Measurement Instruments and Tools

Multimeter

Basic Functions

Voltage Measurement (V):
  DC (DCV): Connect in parallel across the points being measured
  AC (ACV): Connect in parallel; displays RMS value (True RMS meters are more expensive)

Current Measurement (A):
  Must be connected in series! Move probes to the A/mA jack
  Remember to switch back to the V jack after measuring (otherwise, the next voltage measurement will cause a short circuit!)

Resistance Measurement (Ω):
  Measure with power off! Cannot measure in-circuit

Continuity Test (Beep Mode):
  R < ~50Ω triggers beep → Quick check for connections/short circuits

Usage Precautions

1. After measuring current, you must move the probes back to the V/Ω jacks!
   Forgetting to switch back → Next voltage measurement will cause a direct short → Blows fuse/damages meter

2. Use one hand when measuring high voltage (>30V) (keep the other hand in your pocket)
   Prevents current from flowing through the heart

3. Discharge high-voltage capacitors before touching them

4. High-impedance digital meters may induce "ghost voltage"
   Floating wires can pick up surrounding electric fields → Display shows a number, but it does not represent actual voltage

Oscilloscope

Core Parameters

Bandwidth:
  Determines the highest frequency you can see
  Rule of thumb: Bandwidth ≥ 5× signal frequency (for square waves)
       Bandwidth ≥ 3× signal frequency (for sine waves)

  Example: To view a 20MHz square wave → Need at least 100MHz bandwidth
      (3rd/5th harmonics of the square wave must be displayed correctly)

Sample Rate:
  Determines time resolution
  Rule of thumb: Sample rate ≥ 2.5× bandwidth (Nyquist is not enough)
  Example: 100MHz bandwidth → At least 250MSa/s, recommended 1GSa/s

Memory Depth:
  Determines how long a capture can be
  Record duration = Memory Depth / Sample Rate
  1Mpts @ 1GSa/s = 1ms

Probes

1x Probe: Signal passes through directly (1:1)
  Bandwidth limited (~20MHz), high input capacitance

10x Probe: Signal attenuated by 10x
  Higher bandwidth, lower input capacitance → **Use this by default!**
  Must be calibrated (adjust compensation capacitor on the probe)

Measure high voltage (>40V) → Use differential or isolated probes
Measure current → Current probe

Basic Operations

Trigger:
  Determines when the waveform starts displaying
  Common: Rising edge trigger, Level trigger
  Unstable trigger → Waveform runs across the screen

Coupling:
  DC: Displays full signal (AC+DC)
  AC: Filters out DC component, shows only AC
  Must use AC coupling for measuring power supply ripple!

Single Capture:
  Captures once and stops; used for capturing sporadic events/power-on sequences

Keep probe ground leads as short as possible!
  Long ground leads → Antenna effect → Introduces high-frequency noise

Common Measurement Scenarios

1. Power Supply Ripple:
   AC coupling, 20MHz bandwidth limit, short ground spring

2. Clock/Crystal:
   10x probe, place on crystal pins (not on load capacitors)

3. Serial Signals:
   Decode function: Automatic decoding of UART/I2C/SPI

4. MOSFET Switching:
   Use differential probe to measure Vgs and Vds simultaneously → Observe switching losses

Logic Analyzer

Difference from Oscilloscope:
  Oscilloscope: Views analog waveforms (voltage vs. time)
  Logic Analyzer: Views digital timing (0/1 vs. time)

Advantages: Many channels (8/16/32), protocol decoding, long-duration recording
Limitations: Only sees 0/1, cannot see signal quality

Budget Option: Saleae clone (24MHz/8ch) $10
Recommended: DSLogic Plus / Saleae Logic

Typical Usage

Capture SPI (4 lines) + I2C (2 lines) + UART (2 lines) simultaneously
→ Observe timing relationships between buses

Trigger Conditions: Specific protocol packets / Specific logic level sequences

Bench Power Supply

Constant Voltage (CV): Voltage is fixed, current adjusts automatically
Constant Current (CC): Current is fixed, voltage adjusts automatically

First Power-On:
  1. Set voltage → Set current limit → Output OFF
  2. Connect wires → Check for no short circuits
  3. Output ON → Observe if current is normal

Current Limit = The Poor Man's Protection!
  Set a low current limit first, confirm normal operation, then increase

Function Generator

Common Waveforms: Sine, Square, Triangle, Sawtooth, Arbitrary
Uses:
  - Inject test signals
  - Simulate sensor outputs
  - Frequency response testing

Note: Output impedance is typically 50Ω
  Connecting to a high-impedance load → Output voltage will be 2x the displayed value!

Soldering Tools

Soldering Iron

Temperature: Lead-tin 300~330°C, Lead-free 350~380°C
Constant Temperature Station (T12/C245) >> Standard Soldering Iron

Iron Tip Maintenance: Keep tinned, clean with brass wool
          Do NOT sand the tip!

Solder

Lead (63/37 SnPb): 183°C melting point, easy to solder but toxic
Lead-free (SAC305): 217°C melting point, eco-friendly but harder to solder

Flux: Rosin or no-clean flux (extra flux helps significantly)

Soldering Techniques

1. Tin the tip → Preheat pad → Add solder → Remove solder → Remove iron
   Entire process takes 1-3 seconds

2. Heat the pad and pin, not the solder

3. QFP/QFN Packages:
   Drag Soldering: Use plenty of flux, drag the chisel tip
   Hot Air Gun: Essential for BGA/QFN

4. Post-solder Inspection: Inspect each pin with a magnifying glass, check for shorts with a multimeter

ESD Protection

Wrist Strap: Must be grounded! (via 1MΩ resistor)
Desk: Anti-static mat
Storage: Store ICs in anti-static bags/conductive foam

MOSFETs/CMOS ICs are extremely sensitive to static electricity!
(Static electricity you might not even feel is enough to break down the gate oxide layer)

Golden Rules:
- Touch a grounded object before soldering
- Hold ICs by the edges only (do not touch pins)
- Be especially careful in dry weather

Keywords: Multimeter, Oscilloscope, Probe, Trigger, Logic Analyzer, Soldering, ESD, Bench Power Supply