  Temporally modulated metamaterials have attracted significant attention
recently due to their non-reciprocal and frequency converting properties. Here,
a transparent, time-modulated metasurface that functions as a serrodyne
frequency translator, is reported at X-band frequencies. With a simple biasing
architecture, the metasurface provides electrically-tunable transmission phase
that covers $360^\circ$. A sawtooth waveform is used to modulate the
metasurface, allowing Doppler-like frequency translation. Two such metasurfaces
can be cascaded together to achieve magnetless devices that perform either
phase or amplitude non-reciprocity.
