E4A

Oscilloscopes, analyzers and counters

Explain what scopes, spectrum analyzers, antenna analyzers and prescalers show, and how aliasing and probes affect readings.

11 pool questions1 on the exam

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

A digital scope only sees its samples

A digital oscilloscope's converter grabs snapshots, then connects the dots. The sampling rate of the analog-to-digital converter limits the highest frequency it can show accurately. Sample too slowly and you get aliasing: a false, jittery, lower-frequency waveform.

Dots are what the converter captures. The display connects the dots, whatever the real signal did in between.

Signal frequency9 kHz

Displayaliased

RememberFewer than two samples per cycle: the scope invents a slower wave.

Probe habits: compensate, short ground, line trigger

Adjust the probe's trimmer until the flat parts of the square wave are as flat as possible.

Compensate on a square wave: trim until the horizontal portions are as flat as possible.
Keep the probe's ground lead short. A long one adds inductance and distorts fast signals.
Line trigger locks the sweep to the AC power line, so the ripple on a linear supply holds still.

RememberFlat square wave, short ground, line trigger for ripple.

Scope: amplitude over time. Spectrum analyzer: amplitude over frequency

Scope: amplitude vs time. Spectrum analyzer: amplitude vs frequency, so spurs and IMD show up as extra spikes.

A spectrum analyzer plots signal amplitude (vertical) against frequency (horizontal). Every signal is a spike at its own frequency, so it is the tool for spurious signals and intermodulation distortion from an SSB transmitter.

RememberSee other frequencies? Use a spectrum analyzer.

An antenna analyzer reads SWR and impedance directly

An SWR bridge gives a reading you must interpret. An antenna analyzer computes SWR and impedance automatically. It does not tune the antenna for you.

Instrument	Measures SWR?
Directional wattmeter	yes
Vector network analyzer	yes
Antenna analyzer	yes

An antenna analyzer can also measure velocity factor, cable length and the resonant frequency of a tuned circuit.

RememberMeasures SWR, all three ways, and more.

A prescaler divides a signal down so the counter can count it

The prescaler reduces the signal frequency to within the counter's operating range.

A prescaler reduces the signal frequency to within the counter's operating range. It does not amplify or multiply.

RememberPrescaler = divider in front of the counter.

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