Comb Generators

What are Comb Generators?

A comb generator is a specialized reference signal source that produces a fundamental frequency and all of its harmonics simultaneously, creating a "comb" pattern of spectral lines when viewed on a spectrum analyzer. Unlike a signal generator that produces one frequency at a time, a comb generator radiates or conducts multiple frequencies (harmonics) at regular intervals across a wide frequency range, making it an invaluable tool for quick verification and validation of EMC measurement systems.

The term "comb" derives from the appearance of the frequency spectrum, which resembles the teeth of a comb—evenly spaced spectral lines at predictable intervals (such as 1 MHz, 5 MHz, 20 MHz, 100 MHz, or 1 GHz steps, depending on the model).

Technical Principles

Comb generators work on the principle of nonlinear signal processing. A stable oscillator generates a fundamental frequency, which is then processed through nonlinear circuits (such as step recovery diodes or other fast-switching elements) to produce rich harmonic content. This creates a series of equally-spaced frequency components that can extend from the fundamental frequency up to several gigahertz or even tens of gigahertz.

Key characteristics include:

• Frequency Stability: Typically 5-50 ppm, ensuring consistent reference measurements over time
• Amplitude Stability: ±0.1 dB typical, providing reliable amplitude references
• Time Stability: Less than 1 dB drift over 12 months
• Battery Operation: Most models operate 18+ hours on a single charge to eliminate external cabling interference

Applications in EMC Testing

Comb generators serve several critical functions in electromagnetic compatibility (EMC) testing:

• Daily Site Verification: Quick checks (minutes vs. hours/days for full calibration) to ensure test site and measurement system integrity
• Pre-Test Validation: Verify measurement setup before testing expensive equipment under test (EUT)
• System Troubleshooting: Identify which component in the measurement chain has failed (antenna, cable, preamplifier, receiver)
• Site Comparison: Compare multiple test sites or chambers for consistency
• Shielding Effectiveness: Test the effectiveness of shielded enclosures and materials
• Insertion Loss Measurements: Measure attenuation of cables, filters, and attenuators

Radiated Comb Generators (CGO Series)

Radiated comb generators are designed primarily for validating radiated emissions test setups on Open Area Test Sites (OATS), Semi-Anechoic Chambers (SAC), and Fully-Anechoic Chambers (FAC). They feature built-in or attachable antennas and radiate RF energy across their specified frequency range.

The Problem They Solve

EMC test sites undergo formal Normalized Site Attenuation (NSA) calibrations annually or semi-annually—tedious procedures that can take several days and cost thousands of dollars. However, problems can occur between calibrations:

• Underground cables become compromised
• Connectors oxidize or loosen
• Preamplifier gain drifts or fails
• Antenna elements break or corrode
• Turntable or positioning systems malfunction

Without daily verification, you could collect months of invalid data before discovering a problem. A radiated comb generator allows you to take reference measurements at several strategic frequencies in just minutes, comparing results against your baseline data to detect issues immediately.

Radiated Comb Generator Comparison Table

Conducted Comb Generators (CGC Series)

Conducted comb generators are designed to verify conducted emissions measurement systems, particularly Line Impedance Stabilization Networks (LISNs). Unlike radiated comb generators that transmit RF through antennas, conducted comb generators inject RF signals directly into the power lines or measurement circuits.

How They Differ from Radiated Comb Generators

The Problem They Solve

Conducted emissions test setups include LISNs, RF cables, spectrum analyzers, and sometimes preamplifiers. These components are typically calibrated annually, but malfunctions between calibrations can produce erroneous results:

• LISN impedance drifts out of specification
• RF cable develops intermittent connection
• Spectrum analyzer attenuator fails
• Connector corrosion increases loss

A conducted comb generator allows the test engineer to quickly verify the entire conducted measurement chain before each test, ensuring data accuracy and preventing costly retests.

Conducted Comb Generator Comparison Table

Real-World Use Cases and Applications

Use Case 1: Daily OATS Verification (Radiated)

Scenario: An EMC test laboratory performs 3-5 compliance tests daily on an outdoor OATS. Weather, wildlife, and environmental factors can affect measurement integrity.

Solution: Each morning, a technician places a CGO-520 comb generator on the turntable at 3 meters from the receiving antenna. Within 5 minutes, they measure field strength at 30 MHz, 230 MHz, 1 GHz, and 3 GHz, comparing results to baseline data stored in a spreadsheet. A deviation greater than 2 dB triggers investigation.

Cost Savings: Catches a failing preamplifier before running expensive certification tests on customer equipment, saving $15,000+ in potential retesting costs and maintaining customer confidence.

Use Case 2: LISN Verification Before High-Stakes Testing (Conducted)

Scenario: A medical device manufacturer is about to test a $2 million MRI system for conducted emissions compliance. LISN calibration is current, but the last test was 6 months ago.

Solution: Test engineer plugs a CGC-255E into each LISN's EUT port and measures at 150 kHz, 500 kHz, 5 MHz, and 30 MHz. All readings match baseline within 1 dB, confirming system integrity.

Outcome: Testing proceeds with confidence. Had a problem been detected, it would have been identified and corrected before committing the expensive EUT to the chamber.

Use Case 3: Multi-Chamber Correlation (Radiated)

Scenario: An automotive OEM has three SACs for component testing. Engineering suspects Chamber 2 is reading 3-4 dB higher than the other chambers on certain vehicle electronic modules.

Solution: A CGO-515 is measured at identical positions in all three chambers at 1 MHz intervals from 30-1000 MHz. Data reveals Chamber 2's receiving antenna has 3.5 dB higher sensitivity due to a recent repair that changed the antenna's characteristics.

Outcome: The antenna is recalibrated/replaced, and correlation between chambers is restored. The root cause is identified in hours instead of days.

Use Case 4: Cable and Filter Characterization (Conducted)

Scenario: A cable manufacturer needs to verify attenuation characteristics of shielded cables from 150 kHz to 30 MHz for product specifications.

Solution: A CGC-510E is connected to a spectrum analyzer through the cable under test using a coaxial adapter. Measurements at 500 kHz intervals provide insertion loss data across the frequency range in minutes.

Outcome: Production cables are spot-checked quickly without expensive VNA equipment, ensuring consistent manufacturing quality.

Use Case 5: Site Troubleshooting After Failed NSA (Radiated)

Scenario: An NSA calibration fails at 80-200 MHz. The lab needs to determine if the problem is the antenna, cable, preamplifier, or site geometry.

Solution: Using a CGO-505 with 5 MHz steps, technicians systematically test:
1. Full measurement chain (fails)
2. Direct connection to preamp output (passes)
3. Cable between antenna and preamp (fails)
Conclusion: Bad cable connector identified in 30 minutes.

Outcome: Cable is replaced, and NSA is rerun successfully. What could have been days of troubleshooting is resolved in hours.

Use Case 6: Shielding Effectiveness Verification (Radiated)

Scenario: A new shielded enclosure is installed in a facility. Engineering needs to verify shielding effectiveness from 1-18 GHz before housing sensitive equipment.

Solution: A CGO-5100B is placed inside the enclosure, and a spectrum analyzer with horn antenna measures field strength outside at various positions. Attenuation at 100 MHz intervals quantifies shielding performance.

Outcome: Enclosure achieves >80 dB attenuation across the band. Two seam leaks at door gaskets are identified and corrected, then verified with a second measurement.

Selection Guide by Application

Best Practices for Using Comb Generators

Establishing Baseline Data

1. After successful site calibration (NSA), immediately measure your comb generator at strategic frequencies
2. Document: comb generator model/SN, frequency, measured level, date, weather conditions (for OATS)
3. Store baseline data in a spreadsheet or database for historical tracking
4. Repeat measurements quarterly to establish long-term stability trends

Daily Verification Procedure

1. Select 3-5 frequencies spread across your test range (low, mid, high)
2. Avoid frequencies with high ambient signals (especially on OATS)
3. Use consistent positioning: same distance, height, orientation
4. Compare to baseline; investigate if deviation exceeds ±2 dB
5. Log results for trending and regulatory documentation

Frequency Selection Strategy

• Choose frequencies in different antenna/preamp subranges
• Include frequencies near known problematic ranges for your site
• For OATS, avoid broadcast FM (88-108 MHz) and cellular bands
• Include at least one frequency where your limits are strictest

Cost-Benefit Analysis

Investment: $2,500 - $8,000 depending on model and frequency range

Typical Savings (per occurrence):

• Avoided retest of failed unit due to bad measurement: $5,000 - $50,000
• Early detection of failing equipment preventing invalid test data: $10,000+
• Reduced troubleshooting time: $500 - $2,000 (labor savings)
• Customer confidence/reputation: Priceless

ROI Timeline: Most laboratories recover investment cost within 6-12 months through prevented measurement errors and reduced troubleshooting time.

Technical Specifications Summary

Common Features Across All Models

• Frequency Stability: 5-50 ppm (temperature and aging combined)
• Amplitude Stability: ±0.1 dB typical
• Time Stability: <1 dB over 12 months (long-term drift)
• Battery Chemistry: NiMH rechargeable (6V, 1Ah typical)
• Battery Runtime: 18-23 hours continuous operation
• Charge Time: 12-14 hours typical
• Auto Shutoff: Prevents unstable output at low battery voltage
• Indicators: Power LED, Low Battery LED
• Warranty: 3 years standard
• Carrying Case: Custom foam-lined case included

Maintenance and Calibration

Comb generators require minimal maintenance but benefit from periodic verification:

• Annual Calibration: Recommended to maintain NIST traceability (not required for daily use)
• Battery Replacement: Every 2-3 years depending on charge cycles
• Antenna Inspection: Check for physical damage, corrosion, secure connections
• Frequency Verification: Confirm output frequencies annually with frequency counter
• Amplitude Verification: Compare to calibrated signal generator + power meter

Many laboratories maintain two comb generators—one as the working standard for daily checks, and one as a transfer standard that's calibrated and stored for quarterly verification of the working unit.

Conclusion

Comb generators are essential tools for maintaining the integrity of EMC measurement systems. Whether validating radiated test sites or conducted measurement chains, these battery-operated reference sources provide quick, reliable verification that your test setup is functioning correctly. The investment in a comb generator pays dividends through:

• Prevention of costly measurement errors
• Rapid troubleshooting of system problems
• Confidence in test data between formal calibrations
• Regulatory compliance documentation
• Reduced downtime and increased laboratory efficiency

By implementing daily or pre-test verification procedures using appropriate comb generators, EMC laboratories can ensure measurement accuracy, maintain customer confidence, and avoid the significant costs associated with invalid test results.