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Subelement E4
AMATEUR PRACTICES
Section E4A
Test equipment: analog and digital instruments; spectrum analyzers; antenna analyzers; oscilloscopes; RF measurements; computer-aided measurements
Which of the following limits the highest frequency signal that can be accurately displayed on a digital oscilloscope?
  • Correct Answer
    Sampling rate of the analog-to-digital converter
  • Amount of memory
  • Q of the circuit
  • All these choices are correct

A digital oscilloscope operates by digitizing the signal with an analog-to-digital converter (ADC) before processing and displaying the result. An ADC is limited in the range of frequencies that can accurately be converted by what is known as the Nyquist frequency . This is typically 1/2 of the sampling rate of the ADC.

See http://en.wikipedia.org/wiki/Nyquist_frequency

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Which of the following parameters does a spectrum analyzer display on the vertical and horizontal axes?
  • RF amplitude and time
  • Correct Answer
    RF amplitude and frequency
  • SWR and frequency
  • SWR and time

Envision an oscilloscope with a line going from left to right. The line going from left to right represents increments of frequencies. The line going up and down represents the strength (amplitude) of the frequency. Most frequencies appear as spikes in compressed mode. Expand the line out the spikes appear as sharp mountains. This tool is useful in viewing frequencies close to the target frequency. bandwidths, RF shielding assessments,

More info here: https://en.wikipedia.org/wiki/Spectrum_analyzer

-KE0IPR

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Which of the following test instruments is used to display spurious signals and/or intermodulation distortion products generated by an SSB transmitter?
  • A wattmeter
  • Correct Answer
    A spectrum analyzer
  • A logic analyzer
  • A time-domain reflectometer

A spectrum analyzer displays the strength of a signal, and signals above and below the signal's frequency. Since spurious signals and/or intermodulation distortion products appear above and below a SSB signals frequency, the spectrum analyzer is a useful test instrument for displaying these.

You can adjust the vertical and horizontal scales on a spectrum analyzer - the vertical scale is the signal strength in Decibels, and the horizontal scale is the width of the spectrum being displayed.

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How is the compensation of an oscilloscope probe typically adjusted?
  • Correct Answer
    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 high frequency sine wave is displayed and the probe is adjusted for maximum amplitude
  • A frequency standard is displayed and the probe is adjusted until the deflection time is accurate
  • A DC voltage standard is displayed and the probe is adjusted until the displayed voltage is accurate

This adjustment is equivalent to making the probe have uniform attenuation over the range of frequencies to be measured. In passive probes this adjustment is usually a small adjustable capacitor.

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What is the purpose of the prescaler function on a frequency counter?
  • It amplifies low-level signals for more accurate counting
  • It multiplies a higher frequency signal so a low-frequency counter can display the operating frequency
  • It prevents oscillation in a low-frequency counter circuit
  • Correct Answer
    It divides a higher frequency signal so a low-frequency counter can display the input frequency

A prescaler is an electronic counting circuit used to reduce a high frequency electrical signal to a lower frequency by integer division. The prescaler takes the basic timer clock frequency and divides it by some value before feeding it to the timer, according to how the prescaler register(s) are configured.

Mnemonic: think of -divisions- on a -scale-.

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What is the effect of aliasing on a digital oscilloscope caused by setting the time base too slow?
  • Correct Answer
    A false, jittery low-frequency version of the signal is displayed
  • All signals will have a DC offset
  • Calibration of the vertical scale is no longer valid
  • Excessive blanking occurs, which prevents display of the signal

Nyquist's sampling theorem states that the highest frequency that can be unambiguously reconstructed is at half the sampling rate, and above this, aliasing occurs.

If one should sample at 20MHz, signals of up to 10MHz can be reconstructed.

For example, if one supplied a 20,000,000 Hz signal to this oscilloscope, the oscilloscope would sample the same value every cycle, and the signal would totally disappear. If one instead supplied a 20,000,001Hz signal, the signal would drift in phase with the sampling at 1Hz, and a false 1Hz sine wave would be displayed.

Generally, an oscilloscope sampling at 20MHz will have filters to reject signals above 10MHz for this reason.

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Which of the following is an advantage of using an antenna analyzer compared to an SWR bridge to measure antenna SWR?
  • Antenna analyzers automatically tune your antenna for resonance
  • Correct Answer
    Antenna analyzers do not need an external RF source
  • Antenna analyzers display a time-varying representation of the modulation envelope
  • All these choices are correct

An antenna analyser has its own built in signal source.

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Which of the following measures SWR?
  • A spectrum analyzer
  • A Q meter
  • An ohmmeter
  • Correct Answer
    An antenna analyzer

SWR or Standing Wave Ratio is a measure of the relative impedance of an antenna to the expected impedance by the transmitter. This is measured by an antenna analyzer or a SWR meter.

A spectrum analyzer measures signal amplitude in the frequency domain.

A Q meter measures the efficiency of a tuned circuit.

An Ohm meter measures the resistance of a circuit.

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Which of the following is good practice when using an oscilloscope probe?
  • Correct Answer
    Keep the signal ground connection of the probe as short as possible
  • Never use a high-impedance probe to measure a low-impedance circuit
  • Never use a DC-coupled probe to measure an AC circuit
  • All these choices are correct

The probe tip and ground of an oscilloscope acts like a sensitive antenna loop. The bigger the loop the more undesired signals or noise it will pick up. Also, the inductance of the ground wire increases with length, which can distort high-frequency signals. So, keep it short as possible.

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Which of the following displays multiple digital signal states simultaneously?
  • Network analyzer
  • Bit error rate tester
  • Modulation monitor
  • Correct Answer
    Logic analyzer

A logic analyzer is a device which captures and displays several digital signals at a time. Digital signals from a circuit are sampled and stored so that their states can then be displayed visually. This allows the user to analyze the timing relationships between different signals. Logic states are often displayed as wave forms or timing diagrams. Most logic analyzers can be set to trigger on a complex set of input conditions, and can decode many different communication protocols.

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How should an antenna analyzer be connected when measuring antenna resonance and feed point impedance?
  • Loosely couple the analyzer near the antenna base
  • Connect the analyzer via a high-impedance transformer to the antenna
  • Loosely couple the antenna and a dummy load to the analyzer
  • Correct Answer
    Connect the antenna feed line directly to the analyzer's connector

A portable antenna analyzer is a device that is used to analyze the characteristics of an antenna and often the feed line. You can see some pictures of analyzers on the Wikipedia page, but generally it has at minimum a connector to attach an antenna / feed line to, a readout and dial for selecting the frequency, and a readout for the SWR of the antenna/feed line system at that frequency.

There is no need for a dummy load with an antenna analyzer, and the analyzer is designed to connect to an antenna so it generally connects the same way a transceiver would -- by connecting the feedline directly to the analyzer.

-kd7bbc

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