T7 Practical Circuits

B. Sensitivity

Sensitivity is a receiver's ability to detect weak signals: the more sensitive the receiver, the smaller the signal it can pick out of the background noise.

• A. RF gain is a control that sets how much the receiver's RF stage amplifies incoming signals; it is an adjustment, not a measure of the ability to detect a signal.
• C. Selectivity is the ability to separate a desired signal from others on nearby frequencies, not the ability to detect a weak signal.
• D. Total harmonic distortion measures how much unwanted harmonic content an amplifier adds; it describes signal purity, not detection.

A. A device that combines a receiver and transmitter

A transceiver combines a receiver and a transmitter in one unit, usually sharing circuits such as the frequency control, power supply and antenna connection. Most modern amateur radios, from handhelds to HF base stations, are transceivers.

• B. A device that matches feed line impedance to 50 ohms is an antenna tuner (impedance matching network).
• C. Automatically sending and decoding Morse code is the job of a keyer and a code reader or computer software, not the definition of a transceiver.
• D. Converting receive and transmit frequencies to another band is the job of a transverter.

B. Mixer

A mixer combines an incoming signal with a signal from a local oscillator and produces new signals at the sum and the difference of the two frequencies. Selecting one of those products converts the signal to a different frequency, which is how a superheterodyne receiver moves signals to its intermediate frequency.

• A. A phase splitter produces two copies of a signal that are 180 degrees out of phase; the frequency stays the same.
• C. An inverter either changes DC power into AC power or, in digital logic, reverses a logic level; it does not convert a signal to another frequency.
• D. An amplifier increases a signal's strength without changing its frequency.

C. Selectivity

Selectivity is a receiver's ability to pass the signal you want while rejecting signals on nearby frequencies. Narrow filters, such as a CW filter a few hundred hertz wide, improve selectivity.

• A. Discrimination ratio is not the standard term for this receiver property; the ability to separate one signal from others is called selectivity.
• B. Sensitivity is the ability to detect weak signals, not the ability to separate one signal from others.
• D. Harmonic distortion is the creation of unwanted multiples of a signal's frequency; it describes signal purity, not the ability to tell signals apart.

D. Oscillator

An oscillator generates a steady signal at a specific frequency, using feedback around an amplifier and a frequency-determining part such as a tuned circuit or a quartz crystal. Oscillators set the operating frequency of transmitters and the local oscillator frequency in receivers.

• A. A reactance modulator varies a reactance to shift an oscillator's frequency and produce FM; it modulates a signal rather than generating one.
• B. A phase modulator varies the phase of an existing signal to carry information; it does not create the signal.
• C. A low-pass filter passes frequencies below its cutoff and attenuates higher ones; it does not generate a signal.

C. Transverter

A transverter converts a transceiver's transmit signal to a different band and converts received signals from that band back to the transceiver's frequency. For example, a 2-meter transverter driven by a 10-meter transceiver lets an HF radio operate on 2 meters.

• A. A high-pass filter passes frequencies above its cutoff and blocks lower ones; it removes signals but does not move them to another band.
• B. A low-pass filter passes frequencies below its cutoff, often to reduce harmonics; it does not convert signals to another band.
• D. A phase converter changes single-phase electrical power into three-phase power; it does not convert RF signals between bands.

B. Switches transceiver from receive to transmit when grounded

PTT (push-to-talk) is a control line, usually on the microphone or accessory connector, that switches the transceiver from receive to transmit when it is connected to ground. Pressing the microphone's PTT button, a foot switch or a computer interface grounds the line.

• A. A CW key connects to a separate key input; the PTT line switches between receive and transmit.
• C. Grounding the PTT line puts the radio into transmit; it does not generate a tuning tone.
• D. The PTT input has nothing to do with preamplifier tuning tones; it controls transmit and receive switching.

C. Modulation

Modulation is the process of combining information, such as speech, with an RF carrier by varying the carrier's amplitude, frequency or phase. AM, FM and SSB are different methods of modulation.

• A. Impedance matching makes a load present the impedance a source expects, for example with an antenna tuner; it does not add speech to a carrier.
• B. Oscillation is the generation of a signal at a specific frequency; the carrier comes from an oscillator, but adding speech to it is modulation.
• D. Low-pass filtering removes frequencies above a cutoff; it does not combine speech with a carrier.

B. Set the amplifier for proper operation in the selected mode

SSB and FM place different demands on an amplifier. SSB needs linear amplification so the signal's varying amplitude is reproduced without distortion, while FM and CW can use simpler, more efficient operation, so the mode switch sets the amplifier's operation (for example its bias, or how long it stays in transmit between words) to suit the selected mode.

• A. The amplifier only amplifies what the transceiver sends; the transmitted mode is selected on the transceiver, not by the amplifier's switch.
• C. The mode switch does not change the amplifier's frequency range; the amplifier covers the same band in either position.
• D. The switch configures the amplifier for the transmitted mode; it does not reduce received signal noise.

B. An RF power amplifier

An RF power amplifier takes the transceiver's output signal and increases its power, for example raising a 10-watt signal to 100 watts. It connects between the transceiver output and the feed line to the antenna.

• A. A potentiometer is a variable resistor; it can only reduce or divide a signal, never increase its power.
• C. Transforming impedance changes the ratio of voltage to current but cannot add power; only an amplifier, which draws power from its own supply, increases output power.
• D. Only the RF power amplifier increases transmitted power, so not all of these choices are correct.

A. Set the receive and transmit frequency

The VFO is the tunable oscillator that sets the frequency the transceiver receives and transmits on. Turning the main tuning knob changes the VFO frequency, and many radios have two VFOs so they can receive on one frequency and transmit on another.

• B. Automatic frequency control is a separate circuit that keeps an oscillator or receiver locked on frequency; it is not the job of the VFO.
• C. Injecting a frequency to make CW audible is the job of the beat frequency oscillator (BFO), not the VFO.
• D. Generating and demodulating SSB is done by circuits such as a balanced modulator, sideband filter and product detector, not by the VFO.

D. Talk farther away from the microphone

An FM transmitter's deviation rises with the audio level reaching the microphone. Speaking farther from the microphone (or more softly) lowers the audio level and the deviation, so the signal stays within its channel and sounds clear.

• A. Talking louder raises the audio level and increases the deviation, making the problem worse.
• B. Over-deviation comes from too much audio, not from heat, so letting the radio cool off does not change it.
• C. Transmitter power does not set FM deviation; a higher power level makes the over-deviated signal stronger, not cleaner.

A. The receiver is unable to reject strong signals outside the AM or FM band

This is fundamental overload: the amateur signal is so strong at the broadcast receiver that the receiver cannot reject it, even though the amateur station transmits only on its own frequency. The cause is the receiver's limited ability to reject strong signals outside its band, and a filter at the receiver often cures it.

• B. Excessive microphone gain distorts the amateur signal and can cause splatter near the operating frequency, but it does not make a broadcast receiver pick up the transmission.
• C. An overloaded transmitter audio amplifier distorts the transmitted audio; it does not explain why a broadcast radio picks up the amateur signal.
• D. Low FM deviation makes the amateur signal sound weak to other amateurs; it does not cause reception on a broadcast radio.

D. All these choices are correct

Fundamental overload, harmonics and spurious emissions are all ways an amateur transmission can interfere with other equipment. Overload comes from the strong intended signal itself, while harmonics and other spurious emissions are unwanted signals on other frequencies.

• A. Fundamental overload, where a strong signal on the intended frequency overwhelms a nearby receiver, does cause interference, but it is only one of the listed causes.
• B. Harmonics, signals at whole-number multiples of the transmitted frequency, do cause interference, but they are only one of the listed causes.
• C. Spurious emissions, unwanted emissions outside the necessary bandwidth (47 CFR 97.3(a)(43)), do cause interference, but they are only one of the listed causes.

D. High SWR

Solid-state transceivers include protection circuits that reduce output power when SWR rises, because power reflected from a mismatched antenna system can damage the output transistors. A high SWR is therefore a common reason for low RF output.

• A. Noise figure describes how much noise an amplifier adds to received signals; it has no effect on transmitted power.
• B. Poor linearity distorts the signal and can cause splatter, but it does not make the transceiver cut back its output power.
• C. Low SWR means a good match, which lets the transceiver deliver full power.

A. Block the amateur signal with a filter at the antenna input of the affected receiver

When a non-amateur receiver is overloaded by a strong amateur signal, the cure is at the receiver: a filter at its antenna input blocks the amateur frequency while passing the signals the receiver is meant to receive. The amateur transmission itself may be free of harmonics and spurious emissions.

• B. A filter on the transmitter can remove harmonics and spurious emissions, but it cannot block the amateur's own signal without blocking the transmission itself.
• C. Changing from FM to SSB does not reduce the strength of the signal reaching the affected receiver, so an overloaded receiver stays overloaded.
• D. A narrower transmitted bandwidth does not help a receiver that cannot reject a strong signal outside its own band.

A. Make sure that your station is functioning properly and that it does not cause interference to your own radio or television when it is tuned to the same channel

Start by checking your own station: make sure it is working properly and does not interfere with your own radio or TV tuned to the same channel. If your own equipment is unaffected, the problem is more likely in the neighbor's receiver, such as overload, and you can help with filters or other cures.

• B. Shutting down and calling the FCC is not the first step; most interference problems are solved by the people involved, starting with a check of your own station.
• C. A harmonic doubler would generate harmonics, adding unwanted signals and making interference worse.
• D. Only checking your own station is a sensible first step, so not all of these choices are correct.

D. Installing a band-reject filter

A band-reject (notch) filter in the 2-meter radio's antenna line attenuates the strong commercial FM signal while passing 2-meter signals, so the commercial signal no longer overloads the receiver.

• A. An RF preamplifier amplifies the strong commercial signal along with the 2-meter signals, which makes overload worse.
• B. The strong commercial signal arrives through the antenna along with the desired signals, so better coax shielding does not remove it.
• C. Bypass capacitors on the microphone cable keep RF out of the audio circuits; they do nothing about a strong signal arriving at the receiver input.

D. All these choices are correct

Resolving interference from a neighbor's device takes cooperation and care. Under the FCC's Part 15 rules (47 CFR 15.5(b)), unlicensed devices such as chargers and other electronics may operate only if they cause no harmful interference, and you should also confirm your own station meets good amateur practice before asking a neighbor to make changes.

• A. Working with your neighbor to identify the offending device is a good step, but it is only part of the answer.
• B. Politely explaining that FCC rules do not allow devices that cause harmful interference (47 CFR 15.5(b)) is appropriate, but it is only part of the answer.
• C. Making sure your own station meets good amateur practice is appropriate, since the problem could be in your own equipment, but it is only part of the answer.

D. Be sure all TV feed line coaxial connectors are installed properly

Cable TV signals travel inside a closed, shielded coaxial network. A loose or poorly installed connector lets your transmitted signal leak into the cable (ingress), so the first step is to make sure all the TV feed line connectors are installed properly.

• A. A low-pass filter at the TV input would block the TV signals above its cutoff; low-pass filters are used at transmitters to reduce harmonics.
• B. A high-pass filter can help an over-the-air TV antenna reject HF signals, but with cable TV the first step is to stop the signal from leaking in through poor connections.
• C. A preamplifier amplifies the interfering signal too and can make overload worse.

D. All these choices are correct

Distorted or unintelligible audio through an FM repeater has several possible causes: transmitting slightly off frequency, over-deviating by speaking too loudly or too close to the microphone, or transmitting from a poor location where the signal is weak or affected by reflections.

• A. Being slightly off frequency can distort your audio through the repeater, but it is only one of the possible causes.
• B. Speaking too loudly or too close to the microphone causes over-deviation and distortion, but it is only one of the possible causes.
• C. A bad location can make your signal weak and noisy at the repeater, but it is only one of the possible causes.

C. Adding a clip-on ferrite “choke” to the microphone cable to prevent the transmitted signal from feeding back into the transmitter

RF from your own transmitter can travel back along the microphone cable into the audio circuits (RF feedback) and distort your voice. A clip-on ferrite choke on the cable blocks RF current on the cable while letting the audio pass.

• A. Adding feed line does not fix an antenna mismatch; its extra loss can lower the SWR reading at the radio, but it does not stop RF feedback.
• B. Cycling the power does not stop RF from getting into the audio circuits; the distortion returns as soon as you transmit again.
• D. The squelch control only affects receiving; it has no effect on distortion in the transmitted signal.

A. To prevent transmitting signals over the air when making tests

A dummy load absorbs the transmitter's output and turns it into heat instead of radiating it, so you can test and adjust a transmitter without putting a signal on the air or causing interference.

• B. Over-modulation is controlled by audio levels and transmitter settings; a dummy load does not change modulation.
• C. A dummy load replaces the antenna during tests; it does nothing to improve antenna efficiency.
• D. A dummy load is a test load for transmitters; it does not affect a receiver's signal-to-noise ratio.

B. An antenna analyzer

An antenna analyzer sends a low-power signal into the antenna and measures its SWR or impedance across a range of frequencies. The antenna is resonant where its reactance is zero, which is usually close to where the SWR is lowest.

• A. A VTVM (vacuum tube voltmeter) measures voltage; it cannot show an antenna's resonant frequency.
• C. A Q meter measures the quality factor of coils and capacitors, not antenna resonance.
• D. A frequency counter measures the frequency of a signal applied to it; it says nothing about the frequency at which an antenna is resonant.

B. A 50-ohm non-inductive resistor mounted on a heat sink

A dummy load is a 50-ohm resistor that matches the transmitter's output impedance and turns the RF power into heat. It is non-inductive so it stays close to 50 ohms at radio frequencies, and it is mounted on a heat sink (or immersed in oil) to carry the heat away.

• A. A low-voltage power supply and an AC relay switch circuits on and off; they cannot absorb RF power the way a resistor does.
• C. A low-voltage power supply and a DC relay are switching parts; a dummy load needs a resistor that dissipates the RF power as heat.
• D. An inductive reactance stores and returns energy instead of dissipating it, so it would present a mismatch rather than absorb the power.

C. 1:1

SWR is written as a ratio to 1, and a reading of 1:1 means no power is reflected, which is a perfect match between the feed line and the antenna. Any mismatch produces a higher ratio, such as 1.5:1 or 3:1.

• A. SWR is not written as 50:50; the number 50 refers to the usual feed line impedance in ohms, not an SWR reading.
• B. SWR cannot be less than 1:1, so a reading of zero is not possible.
• D. On an SWR meter, a full-scale reading indicates a very high SWR, the opposite of a perfect match.

A. To protect the RF output amplifier transistors

When SWR is high, reflected power returns to the transmitter and can cause excess voltage and heat in the output amplifier transistors. Solid-state transmitters sense this and reduce output power (fold back) to protect those transistors.

• B. Spectral purity rules limit harmonics and spurious emissions; they are not the reason for reducing power at high SWR.
• C. The power supply is not the limit; the transmitter deliberately reduces power to protect its output stage.
• D. SWR depends on the mismatch between the feed line and the antenna, not on the power level, so reducing power does not lower the SWR.

D. Impedance mismatch

Any SWR above 1:1 indicates an impedance mismatch, and 4:1 is a large one. On a 50-ohm feed line, a 4:1 SWR could come from a resistive load of 200 ohms (50 x 4) or 12.5 ohms (50 / 4); the reflection coefficient is (4 - 1) / (4 + 1) = 0.6, and 0.6² = 0.36, so 36% of the forward power arriving at the load is reflected back toward the transmitter.

• A. SWR is a ratio, not a loss in dB; it describes a mismatch rather than an amount of loss.
• B. A good impedance match gives an SWR near 1:1; 4:1 is a poor match.
• C. SWR does not measure gain, and a mismatch never adds power.

C. It is converted into heat

Feed line loss comes from resistance in the conductors and losses in the insulating dielectric. The power lost in the line is converted into heat, so less power reaches the antenna.

• A. Loss does not increase SWR; extra feed line loss actually lowers the SWR measured at the transmitter by weakening the reflected power.
• B. Harmonics are generated in the transmitter, not by feed line loss; power lost in the line becomes heat, not radiation.
• D. Loss reduces the signal's power but does not change its waveform or content.

D. Directional wattmeter

A directional wattmeter measures forward and reflected power separately, and SWR is calculated from the two. For example, 100 W forward and 4 W reflected gives a power ratio of 4 / 100 = 0.04, whose square root is 0.2, so SWR = (1 + 0.2) / (1 - 0.2) = 1.5:1.

• A. A voltmeter measures voltage at one point; it cannot separate forward and reflected power.
• B. An ohmmeter measures DC resistance, which tells you nothing about SWR at radio frequencies.
• C. Iambic pentameter is a rhythm used in poetry, not a measuring instrument.

A. Moisture contamination

Water that gets into coaxial cable, usually through a damaged jacket or poorly sealed connectors, corrodes the shield and center conductor and increases losses. Over time it ruins the cable, which is why outdoor connectors are sealed.

• B. Solder flux left on a connector is a workmanship issue, not a common cause of coaxial cable failure.
• C. Changes in transmitter output power do not harm coax, as long as the power stays within the cable's rating.
• D. Coax is rated for continuous power, so long periods at 100% duty cycle within that rating do not cause failure.

D. Ultraviolet light can damage the jacket and allow water to enter the cable

Ultraviolet light in sunlight breaks down many plastics, so an unprotected jacket can crack. Cracks let water into the cable, where it corrodes the shield and increases losses, so outdoor coax should have a UV-resistant jacket.

• A. Ultraviolet light does not reach the conductors and does not change their resistance; the damage is to the jacket.
• B. The jacket is outside the shield and carries no RF, so jacket losses are not the concern; the concern is cracking that lets water in.
• C. Ultraviolet light and RF signals do not mix inside a cable to create interference.

C. It has less loss per foot

Foam dielectric is mostly gas, which absorbs less RF energy than solid plastic, so foam-dielectric coax has less loss per foot than a similar cable with solid dielectric.

• A. Foam is not more resistant to moisture; water can work its way into foam dielectric, which is a weakness of foam cable.
• B. Solid dielectric has the higher voltage breakdown; foam trades some of that for lower loss.
• D. Both types are made for a specified impedance, such as 50 ohms; foam does not give a better match.

B. A voltmeter

Electric potential (potential difference) is voltage, and a voltmeter measures it in volts.

• A. An ammeter measures current in amperes, not electric potential.
• C. A potentiometer is a variable resistor, often used as an adjustable voltage divider such as a volume control; it is not the meter used to measure voltage.
• D. An ohmmeter measures resistance in ohms, not electric potential.

B. In parallel

Voltage is a difference in potential between two points, so a voltmeter is connected in parallel across the component, with one probe on each side. Its high internal resistance draws very little current, so it barely disturbs the circuit.

• A. A series connection is how an ammeter measures current; a voltmeter in series would insert a large resistance into the circuit.
• C. Quadrature describes two signals that are 90 degrees apart in phase; it is not a way to connect a meter.
• D. In phase describes two signals whose peaks line up in time; it is not a way to connect a meter.

A. In series

To measure current, the meter must carry the current itself, so you open the circuit and connect the meter in series with the component. On its current setting the meter has a very low resistance, so it does not disturb the circuit.

• B. A parallel connection is used to measure voltage; a meter set for current and placed in parallel can short the circuit and blow the meter's fuse.
• C. Quadrature describes two signals that are 90 degrees apart in phase; it is not a way to connect a meter.
• D. In phase describes two signals whose peaks line up in time; it is not a way to connect a meter.

D. An ammeter

Electric current is measured in amperes with an ammeter, which is connected in series so the current flows through it.

• A. An ohmmeter measures resistance, not current.
• B. An electrometer is a specialized laboratory instrument designed mainly for measuring charge and voltage with very high input resistance; the everyday instrument for current is the ammeter.
• C. A voltmeter measures voltage (electric potential), not current.

A. By applying a small current and measuring the resulting voltage

An ohmmeter passes a small, known current from its internal battery through the component and measures the resulting voltage; Ohm's law then gives the resistance, R = E / I. For example, 1 mA producing 2 V means 2 V / 0.001 A = 2,000 ohms.

• B. A variable resistor placed in parallel would change the circuit being measured; an ohmmeter uses a known test current instead.
• C. Placing a variable resistor in series is not how an ohmmeter works; it applies a known current and measures the voltage.
• D. An ohmmeter uses a known, fixed test source; it does not sweep a variable voltage and watch the current change.

C. Attempting to measure voltage when using the resistance setting

On its resistance setting, a multimeter applies its own small test current and expects an unpowered circuit. Connecting it across a voltage can force excess current through its internal circuits and damage the meter.

• A. Measuring resistance on the voltage setting gives a meaningless reading but does not harm the meter, because the voltage ranges have a high input resistance.
• B. Leaving one probe off ground means there is no complete measurement path; it does not damage the meter.
• D. A multimeter is not damaged by being used without a warm-up period.

C. Voltage and resistance

A multimeter combines a voltmeter, an ammeter and an ohmmeter in one instrument, so it measures voltage, current and resistance.

• A. Signal strength and noise are measured with instruments such as an S meter or a spectrum analyzer, not a multimeter.
• B. Impedance and reactance at radio frequencies are measured with instruments such as an antenna analyzer or impedance bridge; a multimeter measures DC resistance.
• D. Of the pairs listed, only voltage and resistance are multimeter measurements, so not all of these choices are correct.

A. Acid-core solder

Acid-core solder contains an acid flux meant for plumbing and sheet metal work. Its residue corrodes electronic components and circuit boards over time, so electronics work uses rosin or other electronics-grade flux.

• B. Lead-tin solder, such as 60/40 or 63/37, is the traditional solder for electronics.
• C. Rosin-core solder is made for electronics; the rosin flux cleans the joint without corroding it.
• D. Tin-copper solder is a lead-free alloy widely used in electronics.

C. A rough or lumpy surface

A cold joint forms when the solder does not get hot enough to flow properly, or the parts move while it cools. The solder looks rough, lumpy or grainy and dull instead of smooth, and the connection can be weak or intermittent.

• A. Dark spots usually come from burned flux or contamination; they are not the typical sign of a cold joint.
• B. A bright, shiny, smooth surface is the sign of a good tin-lead joint.
• D. A greenish tinge indicates corrosion, often of copper, not a cold solder joint.

A. Increasing resistance with time

The ohmmeter's test current starts charging the discharged capacitor. At first current flows freely, so the meter reads a low resistance; as the capacitor charges, the current falls and the reading rises toward a very high resistance.

• B. A charging capacitor draws less current over time, so the resistance reading rises, not falls.
• C. A steady reading means the resistance is not changing, as with a resistor or a short circuit, not a charging capacitor.
• D. A good capacitor charges smoothly; it does not jump back and forth between open and short.

B. Ensure that the circuit is not powered

An ohmmeter supplies its own small test current and assumes the circuit is unpowered. Measuring a powered circuit gives false readings and can damage the meter, so turn off the power (and discharge any capacitors) first.

• A. The circuit's operating voltages do not matter for a resistance measurement, which must be made with the power off.
• C. Grounding the circuit is not the needed precaution; removing power is.
• D. Operating frequency is irrelevant; the ohmmeter needs the circuit to be off, not operating.