Oscilloscope
| Oscilloscope | |
|---|---|
| Also called | Scope, o-scope |
| Displays | Voltage (vertical) against time (horizontal) |
| Main types | Analog (cathode-ray tube) and digital storage oscilloscopes (DSOs) |
| Key specifications | Bandwidth, sample rate, number of channels, memory depth |
| In the shack | |
| Standard probe | 10x passive probe |
| Common uses | Audio and RF waveforms, modulation checks, keying, troubleshooting |
| Entry-level digital models | Two or four channels, roughly 50 to 200 MHz bandwidth |
An oscilloscope is an instrument that draws a graph of voltage over time, showing the actual shape of a signal rather than a single number. Where a multimeter might report that a point carries a few volts of AC, an oscilloscope shows whether it is a clean sine wave, a burst of noise or a signal clipped flat on top. It is one of the most useful instruments for homebrewing and repair, and it gives a direct look at the quality of a transmitted signal.
How it works
The screen is divided into a grid of squares called divisions. The vertical scale is set in volts per division and the horizontal scale, the sweep speed, in time per division, so a signal's amplitude and period can be read straight off the grid. The trigger starts each sweep at the same point on the waveform, so that a repeating signal stands still on the screen instead of drifting.
Older analog scopes draw the trace directly with an electron beam on a cathode-ray tube. A digital storage oscilloscope (DSO) samples the signal with an analog-to-digital converter. That lets it freeze a single event, measure frequency, peak-to-peak voltage and RMS value automatically, and calculate an FFT, a rough frequency-domain view of the signal.
Bandwidth and sample rate
A scope's bandwidth is the frequency at which a sine wave is displayed 3 dB low, at about 70 percent of its true amplitude. Readings fall off well before that point, so a common rule is to choose a bandwidth several times the highest frequency of interest, and more for square waves, whose sharp edges contain high harmonics. Viewing a signal on the 10-meter band with reasonable accuracy calls for something like a 100 MHz scope.
A digital scope must also sample fast enough, several times the highest frequency being viewed, or it can show a false, lower-frequency waveform, an effect called aliasing.
Probes
The standard probe is the 10x passive probe. It divides the signal by ten and presents about 10 megohms with far less capacitance than a direct connection, so it disturbs the circuit less. Many probes switch between 1x and 10x, but the 1x setting often has a bandwidth of only a few megahertz.
A 10x probe must be compensated for the scope it is used with. A trimmer on the probe is adjusted, while viewing the scope's built-in square-wave calibration signal, until the corners are square, with neither rounding nor overshoot. At RF, the long ground lead with its alligator clip acts as an inductor and adds ringing; a short ground spring at the probe tip gives a far cleaner picture.
The ground clip of a bench scope is connected to the safety ground of its power cord. Clipping it to a point that is not at ground potential, such as anywhere in a mains-powered circuit, creates a short circuit. Never remove the ground pin from the scope's plug; use a differential probe or an isolation transformer on the equipment under test instead.
Checking audio, RF and modulation
- Audio. Microphone and audio stages, including a digital mode interface, can be checked for clipping, hum and distortion.
- Transmitter output. The transmitter feeds a dummy load, and the scope looks at the RF through an attenuator or an RF sampler, a tap that delivers a small, fixed fraction of the signal. A hundred watts into 50 ohms is about 200 volts peak to peak, more than many scope inputs and probes should see directly.
- AM. The envelope shows modulation depth. At 100 percent modulation the troughs just touch zero; flat gaps at zero mean overmodulation and splatter. Feeding the audio to the horizontal input instead of the sweep gives the classic trapezoid pattern, whose straight sides indicate linear modulation. See AM.
- SSB. In a two-tone test, two equal audio tones that are not harmonically related are fed into the microphone input, and the output envelope should be a series of smooth, rounded humps. Flattened peaks show overdrive, ALC trouble or an amplifier running out of linearity. See Single sideband.
- CW. The keying envelope should rise and fall over a few milliseconds. Square, abrupt edges produce key clicks heard well away from the operating frequency. See Morse code.
Inexpensive digital scopes
Prices of digital scopes have fallen sharply. Rigol, Siglent and other makers sell four-channel bench scopes with bandwidths of 50 to 100 MHz or more for a few hundred dollars. USB and handheld models are also sold, and tiny kit scopes are fine for audio but not for RF. Used analog scopes from the big laboratory brands turn up at hamfests and remain capable, if heavy.
A scope's FFT function is handy, but many entry-level scopes digitize with only 8 bits, which gives at best about 48 dB of dynamic range. That is too little to judge harmonics reliably, a job for a spectrum analyzer.