This disclosure describes techniques for accurate estimation and de-embedding of the effects of pigtail probes in circuits. An adaptive pigtail model is developed and described that can accurately de-embed the effects of pigtail probes in digital circuits. Example parameters are identified that include pigtail length, tip length, pigtail tilt degree, ground distance, and solder amount. A pigtail simulation model is developed to model the soldered pigtail probe. The dependency of circuit impedance on the identified parameters is determined by experimentation which indicates that circuit performance can be boosted by short ground distance lengths, low tilt angles, thick support wires, short pin and cable lengths, and thick support wires. The pigtail model can be utilized to derive de-embedded results for different pigtail probe configurations without a need for explicitly measuring de-embedded results for those configurations

Chang, Victor

Guu, Ting-Yu

Hung, Yu-Kai

Wu, Hsin-Hung

English

Technical Disclosure Common

Technical Disclosure Commons Defensive Publications Series December 2020 Adaptive RF Pigtail Probe Modeling for De-embedding of RF Measurements Yu-Kai Hung Victor Chang Hsin-Hung Wu Ting-Yu Guu Follow this and additional works at: https://www.tdcommons.org/dpubs_series Recommended Citation Hung, Yu-Kai; Chang, Victor; Wu, Hsin-Hung; and Guu, Ting-Yu, "Adaptive RF Pigtail Probe Modeling for De-embedding of RF Measurements", Technical Disclosure Commons, (December 10, 2020) https://www.tdcommons.org/dpubs_series/3867 This work is licensed under a Creative Commons Attribution 4.0 License. This Article is brought to you for free and open access by Technical Disclosure Commons. It has been accepted for inclusion in Defensive Publications Series by an authorized administrator of Technical Disclosure Commons. Adaptive RF Pigtail Probe Modeling for De-embedding of RF Measurements ABSTRACT This disclosure describes techniques for accurate estimation and de-embedding of the effects of pigtail probes in circuits. An adaptive pigtail model is developed and described that can accurately de-embed the effects of pigtail probes in digital circuits. Example parameters are identified that include pigtail length, tip length, pigtail tilt degree, ground distance, and solder amount. A pigtail simulation model is developed to model the soldered pigtail probe. The dependency of circuit impedance on the identified parameters is determined by experimentation which indicates that circuit performance can be boosted by short ground distance lengths, low tilt angles, thick support wires, short pin and cable lengths, and thick support wires. The pigtail model can be utilized to derive de-embedded results for different pigtail probe configurations without a need for explicitly measuring de-embedded results for those configurations. KEYWORDS ● De-embedding ● High frequency circuit ● Smith chart ● Radio Frequency (RF) trace ● Trace Impedance ● RF probe ● Vector Network Analyzer (VNA) ● Coaxial connector ● SubMiniature version A (SMA) connector 2Hung et al.: Adaptive RF Pigtail Probe Modeling for De-embedding of RF MeasurePublished by Technical Disclosure Commons, 2020BACKGROUND Proper operation of high speed digital circuits within printed circuit boards (PCB) depends on accurate impedance matching. For efficient signal transfer, a target characteristic impedance is usually specified, e.g., 50 Ω. The target characteristic impedance is sought to be realized at key points in the circuit, such as component connections, transmission line junctions, terminators, etc. A prerequisite to matching and tuning a PCB is the determination of an impedance of the traces. Circuits that do not include a standard connector mounted on the board pose challenges for the measurement of radio frequency (RF) parameters. For such circuits, RF pigtail probes (semi-rigid cables) are commonly soldered onto the board to enable measurement of RF parameters. However, accurate measurement of trace impedance requires compensation (de-embedding) of the effects introduced by the RF pigtail probe. Port extension techniques can be utilized to de-embed the effects of pigtails, which entail shifting a measurement reference plane by the addition of an electrical delay in the circuit. A vector network analyzer (VNA) is typically utilized in performing the port extension techniques. However, traditional port extension methods by utilizing a VNA can only be performed for calibration of an open circuit version and neglects the effect of soldered pigtails on the PCB. The error from traditional port extension methods is particularly magnified in PCBs that include high frequency circuits. DESCRIPTION This disclosure describes techniques for the accurate estimation and de-embedding of the effects of pigtail probes in circuits. An adaptive pigtail model is developed and described that can accurately de-embed the effects of pigtail probes in digital circuits. Per techniques of this 3Defensive Publications Series, Art. 3867 [2020]https://www.tdcommons.org/dpubs_series/3867disclosure, a pigtail probe is modeled based on different parameters that characterize the pigtail probe attachment to a circuit. The model includes consideration of parameters that are excluded in conventional port extension methods. Fig. 1: Pigtail probe attached to a printed circuit board (PCB) Fig. 1 depicts an example pigtail probe attached to a circuit. As depicted in Fig. 1(a), a pigtail probe is soldered onto a PCB. A ground (gnd) and signal connections are labeled. Fig. 1(b) is a schematic representation of the pigtail probe connected to the PCB. The pigtail probe includes an SubMiniature version A (SMA) port, a tip, and a PVC extension, and is connected using a soldered joint to a signal pad. A ground pad on the PCB is also depicted. This setup is typically utilized during impedance matching of a PCB while optimizing its design for proper circuit function. 4Hung et al.: Adaptive RF Pigtail Probe Modeling for De-embedding of RF MeasurePublished by Technical Disclosure Commons, 2020Fig. 2: Pigtail probe components and parameters Fig. 2 depicts some example parameters utilized in characterization and modeling of the pigtail probe connection, per techniques of this disclosure. Example parameters that are considered include pigtail length, tip length, pigtail tilt degree, ground distance, and solder amount. Based on the identified parameters, a pigtail simulation model is developed that models the soldered pigtail probe.  Experiments are conducted based on different configurations of pigtail probes to validate the measurements against simulations using the model at different operating frequencies, and to determine optimal configurations. A determination of optimality is made based on comparison of a measured trace impedance with a predetermined target impedance, e.g., 50 Ω. Parameters of the pigtail probe connector are adjusted to identify dominant parameters. 5Defensive Publications Series, Art. 3867 [2020]https://www.tdcommons.org/dpubs_series/3867Fig. 3: Identification of dominant parameters for the pigtail model Fig. 3 depicts example configurations and experiments to determine the dependency of circuit impedance on the identified parameters. For example, various ground path distances between 1.5mm and 9.5 mm are studied before a determination is made that shorter ground distances are more optimal. Similarly, different pigtail tilt degrees between 20o and 50o before it is determined that a lower pigtail tilt degree provides optimal circuit performance. 6Hung et al.: Adaptive RF Pigtail Probe Modeling for De-embedding of RF MeasurePublished by Technical Disclosure Commons, 2020Fig. 4: Suggested configuration parameters for optimal circuit performance Fig. 4 depicts example parameters and a suggested setting for each parameter, based on studies performed per techniques of this disclosure. While the studies indicate that circuit performance is relatively unchanged for different settings for pin solder and PVC length, the studies indicate that circuit performance can be boosted by short ground distance lengths, low tilt angles, thick support wires, short pin and cable lengths, and thick support wires. Of these parameters, ground distance length, title angle, support wire thickness, and pin solder are typically not considered and/or modeled in traditional port extension techniques. 7Defensive Publications Series, Art. 3867 [2020]https://www.tdcommons.org/dpubs_series/3867Fig. 5: Comparison of simulated and measured S-parameters Fig. 5 depicts an example comparison of scattering parameters (S-parameters) for an embedded and de-embedded soldered SMA pigtail probe. Smith charts derived from the simulated and measured results are illustrated for corresponding de-embedded and embedded configurations. While the embedded configuration shows a larger variation between the measured and simulated results, both configurations yield a satisfactory match between the simulated and measured results. 8Hung et al.: Adaptive RF Pigtail Probe Modeling for De-embedding of RF MeasurePublished by Technical Disclosure Commons, 2020Fig. 6: The pigtail model can be used to de-embed measured results Fig. 6 depicts an example of use of the pigtail model to de-embed measurement results. Fig. 6(a) depicts an example pigtail probe configuration and a corresponding schematic representation for the pigtail mode. Impedance measurements are made without a pigtail port extension. The configuration parameters are used in conjunction with the pigtail model to 9Defensive Publications Series, Art. 3867 [2020]https://www.tdcommons.org/dpubs_series/3867determine de-embedded results based on simulations performed using the model. De-embedded results are also obtained via measurements.  Fig 6(b) depicts a comparison (Smith chart) of simulated and measured results that is indicative of agreement of the measured and simulated results, thereby validating use of the pigtail model to derive de-embedded results for different pigtail probe configurations without a need for explicitly measuring de-embedded results for those configurations. CONCLUSION This disclosure describes techniques for accurate estimation and de-embedding of the effects of pigtail probes in circuits. An adaptive pigtail model is developed and described that can accurately de-embed the effects of pigtail probes in digital circuits. Example parameters are identified that include pigtail length, tip length, pigtail tilt degree, ground distance, and solder amount. A pigtail simulation model is developed to model the soldered pigtail probe. The dependency of circuit impedance on the identified parameters is determined by experimentation which indicates that circuit performance can be boosted by short ground distance lengths, low tilt angles, thick support wires, short pin and cable lengths, and thick support wires. The pigtail model can be utilized to derive de-embedded results for different pigtail probe configurations without a need for explicitly measuring de-embedded results for those configurations. REFERENCES 1. RF Pigtail Test Probes, http://p1db.com/page/rf-pigtail-test-probes, accessed 24 Nov 2020. 2. RF-Probes: Plug Connector, Miniature Switch and PCB Contacting, http://www.cotelec.fr/media/ingun_gks_rfcatalogue_5_2_en__070516800_1736_16042018.pdf, accessed 24 Nov 2020. 10Hung et al.: Adaptive RF Pigtail Probe Modeling for De-embedding of RF MeasurePublished by Technical Disclosure Commons, 20203. PQ186 Network Metrology Training Kit, https://www.picotech.com/download/manuals/network-metrology-training-kit-users-guide.pdf, accessed 24 Nov 2020. 11Defensive Publications Series, Art. 3867 [2020]https://www.tdcommons.org/dpubs_series/3867

Adaptive RF Pigtail Probe Modeling for De-embedding of RF Measurements

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Adaptive RF Pigtail Probe Modeling for De-embedding of RF Measurements

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