Comparison between two‐wire and three‐wire models for shielded twisted‐pair cables used in Johnson noise thermometry

Using typical values of cable parameters and typical connections used in Johnson noise thermometry, a comparison was made between a two‐wire and a three‐wire transmission line model of a 30‐m‐long, shielded, twisted‐pair cable over a range of frequencies from 0 to 300 kHz. For a 200‐Ω source impedance and a 1‐MΩ load impedance, the maximum difference between the voltage transfer functions calculated from the two models was 0.415%. For a 500‐Ω source impedance, the maximum difference was 0.607%. For a 200‐Ω source impedance and a 1‐Ω load impedance, the maximum difference between the current transfer functions calculated from the two models was 4.24%. For a 500‐Ω source impedance, the maximum difference was approximately 4.47%.