Amateur Extra · correct answers only, in question-pool order.
The analog-to-digital converter must sample fast enough to represent the waveform accurately. Its sampling rate therefore limits the highest frequency a digital oscilloscope can display correctly.
A spectrum analyzer plots signal amplitude vertically and frequency horizontally.
A spectrum analyzer displays the transmitter's output as a function of frequency, making spurious signals and intermodulation products visible.
Probe compensation is checked with a square wave. Adjust the probe until the flat portions of the displayed waveform are as flat as possible.
A prescaler divides an input signal so a frequency counter with a lower maximum counting frequency can measure a higher-frequency signal.
Aliasing occurs when a waveform is sampled too slowly, causing a false lower-frequency and often jittery display.
An antenna analyzer directly calculates both SWR and impedance, providing more information than a basic SWR bridge.
The official E4A08 source lists a directional wattmeter as an instrument that can be used to measure SWR.
The official E4A08 source lists a vector network analyzer as an instrument that can be used to measure SWR.
The official E4A08 source lists an antenna analyzer as an instrument that can be used to measure SWR.
A long probe ground lead adds inductance and can distort high-frequency measurements. Keeping the ground connection short improves measurement accuracy.
Line triggering synchronizes the oscilloscope sweep to the AC power-line frequency, making power-supply ripple stable on the display.
The official E4A11 source identifies velocity factor as one quantity that can be measured with an antenna analyzer.
The official E4A11 source identifies cable length as one quantity that can be measured with an antenna analyzer.
The official E4A11 source identifies tuned-circuit resonant frequency as one quantity measurable with an antenna analyzer.
A frequency counter ultimately depends on its internal time base as the reference against which cycles are counted. Time-base error therefore becomes frequency-measurement error.
For an analog voltmeter rated in ohms per volt, multiply the selected full-scale voltage by the ohms-per-volt rating to obtain the meter's input impedance.
S21 represents transmission from port 1 to port 2, which corresponds to forward gain in a two-port network.
S11 is the reflection coefficient looking into port 1, so it represents input return loss and is related to VSWR.
A one-port VNA calibration uses known open, short, and 50-ohm load standards to remove systematic measurement errors.
Power absorbed by the load is forward power minus reflected power: 100 W minus 25 W equals 75 W.
The subscripts on S parameters identify the port or ports at which the incident and resulting waves are measured.
For a series-tuned circuit, Q is related to resonant frequency divided by the half-power bandwidth. Measuring the response bandwidth therefore lets you determine Q.
A two-port VNA measures transmission through a network as frequency changes, so it can display a filter's amplitude and phase response.
The standard two-tone SSB test uses two non-harmonically related audio-frequency tones and observes the RF output spectrum for intermodulation products.
The official E4B11 source lists input impedance as a VNA measurement.
The official E4B11 source lists output impedance as a VNA measurement.
The official E4B11 source lists reflection coefficient as a VNA measurement.
Master-clock phase noise in an SDR can mix with strong nearby signals and spread their energy into the desired channel, creating interference.
A front-end filter or preselector rejects strong signals before they reach sensitive receiver stages, making it effective against out-of-band overload.
The FM capture effect is the tendency of a receiver to suppress the weaker of two signals on the same frequency and reproduce the stronger one.
Noise figure compares the noise produced by the receiver with the theoretical minimum noise, expressed as a ratio in decibels.
-174 dBm is the approximate thermal-noise power in a 1 Hz bandwidth at room temperature for an ideal receiver input.
Noise power rises in proportion to bandwidth. Increasing bandwidth from 50 Hz to 1000 Hz is a factor of 20, or about 13 dB.
MDS means minimum discernible signal: the weakest signal level that can be distinguished from the receiver noise.
An SDR overloads when the input drives its analog-to-digital converter beyond the converter's reference-voltage range.
A high first IF moves the image frequency farther from the wanted signal, making the image easier for front-end filtering to reject.
Selectable receiver bandwidth lets the operator match the filter to the modulation bandwidth, maximizing signal-to-noise ratio while excluding nearby interference.
On lower HF bands, atmospheric noise is often much stronger than the receiver's own internal noise. Moderate attenuation reduces signal and external noise together with little change in overall SNR while helping prevent overload.
A narrow roofing filter attenuates strong nearby signals early in the IF chain, improving blocking dynamic range.
Reciprocal mixing occurs when local-oscillator phase noise mixes with a strong nearby signal, producing noise or interference in the desired receive channel.
IF Shift moves the receiver passband relative to the tuned frequency, helping reduce interference from an adjacent signal without changing the transmit frequency.
Blocking dynamic range is the difference between the receiver noise floor and the level of a strong input signal that causes 1 dB of gain compression.
Poor dynamic range lets strong nearby signals cause receiver desensitization and cross-modulation products that appear as spurious signals.
Two nearby repeater transmit signals can mix in a nonlinear final amplifier and generate intermodulation products.
A properly terminated circulator on the repeater transmitter output helps prevent energy from a nearby transmitter from entering the repeater's final amplifier and mixing there.
Desensitization is the reduction of receiver sensitivity caused by a strong nearby signal.
Attenuation placed ahead of the first RF stage reduces the level of strong signals before they can overload the receiver.
Intermodulation is produced when multiple signals pass through a nonlinear circuit or device.
A receiver preselector is a tunable front-end filter that rejects signals outside the band being received.
A third-order intercept point of 40 dBm is the theoretical input level at which extrapolated desired-signal and third-order product output amplitudes would become equal.
Odd-order intermodulation products from two in-band signals tend to fall near the original signals and are therefore likely to land inside the receiver passband.
Start with +40 dBm transmit power, add 10 dB antenna gain, subtract 3 dB cable loss and 136 dB path loss: received level is -89 dBm. A -103 dBm MDS plus 6 dB required SNR means -97 dBm is required, leaving an 8 dB margin.
Add transmitter power and antenna gains, then subtract path loss: +40 dBm + 6 dBi + 3 dBi - 100 dB = -51 dBm.
-100 dBm is 10^-13 watt, which equals 0.1 picowatt.
An automatic notch filter looks for steady narrow carriers. A CW signal can resemble the interference it is designed to remove, so the desired CW may also be notched out.
The official E4E02 source identifies broadband white noise as one type of noise that digital noise reduction can often reduce.
The official E4E02 source identifies ignition noise as one type of noise that digital noise reduction can often reduce.
The official E4E02 source identifies power-line noise as one type of noise that digital noise reduction can often reduce.
A noise blanker is designed primarily for short-duration impulse noise such as ignition pulses.
Ferrite chokes on the charging-system leads add impedance to conducted RF noise currents and help keep alternator or charging noise out of the radio.
A brute-force AC-line filter placed in series with a line-driven motor's power leads can suppress RF interference conducted along the mains wiring.
Digital network equipment can generate discrete, unstable modulated or unmodulated signals at specific frequencies through its clocks and high-speed digital circuitry.
Common-mode current can flow on a cable shield and its conductors together, causing a nominally shielded cable to radiate or receive RF.
Current flowing equally and in the same direction on all conductors of a multiconductor cable is common-mode current.
A noise blanker driven too hard by strong signals can create distortion and spurious-looking products in the receiver.
The official E4E10 source lists arcing thermostat-controlled contacts as a possible source of intermittent loud roaring or buzzing AC-line interference.
The official E4E10 source lists a defective doorbell or transformer as a possible source of intermittent loud AC-line interference.
The official E4E10 source lists a malfunctioning illuminated advertising display as a possible source of intermittent loud AC-line interference.
Corroded metal junctions can behave as nonlinear rectifiers, mixing strong local AM broadcast signals and reradiating new spurious frequencies.
Switch-mode power supplies operate with repetitive switching waveforms that can create combs of carriers at regular frequency intervals.
The station AC surge protector should be mounted on the single-point ground panel so surge paths converge at the same bonding reference.
A single-point ground panel gives lightning protectors a common reference so they rise together during a surge and activate at approximately the same time.