The spectrum analyser
Which of the following parameters does a spectrum analyzer display on the vertical and horizontal axes?
Signal amplitude and frequency — frequency horizontally, amplitude vertically.
Which of the following test instruments is used to display spurious signals and /or intermodulation distortion products from a transmitter?
Spectrum analyzer.
The frequency-domain instrument. A scope shows you a waveform; only a spectrum analyser shows you what else your transmitter is producing, and at what level.
Digital oscilloscopes and their failure mode
Which of the following limits the highest frequency signal that can be accurately displayed on a digital oscilloscope?
Sampling rate of the analog-to-digital converter.
Nyquist again, in a different instrument. And the failure when you exceed it is insidious:
What is the effect of aliasing on a digital oscilloscope when displaying a waveform?
A false, jittery low-frequency version of the waveform is displayed
Not a blank screen, not a warning — a plausible-looking waveform at the wrong frequency. If a digital scope shows you something slow and unstable that you did not expect, suspect aliasing before you suspect the circuit.
Probes
How is compensation of an oscilloscope probe performed?
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 10× probe forms a divider with the scope’s input capacitance, and the divider is only flat with frequency when a trimmer in the probe matches it. Uncompensated, square-wave tops slope up or down and every amplitude reading is wrong.
Every scope has a calibration output for this. It takes twenty seconds and it is skipped constantly.
Which of the following is good practice when using an oscilloscope probe?
Minimize the length of the probe’s ground connection.
The ground lead is an inductor in series with your measurement. The standard long crocodile lead adds enough inductance to make a fast edge ring convincingly — you measure ringing that exists only because of how you measured. For fast signals, use the short ground spring.
Triggering
Which trigger mode is most effective when using an oscilloscope to measure a linear power supply’s output ripple?
Line
Ripple is synchronous with the mains, so triggering on the mains itself gives a stable display. Triggering on the ripple waveform gives an unstable one, because the ripple is small and noisy.
Frequency counters
What is the purpose of using a prescaler with a frequency counter?
Reduce the signal frequency to within the counter’s operating range.
A divider ahead of the counter. A counter good to 100 MHz with a divide-by-10 prescaler reads to 1 GHz, at a tenth the resolution.
Antenna analysers
Which of the following is an advantage of using an antenna analyzer compared to an SWR bridge?
Antenna analyzers compute SWR and impedance automatically
Two advantages, and the second is the significant one: no transmission. You can sweep an antenna without putting a signal on the air, which means no interference, no identification requirement, and no risk to a transmitter into an unknown load.
Which of the following can be measured with an antenna analyzer?
All these choices are correct — SWR, feed line loss, and impedance.
Which of the following is used to measure SWR?
All these choices are correct — a directional wattmeter, an antenna analyser, and an SWR bridge.
Check yourself
- Your digital scope shows a slow wandering waveform on a circuit you know oscillates at 50 MHz. What is happening?
- Square waves on your scope have sloping tops. What do you adjust?
- You need to check an antenna’s resonance without transmitting. What instrument?
Answers
- Aliasing — the sample rate is below twice the signal frequency, so you are seeing a false low-frequency version.
- Probe compensation — the trimmer in the probe, against the scope’s calibration square wave.
- An antenna analyser, which computes SWR and impedance without transmitting.