of the same type.
