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Graphene shows great potential applications in ultrahigh speed electronics due to its high carrier mobility and velocity. Nowadays, many radio frequency circuits based on graphene have been realized. For example, graphene frequency doubler is a promising option for signal generation at high frequencies. Graphene frequency doubler can achieve excellent spectral purity, because of its ambipolar transport and highly symmetric transfer characteristics. Here, we present high performance graphene frequency doublers based on millimeter-scale single-crystal graphene on HfO2 and Si substrates. We achieve a high spectral purity degree of larger than 94% without any filtering and the conversion gain is -23.4 dB at fin=1 GHz. The high conversion gain and spectral purity can be attributed to the high-quality millimeter-scale single-crystal graphene and high-quality high- substrates. Furthermore, we investigate the relation of conversion gain to source-drain voltage Vd and input signal power Pin. The results show that the conversion gain increases with source-drain voltage increasing, and the conversion gain also increases with input signal power increasing. The dependence of conversion gain on Vd and Pin can be attributed to the transconductance increasing with Vd and Pin. We compare the conversion gains and spectral purity degrees of graphene frequency doublers with different transconductances and electron-hole symmetries at different frequencies. The result shows that the conversion gain is larger for device with higher transconductance and the spectral purity has a moderate tolerance for the electron-hole symmetry of the graphene transistor at fin=1 GHz. As the working frequency increases to 4 GHz, the spectral purity of the device with weak electron-hole symmetry decreases dramatically, while the spectral purity of the device with better electron-hole symmetry is kept around 85%. We attribute this phenomenon to the different carrier transit times and different electron-hole symmetries of graphene transistors. In conclusion, the short channel graphene transistor with ultrathin gate dielectric and high electron-hole symmetry is needed in order to achieve high performance graphene frequency doubler.