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This paper presents a study on the effect of impedance mismatch on phase linearity (group delay variations) in grounded coplanar waveguide (GCPW) structures. Two 400 ps GCPW delay lines were designed using a short circuited stub and a trans- mission line. The structures were simulated over a wide frequency range (0.1 GHz- 5 GHz) using both ADS circuit model and CST electromagnetic simulation tool. Based on mathematical analysis and simulation results, impedance mismatch appears to have a large e_ect on group delay variations in stubs when compared to transmission lines. The simulated time delay of the short circuited stub shows a maximum delay deviation of ±0.75% and ±7.4% for 1.6% and 5.8% impedance mismatch values, respectively. On the other hand, the transmission delay line simulation results show only ±0.1% and ±1.5% for the same impedance mismatch. For the electromagnetic simulation, the presented results indicate even larger variation of time delay for GCPW short stub as it reaches ±3.75% and ±7.5% at 2 GHz and 4.5 GHz for 1.6% impedance mismatch, respectively