In Oracle 10g or higher, the AWR report can be generated using selected time interval. The report can be requested using Main menu Report | Performance | AWR Report. If this menu item is not enabled, then make sure that time interval is selected and the focus is on the screen that has time selection. The AWR report can also be requested using most popup menus, in the Reports submenu. The report can be generated using two formats: text and HTML.
Some details how AWR report is generated. The work is done by dbms_workload_repository Oracle package, awr_report_html() function. This function needs snapshot IDs supplied as parameters. Lab128 chooses snapshots based on selected time interval picking closest snapshot before and after time boundaries. The discrepancy between user's requested interval and actual one in AWR is reported in the header. The default time resolution of AWR is 1 hour therefore the discrepancy may be substantial. The frequency of snapshots can be changed, but typically it is not set higher than 1 snapshot per 15 minutes. As alternative, you can use Statspack-style report generated by Lab128 natively from its own repository with high time resolution (1snapshot every 6-12 seconds).