A step-by-step visual guide to what you do in Lab 5 and why you see what you see: reading the scope three ways, sweeping a capacitor filter from 3 kHz down to 1 Hz, building the board, and checking DC voltages with the DMM. Every screen below is a live simulation of the lab circuit, so you can predict your readings before you take them.
The screen is a grid: 10 divisions across (time) and 8 divisions tall (voltage). Every reading starts from two knob settings: volts per division (vertical) and time per division (horizontal). Counting boxes means: value = number of boxes × setting. Change the knobs below and watch the same 900 Hz wave get taller, shorter, wider or narrower. The signal never changes; only your view of it does.
Each box has 5 small tick marks, so by eye you can read to about ±0.1–0.2 of a box. That is why counting boxes is the least precise of the three methods.
The generator assumes a 50 Ω load and splits its voltage half-and-half with its own 50 Ω. Our circuit is about 155 kΩ, so almost nothing is lost inside and you see the full open-circuit voltage: 2 × 1 V rms = 2 V rms ≈ 5.66 Vpp.
Good settings: 1 V/div shows 5.7 boxes tall; 200 µs/div shows 5.6 boxes per period.
You measure Vpp, period and frequency three times. The answers should agree within a few percent; what changes is how precise and how fast each method is. Pick a method to watch it work.
| Counting boxes | Cursors | Automatic | |
|---|---|---|---|
| Vpp | ≈ 5.6 V | ≈ 5.66 V | ≈ 5.66 V |
| Period | ≈ 1.12 ms | ≈ 1.111 ms | ≈ 1.111 ms |
| Frequency | ≈ 893 Hz (calculated) | ≈ 900 Hz (1/ΔX) | ≈ 900 Hz |
| Limited by | your eye, ±0.2 box | where you park the cursor | what is on screen (needs ≥1 full cycle) |
These are ideal values; yours will differ slightly. The scope itself is only ±2% accurate in voltage, so a DMM is better for precise DC, but the scope shows the shape, timing and phase that a DMM cannot.
A sine wave averages to zero over whole cycles. If the screen shows 2.5 cycles, the full-screen average also counts the leftover half cycle (orange) and comes out wrong. The N Cycles version uses only whole cycles. Slide to change how many cycles fit on screen.
Same idea for AC RMS – N Cycles in step 8.7: it ignores the partial cycle at the edge of the screen.
A capacitor stores charge. Pushing charge in takes time, so how a capacitor behaves depends on how fast the voltage is changing, which is the frequency. Its “AC resistance” is called reactance:
R1 = 1 kΩ and C1 = 1 µF, so the time constant τ = RC = 1 ms. Compare it with the period T of the sine.
This is the circuit you build on the plugboard. CH1 (yellow) watches top, which is the generator and stays constant. CH2 (green) watches mid, the voltage across R2. Pick each frequency from the handout, then try Autoscale. Below about 20 Hz it stops helping and you set the knobs yourself, exactly like in the lab.
Dots mark the handout frequencies; the ring is where you are now. Use a log frequency axis in Google Sheets, otherwise everything below 1 kHz is squashed into the left edge (same lesson as pre-lab step 11).
XC is only about 530 Ω, far smaller than R1’s 150 kΩ, so current takes the capacitor like a shortcut. R1 and C1 together act like a wire and mid is almost the same as top.
XC ≈ 1.6 MΩ, so the capacitor is nearly an open switch. Only R1 and R2 remain: a plain voltage divider, 2 V × 4.75k / 154.75k ≈ 0.061 V. That is why CH2 needs a much smaller V/div.
Current through a capacitor peaks when the voltage is changing fastest, a quarter cycle early. When the capacitor carries part of the current, mid rises before top does. The lead is largest around 60 Hz (≈70°) and fades at both ends, where one path (R1 or C1) takes over.
Physically, mid leads top. The handout says the scope shows this as a negative number: channel 2 arrives earlier than channel 1. Record the sign exactly as the scope shows it and say so in your plot.
Autoscale hunts for repeating edges to trigger on, and it wants to see several of them quickly. At 3 Hz one cycle takes 0.33 s, so it cannot find a stable picture. Set it yourself: time/div ≈ (cycles you want × period) / 10, and pick V/div so the wave fills about 6 boxes.
To show 3 cycles of a 1 Hz wave the screen spans 3 seconds, so each new picture takes 3 seconds to draw. Watch the trace crawl across the screen above. Wait for at least one full cycle before reading any measurement.
Calculated for an ideal circuit with 2.00 V rms at top. Real parts are ±1% (resistors) and up to ±10% (capacitor), so expect a few percent difference, most of it near the corner.
You tear down the plugboard and solder the same parts, plus power-supply parts, onto your PCB. Pick a part to see how to recognise it, where it goes, which way round, and what it is for.
The microcontroller and LEDs draw current in short bursts. The power adapter is slow to react, so without help the voltage at the chip dips on every burst. Capacitors next to the chip are small local energy tanks that cover each burst until the supply catches up.
Illustration only, not to scale. 100 nF covers the fastest spikes, 10 µF and 470 µF cover longer dips, and the ferrite bead L1 blocks high-frequency noise from the digital side reaching the analog supply.
Three digits: first two are the value, third is how many zeros to add, in picofarads. So 104 = 10 + 0000 pF = 100 000 pF = 100 nF.
Why the DMM here and not the scope: these are steady DC values and the DMM is far more accurate than the scope’s ±2%.
Check your board’s schematic for which resistor connects to AVDD before you hand in Question 4. The formula is the same either way: VTP2 = AVDD × Rbottom / (Rtop + Rbottom). The 4.3 V limit is the microcontroller’s internal reference; TP2 must stay below it.