Application of KP2021 High-Frequency Electrosurgical Analyzer and Network Analyzer in Thermage Testing

Thermage, a non-invasive radiofrequency (RF) skin tightening technology, is widely used in medical aesthetics. With operating frequencies increasing to 1MHz-5MHz, testing faces challenges such as skin effect, proximity effect, and parasitic parameters. Based on the GB 9706.202-2021 standard, this article explores the integrated application of the KP2021 high-frequency electrosurgical analyzer and vector network analyzer (VNA) in power measurement, impedance analysis, and performance validation. Through optimized strategies, these tools ensure the safety and efficacy of Thermage devices.

Keywords: Thermage; KP2021 high-frequency electrosurgical analyzer; network analyzer; high-frequency testing; 

IEC 60601-2-20 standard; skin effect; parasitic parameters

Thermage is a non-invasive RF skin tightening technology that heats deep collagen layers to promote regeneration, achieving skin tightening and anti-aging effects. As a medical aesthetic device, the stability, safety, and performance consistency of its RF output are critical. According to IEC 60601-2-2 and its Chinese equivalent, GB 9706.202-2021, RF medical devices require testing for output power, leakage current, and impedance matching to ensure clinical safety and efficacy.

High-frequency electrosurgical devices utilize high-density, high-frequency current to create localized thermal effects, vaporizing or disrupting tissue for cutting and coagulation. These devices, typically operating in the 200kHz-5MHz range, are widely used in open surgeries (e.g., general surgery, gynecology) and endoscopic procedures (e.g., laparoscopy, gastroscopy). While traditional electrosurgical units operate at 400kHz-650kHz (e.g., 512kHz) for significant cutting and hemostasis, higher-frequency devices (1MHz-5MHz) enable finer cutting and coagulation with reduced thermal damage, suitable for plastic surgery and dermatology. As higher-frequency devices like low-temperature RF knives and aesthetic RF systems emerge, testing challenges intensify. The GB 9706.202-2021 standard, particularly clause 201.5.4, imposes stringent requirements on measurement instruments and test resistors, rendering traditional methods inadequate.

The KP2021 high-frequency electrosurgical analyzer and vector network analyzer (VNA) play pivotal roles in Thermage testing. This article examines their applications in quality control, production validation, and maintenance, analyzing high-frequency testing challenges and proposing innovative solutions.

The KP2021, developed by KINGPO Technology, is a precision testing instrument for high-frequency electrosurgical units (ESUs). Its key features include:

• Wide Measurement Range: Power (0-500W, ±3% or ±1W), voltage (0-400V RMS, ±2% or ±2V), current (2mA-5000mA, ±1%), high-frequency leakage current (2mA-5000mA, ±1%), load impedance (0-6400Ω, ±1%).
• Frequency Coverage: 50kHz-200MHz, supporting continuous, pulsed, and stimulation modes.
• Diverse Test Modes: RF power measurement (monopolar/bipolar), power load curve testing, leakage current measurement, and REM/ARM/CQM (return electrode monitoring) testing.
• Automation and Compatibility: Supports automated testing, compatible with brands like Valleylab, Conmed, and Erbe, and integrates with LIMS/MES systems.

Compliant with IEC 60601-2-2, the KP2021 is ideal for R&D, production quality control, and hospital equipment maintenance.

The vector network analyzer (VNA) measures RF network parameters, such as S-parameters (scattering parameters, including reflection coefficient S11 and transmission coefficient S21). Its applications in medical RF device testing include:

• Impedance Matching: Evaluates RF energy transfer efficiency, reducing reflection losses to ensure stable output under varying skin impedances.
• Frequency Response Analysis: Measures amplitude and phase responses across a wide band (10kHz-20MHz), identifying distortions from parasitic parameters.
• Impedance Spectrum Measurement: Quantifies resistance, reactance, and phase angle via Smith chart analysis, ensuring compliance with GB 9706.202-2021.
• Compatibility: Modern VNAs (e.g., Keysight, Anritsu) cover frequencies up to 70GHz with 0.1dB accuracy, suitable for RF medical device R&D and validation.

These capabilities make VNAs ideal for analyzing Thermage’s RF chain, complementing traditional power meters.

Clause 201.5.4 of GB 9706.202-2021 mandates that instruments measuring high-frequency current provide true RMS accuracy of at least 5% from 10kHz to five times the device’s fundamental frequency. Test resistors must have a rated power at least 50% of the test consumption, with resistance component accuracy within 3% and an impedance phase angle not exceeding 8.5° in the same frequency range.

While these requirements are manageable for traditional 500kHz electrosurgical units, Thermage devices operating above 4MHz face significant challenges, as resistor impedance characteristics directly impact power measurement and performance evaluation accuracy.

The skin effect causes high-frequency current to concentrate on a conductor’s surface, reducing effective conductive area and increasing the resistor’s actual resistance compared to DC or low-frequency values. This can lead to power calculation errors exceeding 10%.

The proximity effect, occurring alongside the skin effect in closely arranged conductors, exacerbates uneven current distribution due to magnetic field interactions. In Thermage’s RF probe and load designs, this increases losses and thermal instability.

At high frequencies, resistors exhibit non-negligible parasitic inductance (L) and capacitance (C), forming a complex impedance Z = R + jX (X = XL - XC). Parasitic inductance generates reactance XL = 2πfL, increasing with frequency, while parasitic capacitance generates reactance XC = 1/(2πfC), decreasing with frequency. This results in a phase angle deviation from 0°, potentially exceeding 8.5°, violating standards and risking unstable output or overheating.

Reactive parameters, driven by inductive (XL) and capacitive (XC) reactances, contribute to impedance Z = R + jX. If XL and XC are unbalanced or excessive, the phase angle deviates significantly, reducing power factor and energy transfer efficiency.

Non-inductive resistors, designed to minimize parasitic inductance using thin-film, thick-film, or carbon-film structures, still face challenges above 4MHz:

• Residual Parasitic Inductance: Even small inductance produces significant reactance at high frequencies.
• Parasitic Capacitance: Capacitive reactance decreases, causing resonance and deviating from pure resistance.
• Wideband Stability: Maintaining phase angle ≤8.5° and resistance accuracy ±3% from 10kHz-20MHz is challenging.
• High-Power Dissipation: Thin-film structures have lower heat dissipation, limiting power handling or requiring complex designs.

1. Preparation: Connect KP2021 to the Thermage device, setting load impedance (e.g., 200Ω to simulate skin). Integrate VNA into the RF chain, calibrating to eliminate cable parasitics.
2. Power and Leakage Testing: KP2021 measures output power, voltage/current RMS, and leakage current, ensuring compliance with GB standards, and monitors REM functionality.
3. Impedance and Phase Angle Analysis: VNA scans the frequency band, measures S-parameters, and calculates phase angle. If >8.5°, adjust matching network or resistor structure.
4. High-Frequency Effect Compensation: KP2021’s pulse mode testing, combined with VNA’s time-domain reflectometry (TDR), identifies signal distortions, with digital algorithms compensating for errors.
5. Validation and Reporting: Integrate data into automated systems, generating GB 9706.202-2021-compliant reports with power load curves and impedance spectra.

KP2021 simulates skin impedances (50-500Ω) to quantify skin/proximity effects and correct readings. VNA’s S11 measurements calculate parasitic parameters, ensuring a power factor close to 1.

• Low-Inductance Design: Use thin-film, thick-film, or carbon-film resistors, avoiding wire-wound structures.
• Low Parasitic Capacitance: Optimize packaging and pin design to minimize contact area.
• Wideband Impedance Matching: Employ parallel low-value resistors to reduce parasitic effects and maintain phase angle stability.

• True RMS Measurement: KP2021 and VNA support non-sinusoidal waveform measurement across 30kHz-20MHz.
• Wideband Sensors: Select low-loss, high-linearity probes with controlled parasitic parameters.

Regularly calibrate systems using certified high-frequency sources to ensure accuracy.

• Short Leads and Coaxial Connections: Use high-frequency coaxial cables to minimize losses and parasitics.
• Shielding and Grounding: Implement electromagnetic shielding and proper grounding to reduce interference.
• Impedance Matching Networks: Design networks to maximize energy transfer efficiency.

• Digital Signal Processing: Apply Fourier transforms to analyze and correct parasitic distortions.
• Machine Learning: Model and predict high-frequency behavior, auto-adjusting test parameters.
• Virtual Instrumentation: Combine hardware and software for real-time monitoring and data correction.

In testing a 4MHz Thermage system, initial results showed a 5% power deviation and a 10° phase angle. KP2021 identified excessive leakage current, while VNA detected a 0.1μH parasitic inductance. After replacing with low-inductance resistors and optimizing the matching network, the phase angle dropped to 5°, and power accuracy reached ±2%, meeting standards.

The GB 9706.202-2021 standard highlights the limitations of traditional testing in high-frequency environments. The integrated use of KP2021 and VNA addresses challenges like skin effect and parasitic parameters, ensuring Thermage devices meet safety and efficacy standards. Future advancements, incorporating machine learning and virtual instrumentation, will further enhance testing capabilities for high-frequency medical devices.

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