Debug Tools and Techniques

Introduction

Effective debugging of analog circuits requires mastery of specialized test equipment and measurement techniques. Each instrument provides a unique window into circuit behavior, and understanding when and how to apply each tool determines the difference between efficient problem resolution and frustrating guesswork. From oscilloscopes that reveal time-domain waveforms to spectrum analyzers that expose frequency-domain characteristics, the modern engineer has access to powerful diagnostic capabilities that previous generations could only dream of.

This article covers the essential debug tools and techniques used in analog electronics troubleshooting. Beyond simply knowing what buttons to press, effective debugging requires understanding measurement principles, recognizing the limitations of each technique, and knowing how to combine multiple tools to build a complete picture of circuit behavior. Whether tracking down an elusive oscillation, measuring small signals buried in noise, or characterizing the thermal behavior of power circuits, these techniques form the foundation of practical analog debugging.

Oscilloscope Probing Techniques

The oscilloscope remains the most fundamental tool for analog debugging, providing real-time visualization of voltage waveforms. However, the quality of oscilloscope measurements depends critically on proper probing technique. A probe is not simply a wire connecting the circuit to the oscilloscope; it is a measurement system that can significantly affect both the circuit under test and the accuracy of the measurement.

Understanding Probe Loading Effects

Every probe presents some load to the circuit being measured, and this loading can alter circuit behavior:

Resistive loading: A standard 10X probe presents approximately 10 megohms input resistance. While this seems high, measuring a node with 1 megohm source impedance results in roughly 9 percent signal attenuation and can shift DC operating points
Capacitive loading: Probe input capacitance, typically 10 to 15 picofarads for a 10X passive probe, creates a low-pass filter with the source impedance. On a 100 kilohm source, 15 picofarads produces a pole at only 106 kilohertz, severely limiting measurement bandwidth
Ground lead inductance: The probe ground lead introduces inductance, typically 25 nanohenries per inch. This inductance resonates with probe capacitance, causing ringing and overshoot on fast edges. A 6-inch ground lead can produce resonance around 50 megahertz

Passive Probe Selection and Usage

Passive probes remain the workhorse of oscilloscope measurements due to their robustness and wide dynamic range:

1X versus 10X probes: A 1X probe provides no attenuation but presents full oscilloscope input impedance to the circuit. A 10X probe reduces the signal by a factor of 10 while increasing input impedance by the same factor. Use 10X probes for general-purpose measurements to minimize loading
High-attenuation probes: 100X and 1000X probes measure higher voltages and present even higher input impedance with lower capacitance. Essential for measuring power supply rails or high-voltage nodes
Compensation adjustment: Passive probes require compensation to match oscilloscope input capacitance. An improperly compensated probe distorts square waves and introduces frequency-dependent amplitude errors. Always verify compensation using the oscilloscope calibration output before making critical measurements
Bandwidth considerations: Probe bandwidth must exceed the signal bandwidth by at least five times for accurate rise time measurements. A 100 megahertz probe measuring a 20 megahertz signal may show significant roll-off

Active Probes for High-Frequency Measurements

Active probes incorporate a buffer amplifier at the probe tip, dramatically reducing loading on the circuit:

Input characteristics: Active probes typically present 1 megohm or higher resistance with less than 1 picofarad capacitance, compared to 10-15 picofarads for passive probes
Bandwidth capability: Active probes achieve bandwidths exceeding several gigahertz, essential for measuring fast digital signals and high-frequency analog circuits
Dynamic range limitations: The active buffer limits the voltage range that can be measured without distortion, typically a few volts peak-to-peak. Check probe specifications before measuring larger signals
Offset capability: Many active probes include DC offset adjustment, allowing measurement of small signals riding on large DC levels without saturating the probe
Power requirements: Active probes require power, usually supplied through the oscilloscope interface. This adds cable bulk but enables the superior input characteristics

Ground Connection Best Practices

The probe ground connection is as critical as the probe tip connection for accurate measurements:

Minimize ground lead length: For high-frequency measurements, use the shortest possible ground connection. Replace the standard 6-inch ground lead with ground springs or tip-and-barrel accessories
Dedicated ground points: Design circuit boards with probe-accessible ground points adjacent to critical test nodes. A via connected to the ground plane near each test point enables low-inductance connections
Ground near the signal: Always ground the probe as close as possible to the signal source. Grounding at a distant point allows common-mode noise between the two ground locations to appear as signal
Avoid ground loops: When using multiple probes, ensure all ground connections do not create loops that can pick up magnetic fields. Connect all probe grounds at a single point when possible

Special Probing Situations

Certain measurement scenarios require adapted probing approaches:

Oscillator circuits: Probe capacitance can shift oscillator frequency or stop oscillation entirely. Use minimum-capacitance probing and verify the oscillator continues operating normally
High-impedance nodes: Nodes like integrator inputs or sample-and-hold capacitors require active probes or buffered test points. Design in unity-gain buffer amplifiers for critical high-impedance nodes
Floating measurements: Never connect a standard oscilloscope probe ground to a non-ground-referenced point. Use differential probes or isolated oscilloscopes for floating measurements
High-voltage measurements: Use properly rated high-voltage probes with appropriate safety ratings. Never exceed probe voltage limits, and maintain proper clearances

Spectrum Analyzer Usage

While oscilloscopes display signals in the time domain, spectrum analyzers reveal the frequency-domain content of signals. This perspective is invaluable for identifying noise sources, measuring distortion, detecting oscillations, and characterizing filter responses. Many problems invisible on an oscilloscope become obvious when viewed on a spectrum analyzer.

Spectrum Analyzer Operating Principles

Understanding how spectrum analyzers work helps users set appropriate parameters and interpret results:

Swept-tuned architecture: Traditional spectrum analyzers sweep a bandpass filter across the frequency range, measuring power at each frequency. Sweep time affects measurement speed and ability to capture transient events
FFT-based analyzers: Modern analyzers digitize the input and compute frequency content using Fast Fourier Transform algorithms. These capture complete spectra simultaneously but may have limited dynamic range compared to swept analyzers
Resolution bandwidth: The width of the measurement filter determines frequency resolution. Narrower resolution bandwidth reveals closely spaced signals but requires longer sweep times. Use resolution bandwidth at least ten times narrower than the spacing between signals of interest
Video bandwidth: Post-detection filtering smooths the display but can hide rapid amplitude changes. Set video bandwidth equal to or wider than resolution bandwidth for most measurements

Noise and Distortion Measurements

Spectrum analyzers excel at characterizing noise and nonlinear distortion:

Noise floor assessment: Measure circuit output with no input to establish the noise floor. Compare to expected noise based on component noise analysis
Noise spectral density: For accurate noise density measurements, account for resolution bandwidth. Normalize power measurements to 1 hertz bandwidth by subtracting 10 times the log of the resolution bandwidth in hertz
Harmonic distortion: Apply a pure sine wave and measure the power at harmonic frequencies relative to the fundamental. Total harmonic distortion is the ratio of harmonic power to fundamental power
Intermodulation distortion: Apply two tones and measure power at intermodulation product frequencies. Third-order products at 2f1-f2 and 2f2-f1 typically dominate in analog circuits
Spurious signals: Sweep across the full frequency range to identify unexpected signals from oscillation, coupling, or interference

Identifying Noise Sources

The spectral signature of noise often reveals its source:

White noise: Constant power spectral density indicates thermal or shot noise from resistors and semiconductors
1/f noise: Power density that increases at lower frequencies indicates flicker noise in semiconductors. This dominates below the corner frequency, typically a few hertz to a few kilohertz depending on the device
Discrete spectral lines: Signals at specific frequencies indicate switching noise from power supplies, digital clocks, or external interference. Trace the frequency to its source by correlating with known switching frequencies
Broadband humps: Elevated noise across a frequency range may indicate oscillation at the edge of stability, where loop gain approaches unity but does not exceed it

Practical Spectrum Analyzer Techniques

Effective spectrum analyzer use requires attention to measurement setup:

Input attenuation: Set input attenuation to prevent overload while maintaining adequate signal-to-noise ratio. Internal distortion from overload creates spurious signals that mask circuit distortion
Reference level selection: Position the largest signal near the top of the display to maximize dynamic range for measuring smaller signals
Averaging: Use video or trace averaging to reduce display noise and reveal weak signals. More averages increase measurement time but improve signal-to-noise ratio
Marker functions: Use markers to read exact frequency and amplitude. Delta markers measure differences between two points for relative measurements
Triggering: Some analyzers support time-domain triggering to capture transient spectral events. This is valuable for diagnosing intermittent problems

Network Analyzer Applications

Network analyzers characterize circuits by measuring how they modify signals passing through them. Unlike spectrum analyzers that measure signal amplitude versus frequency, network analyzers measure both magnitude and phase of transmission and reflection. This capability is essential for characterizing filters, amplifiers, impedance matching networks, and feedback loops.

Vector Network Analyzer Fundamentals

Vector network analyzers provide complete characterization of linear networks:

S-parameter measurements: Network analyzers measure scattering parameters that describe reflection and transmission at each port. S11 represents input reflection, S21 transmission from port 1 to port 2, S12 reverse transmission, and S22 output reflection
Magnitude and phase: Unlike scalar analyzers that measure only magnitude, vector analyzers capture phase information essential for impedance calculations and stability analysis
Calibration requirements: Accurate measurements require calibration to remove systematic errors from cables and connectors. Standard calibrations use known impedance standards: open, short, and load for reflection; through connection for transmission
Frequency range: Select an analyzer covering the frequency range of interest. Low-frequency vector network analyzers extend down to a few hertz for measuring audio circuits and control loops

Impedance and Return Loss Measurement

Network analyzers measure impedance through reflection coefficient:

Return loss: The ratio of reflected to incident power, expressed in decibels. Higher return loss indicates better impedance match. A 50-ohm load on a 50-ohm system shows infinite return loss; a short or open shows 0 dB
Impedance calculation: The analyzer calculates complex impedance from reflection coefficient. Display formats include impedance magnitude and phase, series resistance and reactance, or parallel conductance and susceptance
Smith chart display: The Smith chart provides a graphical representation of impedance that reveals matching network requirements. Points on the outer circle represent purely reactive impedances; the center represents matched impedance
Time domain option: Many network analyzers include time domain analysis that reveals the physical location of impedance discontinuities along a transmission path

Gain and Phase Margin Measurement

Network analyzers are essential for measuring feedback loop stability:

Loop gain measurement: Break the feedback loop (or use injection techniques) and measure the open-loop gain from feedback node back to that point. The gain and phase at each frequency reveal stability
Phase margin identification: Find the frequency where gain magnitude equals 0 dB (unity gain). The difference between the phase at this frequency and -180 degrees is the phase margin. Phase margin above 45 degrees typically ensures adequate stability
Gain margin identification: Find the frequency where phase reaches -180 degrees. The difference between 0 dB and the gain at this frequency is the gain margin. Gain margin above 10 dB provides good stability margin
Bode plot generation: Display magnitude and phase versus frequency on logarithmic scales to visualize control loop behavior across decades of frequency

Filter and Amplifier Characterization

Network analyzers efficiently characterize transmission characteristics:

Frequency response: Measure insertion loss or gain across the operating frequency range. Identify passband ripple, corner frequencies, and stopband rejection
Group delay: Calculate group delay from the derivative of phase with respect to frequency. Constant group delay indicates linear phase and minimum signal distortion
Input and output impedance: Measure reflection at input and output ports to verify impedance matching across frequency
Isolation: For multi-port devices, measure isolation between ports to verify adequate decoupling

Signal Injection Methods

Debugging often requires injecting known signals into circuits to trace signal paths, verify functionality, or stimulate specific behaviors. The choice of injection method depends on the signal characteristics needed and the circuit topology being tested.

Direct Signal Injection

The simplest injection method connects a signal source directly to a circuit node:

Source impedance matching: Consider whether the signal source output impedance appropriately loads the node. A 50-ohm source may excessively load a high-impedance input
DC blocking: Use coupling capacitors when injecting AC signals into nodes with DC bias. Size the capacitor to pass the lowest frequency of interest without significant attenuation
Amplitude selection: Start with small signals to verify linear operation, then increase amplitude to test dynamic range and clipping behavior
Isolation from existing signals: To test a stage in isolation, disconnect or disable upstream signals. This may require lifting components or using switches

Injection Transformers

Transformers provide isolated signal injection with minimal circuit disruption:

Current transformer injection: Pass a wire through a current transformer and drive the transformer secondary. The primary induces current in the wire without a galvanic connection
Voltage injection transformers: Insert a transformer in series with a signal path. The secondary adds voltage to the signal. This technique is common for control loop analysis
Wideband transformers: For accurate injection across frequency, use wideband transformers with flat response over the measurement range
Injection isolation: Transformers provide galvanic isolation, preventing ground loop problems when injecting from external equipment

Control Loop Injection Techniques

Measuring control loop stability requires injecting signals into the feedback path:

Voltage injection: Insert a small resistance in the feedback path and inject across it. The injected voltage adds to the feedback signal, perturbing the loop
Current injection: Use a transformer to inject current into the feedback network. This works well when the feedback network has low impedance
Injection amplitude: Use the smallest amplitude that provides adequate signal-to-noise ratio. Excessive amplitude may drive the loop into nonlinear behavior
AC coupling considerations: For loops with integrators, injection must account for the high gain at low frequencies. Use sufficient injection amplitude at low frequencies or start measurements above the integrator bandwidth

Noise Injection for Testing

Injecting noise or interfering signals tests circuit immunity:

Power supply noise injection: Modulate power supply voltage with sinusoids or noise to measure power supply rejection ratio at various frequencies
Common-mode injection: Apply common-mode signals to differential inputs to verify common-mode rejection
Conducted immunity testing: Inject RF signals onto signal and power cables to simulate electromagnetic interference
Transient injection: Apply fast transients to verify circuit immunity to electrostatic discharge and switching transients

Current Probing Techniques

Current measurements complement voltage measurements by revealing power flow, identifying excessive current draw, and measuring switching losses. Several techniques enable current measurement without breaking the circuit, each with distinct characteristics and applications.

Current Sense Resistors

Inserting a small resistance and measuring the voltage drop is the most direct current measurement:

Resistance selection: Balance sensitivity against power loss and voltage drop. Typical values range from 1 milliohm for high-current power circuits to 100 ohms for microampere measurements
Four-wire sensing: Use Kelvin connections to separate current-carrying leads from voltage-sensing leads, eliminating lead resistance errors
Inductance considerations: Wire-wound resistors have significant inductance that affects high-frequency accuracy. Use non-inductive types for fast current measurements
Thermal effects: Current flow heats the resistor, changing its value. Select stable resistance types and allow for warmup in precision measurements
High-side versus low-side sensing: Low-side sensing is simpler but shifts the load ground reference. High-side sensing preserves ground reference but requires differential measurement

Current Transformers

Current transformers measure AC current by detecting the magnetic field around a conductor:

Operating principle: The current-carrying conductor passes through a magnetic core. The magnetic field induces proportional current in a secondary winding wrapped around the core
Frequency range: Current transformers respond to AC only, typically from a few hundred hertz to tens of megahertz. They cannot measure DC current
Turns ratio selection: The secondary current equals primary current divided by the turns ratio. Select a ratio that produces easily measurable secondary current
Burden resistor: A resistor across the secondary converts current to voltage. Smaller resistance extends high-frequency response; larger resistance improves low-frequency response
Saturation limits: Magnetic cores saturate at high current levels, limiting measurement range. Large DC components can also cause saturation, distorting AC measurements

Hall Effect Current Probes

Hall effect sensors measure magnetic fields directly, enabling DC current measurement:

DC through RF bandwidth: Hall sensors respond to DC, complementing current transformers. Many probes combine Hall and transformer sensing for extended bandwidth from DC to tens of megahertz
Null-balance operation: High-accuracy probes use feedback to null the magnetic field, improving linearity and bandwidth
Zero offset drift: Hall sensors exhibit temperature-dependent offset. Zero the probe with no current before measurement and periodically during extended measurements
Sensitivity selection: Adjust probe sensitivity to match expected current range. Higher sensitivity improves small-current measurement at the expense of dynamic range
Positioning requirements: Center the conductor in the probe aperture. Off-center positioning introduces errors, especially with nearby conductors carrying return current

Rogowski Coils

Rogowski coils provide unique capabilities for very high current measurement:

Air core advantages: Without a magnetic core, Rogowski coils cannot saturate, enabling measurement of fault currents reaching thousands of amperes
Flexible form factor: Available as flexible loops that wrap around irregularly shaped conductors or multiple parallel conductors
Output characteristics: The output is proportional to di/dt, the rate of change of current. An integrator recovers the current waveform
Lower sensitivity: Air-core construction provides lower sensitivity than ferrite cores, making Rogowski coils best suited for high-current measurements
Non-intrusive measurement: The open construction allows installation without disconnecting conductors

Current Measurement Best Practices

Accurate current measurement requires attention to technique:

Demagnetization: Degauss clamp-on probes periodically and after measuring large transients or DC currents that may have magnetized the core
Propagation delay compensation: Current probes have inherent delay. Deskew against voltage probes when calculating power or observing switching timing
Return current consideration: Position the probe to measure only the conductor of interest. Adjacent return conductors can cancel the magnetic field if too close
Noise immunity: Shielded current probes reject external magnetic fields. Keep probes away from transformers and inductors that radiate strong fields

Thermal Imaging

Thermal analysis reveals power dissipation distribution, identifies hot spots, and validates thermal design. Excessive temperature is often the first indication of circuit problems, from shorted components to design errors that cause excessive current draw. Thermal imaging provides a rapid, non-contact method to assess temperature across an entire circuit board.

Infrared Camera Fundamentals

Infrared thermal cameras form images based on thermal radiation emitted by objects:

Temperature sensitivity: Modern thermal cameras detect temperature differences below 0.1 degrees Celsius, revealing subtle thermal gradients
Spatial resolution: Resolution determines the smallest feature that can be measured. Macro lenses enable component-level imaging on densely populated boards
Frame rate: Standard cameras capture several frames per second. High-speed thermal cameras with frame rates exceeding 100 Hz capture transient thermal events
Measurement accuracy: Absolute temperature accuracy of plus or minus 2 degrees Celsius is typical. Relative measurements between points on the same image are more accurate

Emissivity Considerations

Accurate temperature measurement requires accounting for surface emissivity:

Emissivity definition: Emissivity describes how efficiently a surface radiates thermal energy compared to an ideal blackbody. Values range from near zero for polished metals to near unity for matte black surfaces
Low-emissivity surfaces: Shiny metal surfaces, solder joints, and uncoated copper have low emissivity and appear cooler than actual temperature
Emissivity correction: Apply the correct emissivity setting for each material type when measuring absolute temperature
High-emissivity coating: Apply matte black paint or commercial high-emissivity coating to low-emissivity surfaces for accurate measurement. Ensure the coating does not significantly affect thermal behavior
Reflection artifacts: Low-emissivity surfaces reflect thermal radiation from nearby hot objects, potentially causing measurement errors

Thermal Imaging Applications

Thermal cameras serve multiple debugging purposes:

Hot spot identification: Quickly locate overheating components for investigation. Components running hotter than expected may indicate excessive current, poor thermal design, or impending failure
Thermal coupling assessment: Identify heat flow from power devices to temperature-sensitive components like voltage references and precision amplifiers
Heat sink evaluation: Verify that heat sinks provide adequate thermal spreading and that thermal interface materials function correctly
Current path visualization: High current through thin traces creates localized heating, revealing current distribution through the circuit
Short circuit location: Shorted components draw excessive current and become hot, making them easily identifiable with thermal imaging

Thermal Transient Analysis

Dynamic thermal measurements reveal thermal impedance characteristics:

Step response measurement: Apply a power step and record temperature versus time. The thermal time constants reveal the thermal mass and resistance of each element in the heat path
Structure function analysis: Mathematical analysis of transient data extracts the cumulative thermal structure, identifying individual interfaces and their thermal resistances
Die attach quality: Voids or poor die attach create anomalous thermal transient responses, enabling non-destructive inspection
Periodic power testing: Apply periodic power and observe temperature modulation. This technique is useful when step response is not practical

Complementary Temperature Measurement Methods

Additional techniques provide point measurements or exploit built-in temperature sensing:

Thermocouple measurement: Fine-wire thermocouples attached to components provide accurate absolute temperature. Type K or Type T thermocouples are common for electronics applications
On-chip temperature sensors: Use the forward voltage of on-chip diodes as temperature indicators. The voltage decreases approximately 2 millivolts per degree Celsius
Thermal test chips: Specialized test dice include heaters and temperature sensors for characterizing package thermal performance
Liquid crystal indicators: Temperature-sensitive films change color at specific temperatures, providing visual indication when limits are exceeded

Curve Tracer Usage

Curve tracers display the current-voltage characteristics of electronic devices, enabling verification of device parameters, matching of components, and identification of damaged devices. By sweeping voltage and measuring current, or vice versa, the curve tracer reveals the complete DC behavior of components.

Curve Tracer Operating Principles

Understanding curve tracer operation enables effective measurement setup:

Horizontal axis: Represents collector-emitter voltage for bipolar transistors, drain-source voltage for FETs, or applied voltage for two-terminal devices
Vertical axis: Represents current through the device, typically collector or drain current for transistors
Step generator: For three-terminal devices, a step generator provides stepped base current or gate voltage to produce a family of curves
Pulsed versus DC operation: Some curve tracers use pulsed measurements to avoid device heating. This is essential for power devices that would overheat during sustained DC measurement

Transistor Characterization

Curve tracers reveal transistor parameters directly from the displayed curves:

Current gain (beta): The ratio of collector current to base current, visible as the spacing between curves in a family of characteristics
Saturation voltage: The knee of the curve where the transistor transitions from saturation to active region
Early voltage: The slope of the active region curves indicates output resistance; extrapolating curves backward locates the Early voltage
Breakdown voltage: The voltage where current increases rapidly due to avalanche or Zener breakdown
Leakage current: Current flow with zero base drive indicates collector-base leakage

Diode and Zener Characterization

Two-terminal devices display their complete I-V relationship:

Forward voltage: The voltage drop at a specified forward current, visible on the forward characteristic
Reverse leakage: Current in the reverse direction before breakdown
Breakdown voltage: For Zener diodes, the voltage where reverse current increases sharply
Dynamic resistance: The slope of the curve at the operating point indicates dynamic resistance
Temperature dependence: Heating the device while observing the curve reveals temperature coefficients

Component Matching

Curve tracers enable precise matching of components for differential circuits:

Dual-trace comparison: Many curve tracers display two sets of curves simultaneously, enabling direct visual comparison of matched pairs
Parameter matching: Match beta, transconductance, threshold voltage, or other parameters by selecting devices with overlapping curves
Temperature coefficient matching: Match devices at multiple temperatures to ensure tracking over the operating range
Sorting for matched sets: Rapidly test many devices to find groups with matched characteristics

Failure Analysis with Curve Tracers

Curve tracers reveal device damage and failure modes:

Junction shorts: Shorted junctions appear as linear resistive characteristics instead of diode curves
Open circuits: Open leads result in zero current regardless of applied voltage
Degraded beta: Compare curves to specification or known-good devices to identify gain degradation
Increased leakage: Elevated reverse current indicates junction damage or contamination
Reduced breakdown voltage: Breakdown at lower than rated voltage suggests damage or over-stress

In-Circuit Testing

In-circuit testing verifies component presence, value, and functionality without removing parts from the circuit board. This approach is particularly valuable during manufacturing to catch assembly defects, but the techniques also apply to debugging in development and field service.

In-Circuit Test Fundamentals

Understanding in-circuit test principles guides effective application:

Bed-of-nails fixtures: Production in-circuit testers use fixtures with spring-loaded pins that contact test points across the board simultaneously
Guarding: Adjacent components create parallel paths that can interfere with measurement. Guarding applies voltage to surrounding nodes to minimize current flow through alternate paths
Access considerations: Effective in-circuit testing requires test access to component terminals. Design for testability by including test pads at critical nodes
Powered versus unpowered testing: Unpowered tests measure passive component values. Powered tests verify active device functionality

Passive Component Testing

In-circuit techniques measure resistors, capacitors, and inductors while installed:

Resistance measurement: Apply known current and measure voltage. Account for parallel resistances from other circuit paths
Capacitance measurement: Apply AC signal and measure impedance. Low frequencies minimize effects of series resistance and inductance
Inductance measurement: Higher frequency measurement distinguishes inductive from capacitive reactance
Component verification: Compare measured values to expected values within tolerance limits. Flag components outside acceptable ranges

Active Device Testing

In-circuit tests can verify active devices by forcing operating conditions:

Diode testing: Apply forward current and measure voltage drop. Compare to expected diode characteristic
Transistor junction testing: Test base-emitter and base-collector junctions as diodes. Verify all junctions show correct diode behavior
Functional testing: Apply power and input signals while monitoring outputs. Verify devices produce expected responses
Open and short detection: Missing components, solder bridges, and broken connections are detected by comparing measurements to expected values

Boundary Scan Testing

For digital and mixed-signal devices supporting JTAG, boundary scan provides powerful test access:

JTAG standard: IEEE 1149.1 defines a standard interface for testing integrated circuits through a serial test access port
Interconnect testing: Boundary scan can verify connections between devices by driving patterns from one device and reading at another
Device programming: The JTAG interface also enables programming of flash memory and programmable logic devices
Internal register access: Boundary scan provides access to internal device registers for configuration and diagnostics

Flying Probe Testing

Flying probe testers provide flexibility without custom fixtures:

Operating principle: Movable probes sequentially contact test points under program control, eliminating the need for dedicated fixtures
Prototype and low-volume production: Flying probes excel for prototype testing and short production runs where fixture cost is not justified
Test program development: Easier program modification compared to fixture-based testers enables rapid iteration
Test time considerations: Sequential probing is slower than parallel bed-of-nails testing, limiting throughput for high-volume production

Design for In-Circuit Testability

Design practices that enhance in-circuit test effectiveness:

Test pad placement: Include test pads accessible from one side of the board, placed on a regular grid for fixture alignment
Series isolation: Small series resistors between parallel components enable individual testing by providing guarding points
Power control: Include provisions for separately powering circuit sections during test
Test access documentation: Document which nodes are accessible and what test capabilities each provides

Integrating Multiple Debug Techniques

Effective debugging rarely relies on a single tool or technique. Complex problems require combining information from multiple sources to build a complete understanding of circuit behavior.

Correlation of Time and Frequency Domain

Oscilloscope and spectrum analyzer measurements complement each other:

Identifying periodic noise: An oscilloscope may show noise riding on a signal. The spectrum analyzer reveals the frequency, often pointing to the source
Verifying filter performance: Time-domain measurements show transient response. Frequency-domain measurements verify the transfer function
Capturing intermittent events: An oscilloscope with triggering can capture transient events. The spectrum analyzer shows if they repeat periodically

Correlating Electrical and Thermal Measurements

Thermal and electrical measurements together reveal power-related problems:

Power dissipation validation: Calculate power from voltage and current measurements. Compare to thermal measurements to verify power distribution
Thermal-electrical feedback: Temperature changes affect device parameters. Observe electrical behavior as temperature varies to identify thermal problems
Hot spot investigation: When thermal imaging reveals hot spots, use electrical measurements to determine current flow and power dissipation in those areas

Systematic Debug Strategy

Apply tools in a logical sequence for efficient debugging:

Start with power and bias: Verify power supply voltages and DC operating points before investigating signal behavior
Follow the signal path: Trace signals from input to output, identifying where behavior deviates from expectations
Use appropriate resolution: Start with broad measurements to localize problems, then focus with higher-resolution techniques
Document everything: Record measurements, observations, and hypotheses. This documentation aids analysis and prevents repeating unsuccessful approaches