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E4A

AMATEUR PRACTICES

Test equipment: analog and digital instruments; spectrum analyzers; antenna analyzers; oscilloscopes; RF measurements

E4A011 of 11

Which of the following limits the highest frequency signal that can be accurately displayed on a digital oscilloscope?

Why A digital scope works by taking discrete samples of the waveform and reconstructing it, so the analog-to-digital converter's sampling rate sets the ceiling on what it can faithfully show. The Nyquist criterion says you need at least two samples per cycle just to detect a frequency, and in practice scope makers want roughly 5 to 10 samples per cycle for a usable waveform picture. So a scope sampling at, say, 1 gigasample per second cannot honestly display signals anywhere near that rate; undersampling produces aliasing, a false low-frequency image of the real signal.

Watch out The idea of an 'analog-to-digital converter reference frequency' sounds plausible but is not the limiting spec; ADCs have a voltage reference that sets amplitude scaling, not a frequency reference. Q describes the sharpness of a resonant circuit and has nothing to do with scope bandwidth.

Digital scope = samples. Nyquist: you need at least 2 samples per cycle, so sample rate caps the top frequency.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E4A022 of 11

Which of the following parameters does a spectrum analyzer display on the vertical and horizontal axes?

Why A spectrum analyzer sweeps a narrow receiver across a band of frequencies and plots how much energy it finds at each one, so the horizontal axis is frequency (set by center frequency and span) and the vertical axis is amplitude, usually in dBm per division. That is what lets you see carriers, sidebands, harmonics and spurious emissions spread out side by side across the display.

Watch out Amplitude versus time describes an oscilloscope, which works in the time domain; SWR versus frequency is what an antenna analyzer or SWR sweeper plots.

Spectrum analyzer = frequency domain: amplitude up, frequency across. Scope = time domain.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E4A033 of 11

Which of the following test instruments is used to display spurious signals and/or intermodulation distortion products generated by an SSB transmitter?

Why A spectrum analyzer displays amplitude versus frequency, so every signal a transmitter puts out shows up as a separate peak at its own frequency. Spurious emissions, harmonics, and the third- and fifth-order intermodulation products of a two-tone SSB test appear as sidebands beside the wanted signal, and their level can be read directly in dB below PEP. An oscilloscope, by contrast, works in the time domain and can only show gross envelope flat-topping, not which frequencies are present.

Watch out A network analyzer is the closest tempting choice, but it injects its own known signal to measure a device's gain, loss, phase, and impedance (S-parameters); it characterizes circuits, not the unwanted output of a transmitter. A logic analyzer looks at digital bit streams.

Spurs and IMD are a frequency-domain problem, so use the frequency-domain instrument: the spectrum analyzer.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E4A044 of 11

How is compensation of an oscilloscope probe performed?

Why A 10x probe forms an RC divider with the scope input capacitance, and the probe's trimmer capacitor must be set so the capacitive divider ratio matches the resistive ratio at all frequencies. A square wave is the ideal test signal because it contains a wide spread of harmonics, so any mismatch shows up immediately as rounded or overshooting corners. You connect the probe to the scope's built-in calibrator output (typically a 1 kHz square wave) and tweak the trimmer until the flat tops and bottoms are truly flat.

Watch out Adjusting on a sine wave for maximum amplitude is tempting because compensation does affect high frequency response, but a single sine frequency cannot reveal the low frequency droop or high frequency peaking that a square wave exposes at once. DC voltage and timebase accuracy are separate calibrations that a probe trimmer does not control.

Square wave in, flat tops out. Rounded corners = under, overshoot = over.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E4A055 of 11

What is the purpose of using a prescaler with a frequency counter?

Why A prescaler is a fast digital divider placed ahead of the counter's gate circuitry. It divides the incoming signal by a fixed ratio, typically 10 or 100, so a signal well above the counter's maximum input frequency comes down into a range the counter can resolve. You then multiply the displayed reading by the prescaler ratio to get the true frequency (or the counter does it for you by stretching the gate time).

Watch out The choice about multiplying the signal has it backwards: multiplying would push the frequency even further beyond a low-frequency counter's limit. Amplification is the job of the counter's input preamp, not a prescaler.

Pre-SCALE means divide down. Prescaler = divide by 10 or 100 to fit a fast signal into a slow counter.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E4A066 of 11

What is the effect of aliasing on a digital oscilloscope when displaying a waveform?

Why A digital scope samples the input at discrete intervals; the Nyquist criterion requires the sample rate to be more than twice the highest frequency component present. If the signal exceeds that limit, the samples reconstruct a completely different, much lower frequency waveform that appears to wander or jitter on the screen. That ghost trace is aliasing, and it can look convincingly like a real signal if you do not check the timebase and sample rate.

Watch out Vertical accuracy, DC offset and blanking are all amplitude or display issues; aliasing is purely a time-axis sampling artifact and leaves the vertical calibration untouched.

Aliasing = undersampling. Too few samples makes a fast signal masquerade as a slow, jittery one (like wagon wheels in movies).

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E4A077 of 11

Which of the following is an advantage of using an antenna analyzer compared to an SWR bridge?

Why An antenna analyzer contains its own low-power signal source and measurement bridge with a microprocessor, so it sweeps frequency and directly reads out SWR, resistance and reactance (or impedance magnitude) without needing your transmitter on the air. A plain SWR bridge only compares forward and reflected power while you key a transmitter into it, and it gives you a ratio rather than the R and X values that tell you which way to adjust the antenna. That makes the analyzer both safer (no QRM, no transmitter loading into a bad match) and far more informative for tuning and troubleshooting.

Watch out The analyzer measures and reports; it has no motors or matching network, so it cannot tune the antenna for you. Displaying a time-varying modulation envelope describes an oscilloscope, not an analyzer.

Analyzer = measure, not adjust. It computes SWR plus R and X; you still do the cutting and tuning.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E4A088 of 11

Which of the following is used to measure SWR?

Why SWR is a ratio derived from forward and reflected power or from the complex impedance seen at the feedpoint, and all three instruments can get there. A directional wattmeter separates forward and reflected power, and SWR = (1 + sqrt(Pr/Pf)) / (1 - sqrt(Pr/Pf)). An antenna analyzer injects a low-level signal and reports SWR (and often R and X) versus frequency. A vector network analyzer measures the complex reflection coefficient S11, from which SWR = (1 + |S11|) / (1 - |S11|).

Watch out It is tempting to pick just the antenna analyzer because that is the hobbyist's usual SWR tool, but the other two instruments measure the same quantity, one from power ratios and one from reflection coefficient.

Anything that can see reflected power or reflection coefficient can give you SWR.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E4A099 of 11

Which of the following is good practice when using an oscilloscope probe?

Why An oscilloscope probe's ground lead is a wire loop, and that loop has inductance which forms a resonant circuit with the probe's input capacitance. At high frequencies that resonance causes ringing, overshoot and distorted rise times on the displayed waveform, so keeping the ground return as short as possible (a ground spring or short ground clip right at the test point) gives the most accurate picture of the signal.

Watch out High-impedance probes are exactly what you want on low-impedance circuits because they load the circuit less, and a DC-coupled probe measures AC signals fine, it just also shows any DC offset present. Since two of the listed practices are wrong, the choice claiming all are correct fails too.

Short ground lead = less loop inductance = less ringing. Long ground pigtail makes square waves wiggle.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E4A1010 of 11

Which trigger mode is most effective when using an oscilloscope to measure a linear power supply's output ripple?

Why Ripple on a linear supply is a direct byproduct of the AC mains, appearing at the line frequency or twice it (60 Hz or 120 Hz in the US). The oscilloscope's line trigger derives its timing from the AC power line itself, so any waveform locked to the mains appears stationary and stable on the screen. That makes the small ripple easy to see and measure even though its amplitude may be too low or too noisy to trigger reliably on its own.

Watch out Edge or level triggering on the ripple signal itself is workable in principle, but the ripple is small and often buried in noise and switching spikes, so the trace tends to jitter; single-shot captures just one sweep and is meant for one-time transients, not a repetitive waveform.

Ripple comes from the power line, so trigger on the line. 60 Hz in, 120 Hz ripple out of a full-wave rectifier.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E4A1111 of 11

Which of the following can be measured with an antenna analyzer?

Why An antenna analyzer is a low-power swept RF source with an SWR/impedance bridge, so anything that changes impedance versus frequency shows up on it. Feed a known length of coax with an open or shorted far end and the frequencies where it looks like a short or open give the velocity factor; run that backwards with a known velocity factor and you get the cable's electrical length, which finds faults or trims stubs. Couple a small link coil from the analyzer loosely to a tuned circuit and the dip in the reading marks its resonant frequency. All three measurements are standard uses of the instrument.

Watch out Each single-item choice is a genuine analyzer measurement, so picking any one of them alone leaves out two other things the same instrument does.

Antenna analyzer = impedance vs frequency; if a dip or peak reveals it, the analyzer can measure it.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

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