How the instruments work

LabDeck has two instruments: a two-channel analog oscilloscope and a function generator. Both are modelled on bench instruments, with the same controls in the same places, so what you learn here carries over to the real thing. This page explains what each control does and when to use it.

The oscilloscope in one minute

An oscilloscope draws a graph of voltage against time. The screen is a grid of 10 horizontal by 8 vertical divisions. Two knobs set the scale of the graph:

VOLTS/DIV sets how many volts one vertical division is worth (from 1 mV to 10 V per division, in a 1-2-5 sequence).
TIME/DIV sets how much time one horizontal division is worth (from 100 ns to 500 ms per division).

To read a measurement you count divisions and multiply. A waveform that is 4 divisions tall at 0.5 V/div is 2 V peak-to-peak. A waveform that repeats every 5 divisions at 200 µs/div has a period of 1 ms, so a frequency of 1 kHz.

Vertical section (one set of controls per channel)

Control	What it does	When to use it
VOLTS/DIV	Vertical scale of the channel.	Turn it until the waveform fills most of the screen without leaving it. Larger waveforms make smaller reading errors.
Y-POS	Moves the trace up and down.	To place the zero line where you want it, or to separate two traces.
Coupling: DC / AC / GND	DC shows the whole signal. AC blocks the DC level and shows only the changes. GND disconnects the input and shows a flat line at 0 V.	Use GND first to mark where zero is. Use AC to look at a small ripple riding on a large DC level.
VAR and UNCAL	A fine adjustment of the scale. The UNCAL light turns on when it is not in its calibrated position.	Only to fit a waveform exactly on the graticule. While UNCAL is on, the VOLTS/DIV value on the knob is not the real scale.
CH1, CH2, DUAL, ADD	Which channels are drawn. DUAL draws both. ADD draws the sum of the two.	DUAL to compare an input with an output. ADD with INV CH2 to measure a difference.
INV CH2	Flips channel 2 upside down.	With ADD, CH1 + (−CH2) gives the voltage between two nodes, even when neither is ground.
ALT / CHOP	The two ways of drawing two traces with one beam. ALT draws one whole sweep for channel 1, then one for channel 2. CHOP jumps between the channels 500 000 times a second within each sweep.	ALT is better for fast sweeps. CHOP is better for slow sweeps, where ALT would visibly flicker, but at fast sweeps CHOP shows the beam as a dashed line.

Horizontal section

TIME/DIV is the time scale. A slow setting (milliseconds per division) shows many cycles; a fast one (microseconds or less) stretches a single edge so you can see how it rises.
X-POS moves the whole trace left and right, for example to bring the start of an edge to a grid line.
×10 MAG stretches the sweep ten times around the centre of the screen, to inspect a detail of what you triggered on.
X-Y stops drawing against time. Channel 1 now drives the horizontal axis and channel 2 the vertical one. Two sine waves of the same frequency draw an ellipse whose shape tells you their phase difference; a straight line means they are in phase, a circle means 90° apart.

The trigger

The trigger is what makes a repeating signal look still. Each sweep starts when the signal crosses a chosen level in a chosen direction; if every sweep starts at the same point of the waveform, the traces pile up on top of each other. Without a good trigger the waveform seems to run across the screen.

SOURCE selects which channel the trigger listens to (CH1 or CH2). It can be a channel that is not drawn.
SLOPE chooses a rising (+) or falling (−) crossing.
LEVEL sets the voltage that must be crossed, within ±4 divisions of the centre of the screen. If the level is above the highest point of the signal, the trigger never fires.
MODE AUTO keeps sweeping even when nothing triggers, so you always see a trace (useful for finding a signal and for DC). NORM waits for a trigger and draws nothing new until one arrives, which is what you want for rare events.
The TRIG’D light is on while the trigger is firing. If it is off in NORM mode, adjust LEVEL or SOURCE.

Display controls

POWER turns the instrument on. Nothing is drawn while it is off.
INTENS (brightness), FOCUS (beam sharpness) and ILLUM (graticule lighting) change how the screen looks, as on a cathode-ray tube. A very bright trace blooms and a badly focused one looks blurred.
BEAM FIND brightens the beam and squeezes a trace that went off-screen back into view, so you can see where it is.
SERVICE opens the tube adjustments: VAR PERSIST sets how long the phosphor glows after the beam has passed, and NOISE adds a little background noise, as in an old instrument.

Probes and loading

You connect the instruments to the circuit with probes placed on the schematic. Each oscilloscope probe has a tip, which you rest on a node, and an alligator clip, which goes on ground. The tip measures the voltage of that node with respect to the clip.

The input of the oscilloscope is not invisible: it is a 1 MΩ resistance to ground on every channel, and that resistance loads the circuit under test. In a low-impedance circuit this changes nothing. In a high-impedance one, for example a divider made with megaohm resistors, it can change the reading by a lot. Real probes and real instruments do the same.

The Ideal instruments option in the Instruments panel switches this loading off, so you can compare the same circuit with and without the instrument attached.

Taking a measurement, step by step

Press POWER on the oscilloscope and OUTPUT on the generator, and rest the probe tips on the nodes you want to see.
Set the coupling to GND and move Y-POS to put the zero line on a grid line. Then set it back to DC.
Adjust VOLTS/DIV until the waveform is as large as the screen allows.
Adjust TIME/DIV until you see one to three cycles.
Set the trigger: source on the channel you are looking at, slope +, LEVEL near the middle of the waveform, mode AUTO. The TRIG’D light should turn on and the picture should stop moving.
Read the amplitude by counting divisions vertically and the period by counting divisions horizontally. The frequency is 1 divided by the period.

A common mistake is to read a value with UNCAL on. Check the light before you trust the numbers.

The function generator

Waveform: sine, square or triangle.
Frequency is set in two steps, like on a bench generator: the decade buttons (×1 Hz up to ×100 kHz) choose the range and the FINE knob covers 0.1 to 1.1 times that range, so neighbouring ranges overlap. The display reads the frequency with four significant digits.
Amplitude: the knob goes from zero to 10 V peak-to-peak, and the attenuator divides it by 1, 10 or 100 (0, −20 or −40 dB). Offset adds a DC level of up to ±5 V.
OUTPUT connects the generator to the circuit. When it is off, the generator is disconnected.
LOAD tells the generator what it is connected to. The output has an internal resistance of 50 Ω, so the voltage at the output depends on the load. With LOAD set to 50 Ω, the display shows what a 50 Ω load receives; a circuit with a much higher impedance sees twice that amplitude. With LOAD set to HI-Z the display shows the voltage on a high-impedance load. If your measurement is twice what the generator displays, this is the reason.

How LabDeck simulates them

LabDeck is not a general-purpose circuit simulator. For circuits made of resistors, capacitors, inductors and sources, the engine computes the response exactly, including the transient after a change; the top left of the oscilloscope screen then says “exact mode”. When the circuit contains a diode, which is not linear, the engine integrates the circuit numerically over time and the screen says “numeric mode”. The goal is not to replace a professional simulator but to give you instruments that behave like the real ones.

Practice

The example experiments are good places to try all this: the RC filter shows the effect of TIME/DIV on an exponential edge, and the series RLC circuit shows how an amplitude can be larger than the generator voltage at resonance. The quick guide explains how to place the probes.