The oscilloscope
What item of test equipment contains horizontal and vertical channel amplifiers?
An oscilloscope.
Which of the following is an advantage of an oscilloscope versus a digital voltmeter?
Complex waveforms can be measured.
A voltmeter reduces a signal to one number. A scope shows its shape over time, which is the only way to see distortion, clipping, ringing, or a keying waveform.
Which of the following is the best instrument to use for checking the keying waveform of a CW transmitter?
An oscilloscope.
Keying waveform is a shape question: how fast the envelope rises and falls. Too fast and you generate key clicks — splatter either side of your signal. Only a scope shows it.
What signal source is connected to the vertical input of an oscilloscope when checking the RF envelope pattern of a transmitted signal?
The attenuated RF output of the transmitter.
Attenuated. A scope input is not built for a hundred watts, and connecting one directly destroys it. Use a proper sampler or a dummy load with a tap.
Meters
Why do voltmeters have high input impedance?
It decreases the loading on circuits being measured.
A voltmeter is in parallel with what it measures, so it draws some current from it. High input impedance keeps that current negligible — otherwise the act of measuring changes the voltage you are trying to read. This is the classic instrumentation problem, and it is why an old 20,000 ohms-per-volt meter gives wrong answers on high-impedance circuits.
What is an advantage of a digital multimeter as compared to an analog multimeter?
Higher precision.
When is an analog multimeter preferred to a digital multimeter?
When adjusting circuits for maximum or minimum values.
Both true, and the pairing is the useful part. Digital gives you more digits; analogue gives you a moving needle, and a needle shows a trend in a way flickering digits do not. Peaking or nulling an adjustment is far easier watching a needle — which is why analogue meters survive on SWR bridges and amplifier front panels.
The two-tone test
What signals are used to conduct a two-tone test?
Two non-harmonically related audio signals.
What transmitter performance parameter does a two-tone test analyze?
Linearity.
Feed two audio tones into an SSB transmitter and look at the output on a spectrum analyser. A perfectly linear transmitter produces exactly two RF tones. A non-linear one produces intermodulation products either side — and the levels of those products are the linearity measurement.
Non-harmonically related is essential. If the tones were harmonically related, their distortion products would land on top of the original tones and hide the distortion.
This is the laboratory version of the waterfall check from the digital-modes lesson. Same phenomenon, measured properly.
Directional wattmeter
Which of the following can be determined with a directional wattmeter?
Standing wave ratio.
It measures forward and reflected power separately; SWR follows from the ratio. “Directional” means exactly that ability to distinguish the two directions.
Antenna analyser
Which of the following must be connected to an antenna analyzer when it is being used for SWR measurements?
Antenna and feed line.
Both. An analyser on the bench with nothing attached measures nothing useful; what you want is the whole system as the transmitter will see it.
Which of the following can be measured with an antenna analyzer?
Impedance of coaxial cable.
Not just SWR — impedance, and against frequency. It is a small vector network analyser, which is why a NanoVNA has largely replaced the dedicated instrument.
And the limitation that produces confusing results:
What effect can strong signals from nearby transmitters have on an antenna analyzer?
Received power that interferes with SWR readings.
An analyser transmits a tiny signal and measures what comes back. A strong nearby transmitter — a broadcast station, a neighbour on the air — swamps that measurement. Readings that wander, or differ between sweeps, usually mean this rather than a faulty antenna. Move, or measure at a quieter time.
The selection table
| Question | Instrument |
|---|---|
| What shape is this waveform? | oscilloscope |
| Is my keying too sharp? | oscilloscope |
| Is my amplifier linear? | two-tone test into a spectrum analyser |
| What is this DC voltage, precisely? | digital multimeter |
| Peak or null this adjustment | analogue multimeter |
| What is my SWR? | directional wattmeter |
| Where is my antenna resonant? | antenna analyser or VNA |
Check yourself
- You want to check whether your CW keying is generating clicks. What instrument?
- Your antenna analyser gives different SWR readings on successive sweeps. Faulty antenna?
- Why must two-tone test tones be non-harmonically related?
Answers
- An oscilloscope — keying waveform is a shape, and only a scope shows it.
- Probably not. Strong nearby signals interfere with an analyser’s measurement. Check whether something local is transmitting.
- So their intermodulation products do not land on the original tones, where they would be invisible.