Repetitive Pulse Generator Calibration: Waveform Quality Metrics and Performance Verification
Calibration of a repetitive pulse generator extends beyond verifying peak voltage. For high-voltage test applications, waveform quality parameters—rise time, pulse flatness, voltage droop, overshoot, and pulse-to-pulse jitter—directly influence test repeatability and compliance with standards like IEC 60060-2 and IEEE 4. This article provides a practical framework for performance verification of repetitive pulse generators used in insulation testing and component qualification.
Critical Waveform Parameters for Repetitive Operation
Unlike single-shot impulse generators, RPGs must maintain consistent pulse shape across thousands or millions of cycles. The following parameters require regular verification:
Peak voltage accuracy: ±2% of setpoint across the operating range.
Rise time (10% to 90%): Stability within ±5% over 1 hour of continuous operation.
Pulse flatness (top voltage variation): ≤3% deviation from peak during the pulse width.
Voltage droop: ≤5% from start to end of pulse (for pulse widths >1 μs).
Overshoot and ringing: First peak overshoot ≤5% of setpoint; ringing decay to <2% within 200 ns.
Pulse-to-pulse jitter (time domain): ≤1% of pulse width or 1 ns, whichever is larger.
Amplitude jitter (shot-to-shot variation): ≤1% standard deviation over 1000 consecutive pulses.
Required Calibration Equipment
Accurate RPG calibration requires specialized high-bandwidth measurement systems:
Digital storage oscilloscope (DSO) with ≥500 MHz bandwidth and ≥5 GS/s sample rate.
High-voltage differential probe or resistive divider rated for maximum RPG output, with bandwidth ≥200 MHz.
Current transformer or Rogowski coil (for current-mode pulses), bandwidth 100 Hz to 200 MHz.
Precision 50 Ω load resistor, non-inductive design, power rating matching RPG output.
High-frequency coaxial cable (RG-400 or RG-223), length ≤2 meters to minimize reflections.
Thermal imaging camera to monitor load and generator internal temperature during extended runs.
Step-by-Step Calibration Procedure
Perform full calibration annually or after any repair. A simplified verification check should be completed before each major test campaign.
Step 1: Setup and Warm-Up
Connect the RPG output through a precision 50 Ω load to the high-voltage probe and oscilloscope. Allow the generator to warm up for 30 minutes at 50% of maximum repetition rate without applied high voltage. This stabilizes internal power supplies and cooling systems.
Step 2: Low-Voltage Reference Check
Set the RPG to produce a 100 V pulse at 100 Hz with 1 μs pulse width. Record 100 consecutive waveforms. Calculate mean peak voltage, standard deviation, and rise time. Compare against manufacturer specifications. If deviation exceeds 5%, investigate probe calibration or generator output stage health.
Step 3: Full-Voltage Single-Pulse Measurement
Disable repetition mode. Generate a single pulse at maximum rated voltage. Capture and save the waveform. Measure rise time, overshoot, pulse flatness, and droop. Acceptable ranges are defined above.
Step 4: High-Repetition Rate Stability Test
Configure the RPG at 50% of maximum voltage and 80% of maximum rated repetition rate. Operate continuously for 60 minutes. Capture and record 1000 pulses at minute 1, minute 30, and minute 60. Compare amplitude stability and jitter between these measurement points. Temperature drift in solid-state switches may cause amplitude changes up to 2% over one hour—document this drift for test report corrections.
Step 5: Load Variation Test
Replace the 50 Ω load with loads representing typical test objects: 100 Ω, 200 Ω, and a 10 nF capacitor (simulating insulation capacitance). Verify that the RPG maintains specified waveform parameters into each load. Significant waveform change suggests output impedance mismatch requiring investigation.
Waveform Quality Troubleshooting Guide
| Observed Anomaly | Likely Cause | Recommended Action |
|---|---|---|
| Excessive overshoot (>10%) | Impedance mismatch or parasitic inductance in load path | Check cable length, add series damping resistor |
| Droop >8% during pulse | Insufficient storage capacitance or DC supply sag | Measure power supply ripple, inspect capacitor bank |
| Increasing jitter with repetition rate | Trigger circuit instability or thermal effect on timing components | Check trigger cable shielding, monitor internal temperature |
| Amplitude drops after 10 minutes operation | Thermal derating of solid-state switches or power supply | Improve cooling, reduce repetition rate, or replace aging components |
| High-frequency ringing on pulse top | Parasitic resonance in test leads or load | Shorten leads, add ferrite beads at load connection |
Documentation and Traceability Requirements
For ISO/IEC 17025 accredited laboratories, calibration records must include:
Ambient temperature and humidity at time of calibration (RPG performance varies by ±0.2%/°C typically).
Traceability statement linking measurement standards to national metrology institutes.
Uncertainty budget for each measured parameter (e.g., peak voltage uncertainty ±3.5% including probe and oscilloscope contributions).
As-left adjustment record if any internal trims were modified during calibration.
Date of next calibration based on manufacturer recommendation and usage hours (typically 12 months or 2000 operating hours).
Field Verification vs. Full Calibration
Between annual calibrations, perform weekly field verification using a portable reference pulser (a known-good, low-voltage pulse source with stable amplitude). Connect the portable reference through the same measurement chain used for RPG testing. If the measured amplitude deviates by more than 1% from the reference value, clean all high-voltage connections and repeat the test. Persistence of deviation indicates need for full recalibration.
Automated Calibration Software Considerations
Modern repetitive pulse generators often include built-in calibration routines accessible via Ethernet or GPIB. While convenient, always verify automated results with an independent oscilloscope measurement. Software calibration corrects for gain and offset errors but cannot detect bandwidth limitations or defective output stages that only manifest at high repetition rates.
Regular calibration and waveform quality verification transform a repetitive pulse generator from an untrusted black box into a precisely characterized test instrument. Laboratories that implement these procedures produce defensible test data, reduce false failures from generator anomalies, and extend equipment lifespan through early detection of developing faults.
