Equivalent circuit models can be found in the P7708, P7713, P7716, P7720 Users manual.  You can download manuals from Tek.com by going to the "Downloads" icon on the right side of the web page.  Select the download type as "manuals", enter in your instrument model number and then press "search".  At the search results use the filter on the left to filter for the manual type you are looking for.