An instrument that amateur radio operators frequently use when experimenting or when debugging equipment is the oscilloscope, or simply just “scope.” Oscilloscopes have become more common in amateur radio shacks as prices have fallen, and the technology has moved from analog to digital.
One of the most important oscilloscope specifications is its bandwidth. The bandwidth of an oscilloscope determines the maximum frequency at which the oscilloscope can accurately measure a signal. Several factors determine the highest frequency signal that can be accurately displayed on a digital oscilloscope, including the characteristics of the analog signal processing circuits and the sampling rate of the scope’s analog-to-digital converter.
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QUESTION: Which of the following limits the highest frequency signal that can be accurately displayed on a digital oscilloscope? (E4A01) ANSWER: Sampling rate of the analog-to-digital converter
While digital scopes have many advantages over analog scopes, you have to know how to use them properly. For example, because digital oscilloscopes sample an input signal at discrete time intervals, it is possible to fool them into displaying an incorrect waveform. This phenomenon is called aliasing. If you set the time base too slow, the scope may display a false, jittery version of the input signal.
QUESTION: What is the effect of aliasing on a digital oscilloscope caused by setting the time base too slow? (E4A06) ANSWER: A false, jittery low-frequency version of the signal is displayed
Oscilloscope probes
When making measurements at RF frequencies, it’s important to connect the probe’s ground connection as close to the location of the measurement as possible. Keeping the signal ground connection as short as possible reduces the noise picked up by the probe and reduces the inductance of the connection, which in turn, makes the measurement more accurate.
QUESTION: Which of the following is good practice when using an oscilloscope probe? (E4A09) ANSWER: Keep the signal ground connection of the probe as short as possible
Good quality passive oscilloscope probes have an adjustable capacitor in them that needs to be adjusted so that the probe capacitive reactance is at least nine times the scope input capacitive reactance. When this capacitor is adjusted properly, we say that the probe is properly compensated, and the scope will display the waveform with as little distortion as possible.
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Most oscilloscopes have a special square-wave output specifically for the purpose of compensating probes. To adjust the compensation, you connect the probe to this output and then adjust the probe until the horizontal portions of the displayed wave are as nearly flat as possible.
QUESTION: How is the compensation of an oscilloscope probe typically adjusted? (E4A04) 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
Spectrum analyzers
While oscilloscopes display the amplitude of a signal over time, spectrum analyzers display the amplitude of the frequency components of a signal. As shown in the figure below, the horizontal axis of an oscilloscope display represents time, while the horizontal axis of a spectrum analyzer display represents frequency. For both instruments, the vertical axis represent the amplitude of the signal being measured.
Spectrum analyzers are very useful for troubleshooting problems. For example, a spectrum analyzer is used to display spurious signals or intermodulation distortion products generated by an SSB transmitter.
QUESTION: Which of the following parameters does a spectrum analyzer display on the vertical and horizontal axes? (E4A02) ANSWER: RF amplitude and frequency QUESTION: Which of the following test instruments is used to display spurious signals and/or intermodulation distortion products generated by an SSB transmitter? (E4A03) ANSWER: A spectrum analyzer
Antenna analyzers
One of the instruments that I think every amateur radio operator should have (or at least have access to) is the antenna analyzer. Antenna analyzers are versatile instruments that allow amateur radio operators to easily make antenna measurements, as well as other impedance measurements. They can even be used as low power RF signal generators. One of the most common uses for an antenna analyzer is measuring the SWR of an antenna system.
An advantage of using an antenna analyzer compared to an SWR bridge to measure antenna SWR is that antenna analyzers do not need an external RF source. What this means is that you don’t need to connect your transmitter to the antenna to tune it. This is because antenna analyzers have internal RF signal generators.
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You can also make related measurements, such as the antenna resonant frequency and feed point impedance with an antenna analyzer. To make these measurements you connect the antenna feed line directly to the analyzer’s connector.
QUESTION: Which of the following measures SWR? (E4A08) ANSWER: An antenna analyzer
QUESTION: Which of the following is an advantage of using an antenna analyzer compared to an SWR bridge to measure antenna SWR? (E4A07) ANSWER: Antenna analyzers do not need an external RF source
QUESTION: How should an antenna analyzer be connected when measuring antenna resonance and feed point impedance? (E4A11) ANSWER: Connect the antenna feed line directly to the analyzer’s connector
Frequency counters, logic analyzers
To measure the frequency of a signal, you use an instrument called a frequency counter. When selecting a frequency counter, an important specification is the maximum frequency. If you want to measure the frequency of a signal whose frequency is higher than the maximum frequency of your counter, you might use a prescaler. A prescaler divides a higher frequency signal so a low-frequency counter can display the input frequency.
QUESTION: What is the purpose of the prescaler function on a frequency counter? (E4A05) ANSWER: It divides a higher frequency signal so a low-frequency counter can display the input frequency
The proper operation of a digital circuit depends on the output state of many digital ICs at specific times. To ensure that a circuit is working properly, or to troubleshoot a circuit, you may want to use a logic analyzer because it displays multiple digital signal states simultaneously.
QUESTION: Which of the following displays multiple digital signal states simultaneously? (E4A10) ANSWER: Logic analyzer
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