The　circuit　topologies　of　efficient　single-phase　grid-connected　photovoltaic　systems　are　introduced.　The　performance　of　the　reference　sinusoid　circuit,　which　should　have　the　same　frequency　and　phase　angle　as　those　of　grid　voltage,　has　great　influence　on　the　proper　operation　and　output　current　quality　of　system.　A　reference　sinusoid　circuit　of　software　phase　lock　technique　based　on　optimal　time　is　proposed,　which　consists　of　sampling　circuit,　signal　conversion　circuit,　software　phase　lock　loop　and　D/A　conversion　circuit.　The　parameter　design　principle　of　these　circuits　and　their　test　results　are　given,　which　show　that,　its　output　sinusoidal　voltage　is　synchronous　with　grid　voltage　with　low　THD　(Total　Harmonic　Distortion).　It　is　simple　and　practical　with　low　price.