E4 Amateur Practices

A. Sampling rate of the analog-to-digital converter

A digital oscilloscope samples the input with an analog-to-digital converter. By the Nyquist criterion it must take more than two samples per cycle of the signal, so the sampling rate limits the highest frequency it can display accurately; practical scopes sample well above that minimum.

• B. An analog-to-digital converter has a reference voltage that sets its input range, not a reference frequency that limits the displayed frequency.
• C. Q describes the sharpness of a tuned circuit's response; it does not set the frequency limit of an oscilloscope.
• D. Only the sampling rate in this list limits the displayed frequency, so not all of these are correct.

B. Signal amplitude and frequency

A spectrum analyzer shows signals in the frequency domain: amplitude on the vertical axis and frequency on the horizontal axis. Each signal appears as a peak at its frequency, with its height showing its strength.

• A. Amplitude versus time is the time-domain display of an oscilloscope.
• C. SWR versus frequency is the sweep display of an antenna analyzer.
• D. A spectrum analyzer does not measure SWR, and its horizontal axis is frequency, not time.

B. Spectrum analyzer

A spectrum analyzer displays every signal present in a transmitter's output by frequency and amplitude, so spurious signals and intermodulation products appear as separate peaks beside the desired signal. Their levels can be read directly relative to the main signal.

• A. A resolver is an electromechanical sensor for measuring shaft angle, not an RF test instrument.
• C. A logic analyzer captures digital logic states over time; it cannot display an RF spectrum.
• D. A network analyzer measures how a device such as a filter or antenna responds to the analyzer's own test signal; it does not show the spectrum a transmitter produces.

A. A square wave is displayed, and the probe is adjusted until the horizontal portions of the displayed wave are as nearly flat as possible

A 10X oscilloscope probe has an adjustable capacitor that must be balanced against the scope's input capacitance. To compensate it, connect the probe to a square wave, usually the scope's built-in calibrator output, and adjust until the top and bottom of the wave are flat, with no rounded corners or overshoot.

• B. Peaking one sine wave for maximum amplitude does not show whether the probe's response is flat across frequencies; a square wave reveals that at a glance.
• C. Timing accuracy depends on the oscilloscope's time base, not on probe compensation.
• D. Probe compensation affects the response to fast changes, not DC accuracy, so a DC voltage standard cannot show whether it is correct.

D. Reduce the signal frequency to within the counter's operating range

A prescaler is a frequency divider placed ahead of a counter. Dividing the input by a fixed factor brings a frequency above the counter's limit into its range: a 1.2 GHz signal divided by 10 becomes 120 MHz, and the reading is multiplied by 10 again for display.

• A. Amplifying weak signals is the job of a preamplifier, not a prescaler.
• B. A prescaler divides the frequency; multiplying it would push the signal even further above the counter's range.
• C. A prescaler has nothing to do with preventing oscillation in the counter.

A. A false, jittery low-frequency version of the waveform is displayed

Aliasing happens when a digital oscilloscope samples a signal fewer than two times per cycle. The samples then trace out a false waveform at a lower frequency than the real signal, often unstable or jittery on the screen, so the display does not represent the actual waveform.

• B. Aliasing is a sampling error in time; it does not change the measured DC offset.
• C. Aliasing affects the frequency and shape shown, not the calibration of the vertical (voltage) scale.
• D. Aliasing produces a false waveform rather than a blank screen; blanking refers to turning off the trace beam in analog displays.

B. Antenna analyzers compute SWR and impedance automatically

An antenna analyzer contains its own low-power signal source and measuring circuits, so it calculates and displays SWR and impedance (resistance and reactance) directly at any frequency. An SWR bridge only compares forward and reflected signals and does not show impedance.

• A. An antenna analyzer measures the antenna; it does not adjust or tune it, which you must do yourself.
• C. Displaying the modulation envelope over time is a job for an oscilloscope, not an antenna analyzer.
• D. Only automatic computation of SWR and impedance is an advantage here; the other statements are false, so not all are correct.

D. All these choices are correct

SWR can be measured in several ways. A directional wattmeter reads forward and reflected power, from which SWR is calculated; a vector network analyzer measures the reflection coefficient and displays SWR; and an antenna analyzer displays SWR directly. All three work.

• A. A directional wattmeter does give SWR from its forward and reflected power readings, but it is only one of the listed instruments that can.
• B. A vector network analyzer measures reflection coefficient and SWR, but the other two instruments can measure SWR too.
• C. An antenna analyzer displays SWR directly, but the other two instruments can measure it as well.

A. Minimize the length of the probe's ground connection

A long ground lead on an oscilloscope probe adds inductance and forms a loop that picks up noise and causes ringing on fast edges, distorting the displayed waveform. Keeping the ground connection as short as possible gives a more accurate measurement, especially at high frequencies.

• B. High-impedance probes are standard and work well on low-impedance circuits; loading is a concern when a low-impedance probe is used on a high-impedance circuit.
• C. A DC-coupled probe passes both AC and DC, so it measures AC circuits without trouble; AC or DC coupling is normally selected at the oscilloscope input.
• D. Only keeping the ground connection short is good practice; the other two statements are false.

D. Line

Ripple from a linear power supply comes from rectifying the AC line, so it repeats at the line frequency or twice it (60 or 120 Hz in the US). Line triggering starts each sweep in step with the AC line, so the ripple displays as a stable pattern even when it is small and noisy.

• A. Single-shot mode captures one event and stops, which suits a one-time transient, not continuous ripple.
• B. Edge triggering on small ripple riding on the supply's DC output can be unreliable; triggering from the AC line locks the display to the ripple's source.
• C. The trigger level is the voltage threshold used with edge triggering; it is a setting, not a mode tied to the line frequency.

D. All these choices are correct

An antenna analyzer is a low-power signal source combined with an impedance and SWR meter, so it can do more than check antennas. It can find the resonant frequency of a tuned circuit, measure a cable's electrical length, and determine its velocity factor by comparing electrical and physical lengths.

• A. Velocity factor can be measured by finding the frequency at which a known length of open or shorted cable resonates, but this is only one of the listed measurements.
• B. Cable length can be found from the cable's resonant frequencies and velocity factor, but this is only one of the listed measurements.
• C. A tuned circuit's resonant frequency can be found by coupling the analyzer to it, but this is only one of the listed measurements.

B. Time base accuracy

A frequency counter counts input cycles during a gate interval set by its time base, a reference oscillator. Any fractional error in the time base appears as the same fractional error in the reading: a time base off by 1 part per million gives a 14.2 Hz error at 14.2 MHz.

• A. The input attenuator sets signal level, not timing, so its accuracy does not change the frequency count.
• C. Decade dividers are digital counters that divide exactly, so they add no frequency error.
• D. Temperature matters mainly through its effect on the time base oscillator, not through the temperature coefficient of the counting logic.

A. The full scale reading of the voltmeter multiplied by its ohms per volt rating is the input impedance of the voltmeter

An analog voltmeter's ohms-per-volt rating tells how much it loads a circuit: multiply the rating by the full-scale voltage of the range in use to get the meter's input resistance. A 20,000 ohms-per-volt meter on its 10 V range has an input resistance of 20,000 x 10 = 200,000 ohms; a higher rating loads the circuit less.

• B. Multiplying the reading by the ohms-per-volt rating does not give power; the meter's resistance comes from the full-scale value of the range, not the reading.
• C. Dividing a resistance reading by the ohms-per-volt rating does not give the applied voltage or any other meaningful value.
• D. Ohms per volt describes voltmeter loading, not the shunt needed to extend an ammeter range.

C. S21

S parameters describe how a device transmits and reflects signals at its ports. S21 is the signal coming out of port 2 compared with the signal applied at port 1, which is the forward gain (or loss) through an amplifier or filter.

• A. S11 is the input reflection coefficient, related to input SWR and return loss.
• B. S12 is reverse transmission, from port 2 back to port 1, which describes reverse gain or isolation.
• D. S22 is the reflection coefficient at the output, port 2.

A. S11

S11 compares the signal reflected from port 1 with the signal applied to port 1, so it is the input reflection coefficient. Input return loss and VSWR are calculated from it: a reflection coefficient magnitude of 0.2 gives a return loss of about 14 dB and an SWR of 1.5:1.

• B. S12 is reverse transmission, from port 2 to port 1, not reflection at the input.
• C. S21 is forward transmission, the gain from port 1 to port 2.
• D. S22 is reflection at the output port, not the input port.

B. Short circuit, open circuit, and 50 ohms

A vector network analyzer is calibrated with standards whose behavior is known exactly: a short circuit, an open circuit, and a precision load equal to the system impedance, usually 50 ohms. Measuring these three lets the analyzer remove the errors of its own cables and connectors from later measurements; two-port measurements also add a through connection.

• A. Calibration uses the extremes of a short and an open plus one matched load, not three different resistive loads.
• C. A resonant circuit's impedance changes sharply with frequency, so it cannot serve as a known reference across the measurement range.
• D. Lengths of coax transform impedance differently at each frequency and are not the standard calibration loads; the standards are short, open and load.

D. 75 watts

Reflected power returns from the load instead of being absorbed, so the power absorbed equals forward power minus reflected power: 100 W - 25 W = 75 W.

• A. 100 watts is the forward power; it ignores the 25 W reflected by the load.
• B. 125 watts adds the reflected power instead of subtracting it.
• C. 112.5 watts adds half the reflected power, which has no basis; reflected power is subtracted.

A. The port or ports at which measurements are made

S parameter subscripts are port numbers. The first number is the port where the signal is measured and the second is the port where the signal is applied, so S21 is the output at port 2 for a signal applied at port 1.

• B. The subscripts name ports, not the time between measurements.
• C. S parameter subscripts carry no information about data quality; they name the ports involved.
• D. Frequency is what the analyzer sweeps; it is not encoded in the subscripts, which are port numbers.

C. The bandwidth of the circuit's frequency response

The Q of a tuned circuit equals its resonant frequency divided by its bandwidth between the half-power (-3 dB) points: Q = f0 / BW. A circuit resonant at 7.1 MHz with a 3 dB bandwidth of 71 kHz has Q = 7,100 kHz / 71 kHz = 100.

• A. At resonance the inductive and capacitive reactances are equal, so their ratio is always 1; the Q of a series circuit is reactance divided by resistance.
• B. A frequency shift by itself says nothing about Q; you need the bandwidth relative to the resonant frequency.
• D. Resonant frequency alone does not give Q; two circuits at the same frequency can have very different bandwidths.

B. Filter frequency response

A two-port vector network analyzer sends a swept test signal into a device and measures what comes out of the other port and what reflects back. With a filter connected between the ports it shows the filter's frequency response (S21, gain or loss versus frequency) along with its input and output match.

• A. Phase noise is a property of a signal source, measured with a spectrum analyzer or phase noise test set; a VNA measures how a device responds to its own test signal.
• C. Pulse rise time is a time-domain measurement made with an oscilloscope.
• D. Forward power from a transmitter is measured with a wattmeter; a VNA works with low-level test signals and is not a transmitter power meter.

B. Modulate the transmitter using two AF signals having non-harmonically related frequencies and observe the RF output with a spectrum analyzer

A two-tone test feeds an SSB transmitter's audio input with two audio tones that are not harmonically related, such as 700 Hz and 1900 Hz. The output then contains two RF signals, and a spectrum analyzer shows the intermodulation products (such as 2f1 - f2 and 2f2 - f1) beside them so their levels can be measured.

• A. An SSB transmitter is modulated through its audio input, so the test tones are audio frequencies, not RF signals.
• C. With harmonically related tones, audio harmonics land on the same frequencies as the products, and a peak-reading wattmeter cannot show individual products anyway.
• D. The test uses audio tones, not RF, and a logic analyzer shows digital states, not the RF spectrum.

D. All these choices are correct

A vector network analyzer measures both the magnitude and phase of reflected and transmitted signals. From reflection at port 1 it finds the reflection coefficient and input impedance, and from reflection at port 2 it finds output impedance, so all three can be measured.

• A. Input impedance is calculated from S11, but the other two quantities can be measured too.
• B. Output impedance is calculated from S22, but the other two quantities can be measured too.
• C. Reflection coefficient is what the analyzer measures directly, but input and output impedance can be measured too.

D. It can combine with strong signals on nearby frequencies to generate interference

Phase noise is random jitter in an oscillator's phase that spreads its energy into noise sidebands. In an SDR the master clock acts as the local oscillator, so its noise sidebands mix with strong signals on nearby frequencies and put noise on top of weak desired signals; this is reciprocal mixing.

• A. Phase noise adds noise around strong signals; it does not stop the receiver from receiving strong signals.
• B. Frequency calibration depends on the clock's average frequency accuracy, not on its short-term phase noise.
• C. The third-order intercept point measures the linearity of the receiver's amplifiers, mixers and converter, which clock phase noise does not change.

A. A front-end filter or preselector

A front-end filter or preselector sits ahead of the first amplifier and mixer and passes the band being received. Strong signals outside that band, such as nearby broadcast stations, are attenuated before they can overload the receiver and create intermodulation or other interference.

• B. A narrow IF filter acts after the mixer, so a strong out-of-band signal has already reached, and possibly overloaded, the earlier stages.
• C. A notch filter removes a single steady tone within the passband, not strong out-of-band signals.
• D. A product detector demodulates SSB and CW; adjusting it does not reject out-of-band signals.

C. Capture effect

An FM receiver tends to lock onto the stronger of two signals on the same frequency and suppress the weaker one; this is the capture effect. A signal only a few dB stronger than another can take over the receiver almost completely.

• A. Desensitization is a loss of sensitivity caused by a strong signal on a nearby frequency, not one signal on the same frequency taking over from another.
• B. Cross-modulation is the modulation of a strong unwanted signal appearing on the desired signal, not suppression of a weaker signal.
• D. Frequency discrimination is how an FM detector converts frequency changes to audio; it is not the name of this effect.

D. The ratio in dB of the noise generated by the receiver to the theoretical minimum noise

Noise figure compares the noise a receiver actually produces with the theoretical minimum, the thermal noise of a matched input at room temperature, expressed in dB. A perfect receiver would have a noise figure of 0 dB, and each dB of noise figure raises the noise floor by 1 dB.

• A. Atmospheric noise and phase noise are separate noise sources; noise figure is not their ratio.
• B. Noise figure compares noise powers, not bandwidths.
• C. Noise figure uses the theoretical thermal noise minimum as its reference, not atmospheric noise, which varies with frequency, time and location.

B. The theoretical noise in a 1 Hz bandwidth at the input of a perfect receiver at room temperature

Thermal noise power is kTB: Boltzmann's constant times temperature times bandwidth. At room temperature (290 K) in a 1 Hz bandwidth this is about 4 x 10⁻²¹ W, or -174 dBm, the noise floor of a perfect, noiseless receiver. Real receivers add bandwidth and noise figure: noise floor = -174 dBm + 10 log(bandwidth in Hz) + noise figure.

• A. A receiver 6 dB above the theoretical minimum has a 6 dB noise figure and a 1 Hz noise floor of -168 dBm, not -174 dBm.
• C. Noise figure is a ratio in dB, not an absolute level in dBm; -174 dBm is a noise power.
• D. A receiver 3 dB above the theoretical minimum has a 1 Hz noise floor of -171 dBm; -174 dBm is the minimum itself.

D. 13 dB

Noise power is proportional to bandwidth, so the noise floor rises by 10 log(new bandwidth / old bandwidth). Going from 50 Hz to 1,000 Hz is a factor of 20, and 10 log(20) = 13 dB.

• A. 3 dB corresponds to doubling the bandwidth, such as 50 Hz to 100 Hz, not a twentyfold increase.
• B. 5 dB corresponds to about a 3.2 times increase in bandwidth, much less than the factor of 20 here.
• C. 10 dB corresponds to a tenfold increase, such as 50 Hz to 500 Hz, while 50 Hz to 1,000 Hz is twentyfold.

B. The minimum discernible signal

MDS stands for minimum discernible signal, the weakest signal a receiver can detect, usually defined as the signal level equal to the receiver's noise in a given bandwidth. A lower (more negative) MDS in dBm means a more sensitive receiver.

• A. Meter display sensitivity is not what MDS stands for; MDS refers to the weakest signal the receiver can detect.
• C. Modulation distortion specification is not what MDS stands for; MDS is a sensitivity measurement.
• D. MDS describes the weakest detectable signal level, not a range of spectrum.

D. The reference voltage of the analog-to-digital converter

An SDR's analog-to-digital converter can represent input voltages only up to its reference voltage, its full-scale input. A signal above that level clips at the maximum digital value, creating distortion and spurious signals across the spectrum, so that is the point of overload.

• A. Half the sample rate is the Nyquist frequency, a limit on frequency, not on signal level.
• B. Buffer size is the memory that holds samples, not the signal level at which the converter overloads.
• C. The maximum count is the digital output at full scale, not an input signal level; the input voltage that produces it is the reference voltage.

C. Easier for front-end circuitry to eliminate image responses

In a superheterodyne receiver the image frequency lies twice the IF away from the desired signal. A high IF puts the image far away, where front-end filters reject it easily: with a 9 MHz IF and a 14.2 MHz signal (local oscillator at 23.2 MHz) the image is at 32.2 MHz, while a 455 kHz IF puts it only 0.91 MHz away, at 15.11 MHz.

• A. A high IF does not reduce the number of components; a receiver may even need an extra conversion stage to get narrow selectivity.
• B. Drift depends on oscillator stability, not on choosing a high IF.
• D. Receiver noise figure is set mainly by the first amplifier and mixer, not by the choice of IF.

C. Receive bandwidth can be set to match the modulation bandwidth, maximizing signal-to-noise ratio and minimizing interference

Noise power grows with bandwidth, so a receive bandwidth wider than the signal lets in extra noise and nearby interference with no gain in signal. Matching the bandwidth to the mode, such as about 2.4 kHz for SSB or a few hundred hertz for CW, maximizes signal-to-noise ratio and keeps out adjacent signals.

• A. The noise figure of the RF amplifier is fixed by its design; selecting a receive bandwidth does not change it.
• B. Selectable bandwidths are for signal-to-noise ratio and interference rejection, not for reducing power consumption.
• D. Bandwidth selection does not allow receiving several frequencies at once; its advantage is better signal-to-noise ratio and less interference.

D. Atmospheric noise is generally greater than internally generated noise even after attenuation

On the lower HF bands, atmospheric and man-made noise arriving from the antenna is usually much stronger than the noise the receiver generates. An attenuator reduces the signal and that external noise equally, so as long as the external noise stays above the receiver's own noise floor, the signal-to-noise ratio does not change while the lower signal levels reduce overload.

• A. An attenuator weakens signals; it contains no filter that makes lower frequency signals stronger.
• B. An attenuator has no noise filter; it reduces the signal and the noise together.
• C. An attenuator cannot separate signals from noise; both arrive from the antenna and are reduced by the same amount.

C. It improves blocking dynamic range by attenuating strong signals near the receive frequency

A roofing filter is a narrow filter placed right after the first mixer, ahead of the amplifiers and later stages. By attenuating strong signals near, but outside, the receive passband before they reach those stages, it keeps them from overloading the receiver, which improves blocking dynamic range.

• A. Sensitivity is set mainly by the noise figure of the stages ahead of the roofing filter, which the filter does not change.
• B. Roofing filters use high-Q circuits, such as crystal filters, to be narrow; low-Q circuits would make them broad and ineffective.
• D. Only the improvement in blocking dynamic range is true, so not all of these are correct.

D. Local oscillator phase noise mixing with adjacent strong signals to create interference to desired signals

Reciprocal mixing happens when the noise sidebands (phase noise) of a receiver's local oscillator mix with a strong signal near the desired frequency. The mixing transfers that noise to the desired frequency, raising the noise floor and masking weak signals even though the strong signal is outside the passband.

• A. Two out-of-band signals mixing to produce an in-band product describes intermodulation distortion.
• B. Reciprocal mixing adds noise from the local oscillator; it does not involve in-phase signals cancelling in the mixer.
• C. Signals sharing alternate time slots describes time-division multiplexing, as used in some digital voice systems, not reciprocal mixing.

C. To reduce interference from stations transmitting on adjacent frequencies

IF Shift moves the IF filter's passband up or down relative to the signal without changing the receive frequency. Sliding the passband away from an interfering station on one side cuts its signal while keeping the desired signal inside the filter.

• A. Listening on a frequency different from the transmit frequency is split operation, not IF Shift.
• B. Changing frequency rapidly is done with the tuning controls, not IF Shift.
• D. Tuning a station slightly off frequency without moving the transmit frequency is receiver incremental tuning (RIT).

A. The difference in dB between the noise floor and the level of an incoming signal that will cause 1 dB of gain compression

Blocking dynamic range spans from the receiver's noise floor up to the level of a strong nearby signal that reduces the gain for a desired signal by 1 dB (1 dB compression). A receiver with a -130 dBm noise floor that compresses with a -5 dBm signal has a blocking dynamic range of -5 - (-130) = 125 dB.

• B. One FM signal suppressing another describes the capture effect, not blocking dynamic range.
• C. The span from the noise floor to the third-order intercept is not a dynamic range specification; the intercept is a theoretical point found by extrapolation.
• D. This garbles the definition of two-tone third-order intermodulation dynamic range, a different specification from blocking dynamic range.

A. Spurious signals caused by cross modulation and desensitization from strong adjacent signals

Dynamic range is the span of signal levels a receiver handles without distortion. With poor dynamic range, strong signals near the desired one overload the front end, causing cross-modulation (their modulation appears on other signals), spurious intermodulation signals, and desensitization (loss of sensitivity).

• B. Oscillator instability and trouble recovering the opposite sideband come from oscillator and detector design, not dynamic range.
• C. Insufficient local oscillator injection is a mixer drive problem, not a result of poor dynamic range, which shows up with strong signals.
• D. Oscillator instability and distortion of weaker signals are not the symptoms of poor dynamic range, which appear when strong signals overload the receiver.

B. The output signals mix in the final amplifier of one or both transmitters

When two repeater transmitters are close together, each one's signal couples into the other's antenna and reaches its final amplifier. The amplifier's nonlinearity mixes the two signals, producing intermodulation products such as 2f1 - f2 that are transmitted and can land on other channels.

• A. Feedback in an amplifier can cause oscillation, but intermodulation comes from mixing two signals in a nonlinear stage.
• C. No harmonic relationship between input frequencies is needed; the products come from the transmitter outputs mixing in a final amplifier.
• D. Intermodulation products are sums and differences of multiples of the two frequencies, so they occur whether or not the outputs are harmonically related.

B. A properly terminated circulator at the output of the repeater’s transmitter

A circulator is a one-way device: it passes the transmitter's output to the antenna but routes any signal coming back from the antenna into a terminating load on its third port. With that port properly terminated, a nearby transmitter's signal picked up by the antenna cannot reach the repeater's final amplifier and mix there.

• A. A band-pass filter between transmitter and receiver does not keep the outside signal, arriving through the antenna, out of the transmitter's final amplifier.
• C. A Class C amplifier is highly nonlinear, so it readily mixes incoming signals rather than reducing intermodulation.
• D. A Class D switching amplifier is also strongly nonlinear and does not prevent mixing of an outside signal.

C. Desensitization

Desensitization is the loss of receiver sensitivity caused by a strong signal near the receive frequency. The strong signal overloads the front end or drives down its gain, so weak signals that were readable fade into the noise.

• A. Reciprocal mixing is local oscillator phase noise mixing with strong nearby signals to add noise, a different mechanism.
• B. Quieting is the drop in FM receiver noise when a signal is present, not a loss of sensitivity.
• D. Cross-modulation interference is the strong signal's modulation appearing on the desired signal, not a general loss of sensitivity.

A. Insert attenuation before the first RF stage

Desensitization happens when a strong signal overloads the receiver's first stages. An attenuator ahead of the first RF stage lowers the strong signal below the overload point, and where outside noise is well above the receiver's own noise, weak signals remain readable.

• B. Raising the IF improves image rejection; it does not reduce overload from a strong signal near the receive frequency.
• C. More front-end gain makes the strong signal even stronger in the receiver, increasing overload and desensitization.
• D. AGC speed changes how quickly gain recovers after a signal; it does not prevent front-end overload.

C. Nonlinear circuits or devices

Intermodulation is created when two or more signals pass through a nonlinear circuit or device, such as an overdriven amplifier, a mixer or a corroded junction. The nonlinearity produces new signals at sums and differences of the inputs and their harmonics, such as 2f1 - f2.

• A. Negative feedback makes amplifiers more linear, which reduces intermodulation.
• B. Lack of neutralization can make an amplifier unstable or oscillate, but intermodulation comes from nonlinearity.
• D. Positive feedback leads to oscillation, not intermodulation between signals.

C. To increase the rejection of signals outside the band being received

A preselector is a tunable band-pass filter at the receiver input that passes the band being received and attenuates signals outside it. Keeping strong out-of-band signals away from the first stages reduces overload, intermodulation and image responses.

• A. Storing often-used frequencies is the job of memory channels, not a preselector.
• B. A preselector is a selective filter, not a broadband attenuator; broadband attenuation reduces wanted and unwanted signals equally.
• D. Choosing the RF amplifier device is a design decision, not something a preselector does.

C. A pair of 40 dBm input signals will theoretically generate a third-order intermodulation product that has the same output amplitude as either of the input signals

As input level rises, third-order intermodulation products grow 3 dB for every 1 dB of input, three times as fast as the desired signals. The third-order intercept is the theoretical level where the two would be equal if their lines were extended, so with a 40 dBm intercept, two 40 dBm input signals would in theory produce a product as strong as either signal. Real receivers overload well before that level.

• A. Third-order products are generated at any input level; below the intercept they are weaker than the signals, not absent.
• B. The intercept point is an absolute power level in dBm, not a level relative to the noise floor, and products occur at all levels.
• D. With a 40 dBm intercept, two 1 mW (0 dBm) signals produce third-order products about 80 dB weaker than the inputs, not 40 dB stronger.

A. Odd-order products of two signals in the band being received are also likely to be within the band

Third-order products such as 2f1 - f2 and 2f2 - f1 fall close to the original signals. Signals at 14.200 and 14.210 MHz produce products at 14.190 and 14.220 MHz, right in the same band, while the second-order products (10 kHz and 28.410 MHz) fall far away and are easily filtered out.

• B. Odd-order products matter because they fall in the receive band, not because they overload IF filters.
• C. Image rejection is a separate receiver property involving the image frequency, not intermodulation.
• D. Two signals produce two nearby third-order products (2f1 - f2 and 2f2 - f1), not three products for every input signal.

C. +8dB

Link margin is how far the received signal exceeds the minimum the receiver needs. Received signal = 40 dBm + 10 dBi - 3 dB - 136 dB = -89 dBm; required signal = MDS + required signal-to-noise ratio = -103 dBm + 6 dB = -97 dBm; margin = -89 dBm - (-97 dBm) = +8 dB.

• A. -8 dB has the sign reversed; the received -89 dBm is 8 dB stronger than the required -97 dBm, so the margin is positive.
• B. -14 dB reverses the sign and also leaves out the required 6 dB signal-to-noise ratio.
• D. +14 dB compares the received signal with the MDS alone and leaves out the required 6 dB signal-to-noise ratio.

A. -51 dBm

Received signal level = transmit power + transmit antenna gain + receive antenna gain - path loss. Here that is 40 dBm + 6 dB + 3 dB - 100 dB = -51 dBm.

• B. -54 dBm leaves out the 3 dB receive antenna gain.
• C. -57 dBm leaves out the 6 dB transmit antenna gain.
• D. -60 dBm leaves out both antenna gains (40 dBm - 100 dB).

D. 0.1 picowatts

dBm is power relative to 1 milliwatt, and every 10 dB is a factor of 10. -100 dBm is 100 dB, a factor of 10¹⁰, below 1 mW: 1 mW / 10¹⁰ = 10⁻¹³ W = 0.1 picowatt.

• A. 100 microwatts is -10 dBm, far stronger than -100 dBm.
• B. 0.1 microwatt is -40 dBm, far stronger than -100 dBm.
• C. 0.001 microwatt (1 nanowatt) is -60 dBm, still 40 dB stronger than -100 dBm.

A. Removal of the CW signal as well as the interfering carrier

An automatic notch filter finds steady tones in the passband and removes them. A CW signal is itself a steady carrier keyed on and off, so the notch can treat it as an interfering carrier and remove the signal you want to copy, which is why automatic notch filters are normally used on voice modes.

• B. Signals outside the notch pass through normally; an automatic notch does not cause nearby signals to overwhelm the desired one.
• C. Ringing is associated with very narrow bandpass filters; the characteristic problem of an automatic notch in CW reception is that it removes the CW signal itself.
• D. Removal of the CW signal is the characteristic problem of an automatic notch in CW reception; the other listed effects are not, so not all of these are correct.

D. All these choices are correct

Digital noise reduction uses DSP to separate a signal from noise based on their different characteristics, such as random noise compared with the structure of speech or CW. It can often reduce broadband white noise, ignition noise and power line noise, so all of these are correct.

• A. Broadband white noise is one type digital noise reduction can reduce, but it also helps with ignition and power line noise.
• B. Ignition noise can often be reduced by digital noise reduction, but it is only one of the listed types.
• C. Power line noise can often be reduced by digital noise reduction, but it is only one of the listed types.

B. Impulse noise

A noise blanker detects short, strong pulses, such as ignition noise, and briefly mutes the receiver for the duration of each pulse. Because it works on brief pulses, it removes impulse noise but cannot help with continuous noise.

• A. Broadband white noise is continuous, so it has no pulses for a blanker to gate out.
• C. Hum and buzz are continuous interference, not short pulses, so a noise blanker does not remove them.
• D. A noise blanker removes only impulse noise, so not all of these are correct.

D. By installing ferrite chokes on the charging system leads

Ferrite chokes on the alternator and charging system leads add impedance to RF noise currents while passing the DC charging current, so they suppress conducted noise from the alternator and regulator. Filter capacitors connected from the leads to ground are often used with them.

• A. A capacitor in series with an alternator lead would block the DC charging current; filter capacitors are connected from the lead to ground, not in series.
• B. Suppression resistors belong in the ignition system, such as resistor spark plugs and wires, and a blocking capacitor in the charging path would stop the DC current.
• C. A high-pass filter in series with the power lead would block the DC the radio needs, and a filter across the antenna feed line does nothing about noise conducted on the power wiring.

B. A brute-force AC-line filter in series with the motor’s power leads

A brute-force AC-line filter is a low-pass filter, with series inductors and capacitors to ground, that passes 60 Hz power while blocking RF. Installed in series with the motor's power leads, it keeps RF noise from sparking brushes or contacts from traveling along the power wiring and radiating.

• A. A high-pass filter would block the 60 Hz power the motor needs and pass the RF noise, the opposite of what is wanted.
• C. A bypass capacitor works when connected in parallel to shunt RF to ground; placed in series it would carry the motor current and bypass nothing.
• D. A choke blocks RF when placed in series with a lead; connected in parallel with the winding it does not stop RF from traveling on the power wiring.

C. The appearance of unstable modulated or unmodulated signals at specific frequencies

Computer and network equipment uses clock oscillators and fast digital signals whose harmonics radiate from the equipment and its cables. In a receiver these appear as carriers, plain or carrying data, at specific frequencies, often drifting or unstable.

• A. A loud AC hum points to power supply problems or ground loops in the station, not radiation from network equipment.
• B. Clicks at regular intervals point to pulsed devices such as electric fence chargers, not network equipment.
• D. A whining noise that pulses on and off points to a motor or other intermittently running device, not to the clock signals of network equipment.

B. Common-mode currents on the shield and conductors

Ideally, signal current flows out on a cable's inner conductor and returns on the inside of the shield, so the fields cancel. Common-mode current flows in the same direction on the outside of the shield and on the conductors together, turning the cable into an antenna that radiates and picks up interference.

• A. Low-inductance ground connections at both ends of the shield help keep RF off the cable; they reduce interference rather than cause it.
• C. Braided shield is a standard, effective shielding material and does not by itself cause radiation.
• D. Tying grounds to a common point is recommended practice that keeps equipment at the same potential, and the troublesome current on a shield is common-mode, not differential-mode.

B. Common-mode current

Common-mode current flows equally, and in the same direction, on all conductors of a cable, as though the cable were a single wire. Because these currents do not cancel, they radiate and pick up interference; ferrite chokes are a common cure.

• A. Differential-mode current flows in opposite directions on the conductors, out on one and back on another; it is the normal signal current.
• C. Reactive current is current out of phase with voltage in a reactance, a different concept from how current divides among a cable's conductors.
• D. Magnetic coupling is one way a current can be induced, not the name for current flowing equally on all conductors.

C. Strong signals may be distorted and appear to cause spurious emissions

A noise blanker gates the receiver off during pulses, but a strong signal near the receive frequency can also trigger it. The rapid gating then mixes with the strong signal, creating distortion products that make the signal look and sound as though it has spurious emissions.

• A. A noise blanker does not create echoes in speech audio; its side effect involves distortion of strong signals.
• B. A noise blanker does not change the audio bandwidth of the received signal.
• D. FM signals can still be demodulated with a noise blanker on; the blanker does not disable demodulation.

D. All these choices are correct

AC line interference comes from arcing or sparking in equipment connected to the power lines. Arcing thermostat contacts, a defective doorbell or doorbell transformer, and a malfunctioning illuminated sign can all produce intermittent loud roaring or buzzing on the bands, so all of these are correct.

• A. Arcing contacts in a thermostatically controlled device, such as a heater, can cause this interference, but they are only one of the listed sources.
• B. A defective doorbell or doorbell transformer can cause this interference, but it is only one of the listed sources.
• C. A malfunctioning illuminated advertising display can cause this interference, but it is only one of the listed sources.

B. Nearby corroded metal connections are mixing and reradiating the broadcast signals

Corroded metal joints, such as rusty bolts in fences, gutters or towers, can act as crude diodes. Strong local broadcast signals induced in the metal mix in these nonlinear junctions, and the structure reradiates the mixing products, which can appear on the MF or HF bands; this is often called the rusty bolt effect.

• A. Overmodulation causes splatter next to that station's own frequency; it does not combine signals from several stations into new spurious signals.
• C. A distant station received by skywave is a real signal on its own frequency, not a product of local stations combining.
• D. The IF amplifier handles only signals already converted to the IF, so its overload does not combine broadcast-band signals into new MF or HF signals.

A. Switch-mode power supplies

A switch-mode power supply switches current on and off at a fixed rate, producing harmonics at every multiple of the switching frequency. A supply switching at 100 kHz, for example, puts a carrier every 100 kHz across a wide range of the spectrum.

• B. Radar transmits pulses on a particular frequency, heard as a buzz or rasp, not a series of carriers at regular intervals across a wide range.
• C. Wireless security cameras transmit on a specific channel, not a series of carriers spread at regular intervals.
• D. Electric fences produce broadband clicks or pops at regular time intervals, not steady carriers at regular frequency intervals.

C. On the single point ground panel

A station AC surge protector belongs on the single point ground panel, with the protectors for feed lines, rotator cables and phone lines. Bonding all the protectors at one point keeps them at the same potential during a surge, so they act together and no large voltage develops between the AC wiring and the other cables.

• A. A protector at the AC service panel guards the building's wiring, but the station's protector belongs at the single point ground panel with the other protectors.
• B. A plug-in protector at an outlet refers to a different ground point than the station's other protectors, so a surge can create voltage differences between equipment.
• D. The AC protector must connect to the station's AC wiring and share the ground panel with the other protectors; a ground rod is a grounding electrode, not a mounting point for it.

D. Ensure all lightning protectors activate at the same time

A single point ground panel bonds all of the station's surge protectors (feed line, rotator, AC and telephone) to one ground connection. Because they share the same reference, a lightning surge raises them all together, so they activate at the same time and no damaging voltage builds up between cables entering the station.

• A. Removing AC power during a short circuit is the job of fuses and circuit breakers, not a ground panel.
• B. The panel does not prevent transients; it gives the protectors a common ground so they respond together when a transient arrives.
• C. Eliminating air gaps is not a function of the ground panel; its purpose is a common bonding point for the protectors.