Invitation letter of Duly authorized unit?

Hi, I want to apply for a F visa and I went to the Chinese embassy in Tokyo to ask some questions and they said that I needed an invitation letter of Duly authorized unit.

Does this mean that I need to get an invitation letter from the person in China that I will be studying with and take it to get approved by a 'Duly authorized unit'? woman at the embassy said it could take between 1 and 6 months to process but I think I might have misunderstood her as I couldn't hear her properly

If so do you know where in tokyo I can get this done? I will be staying in Beijing.

Also, do you know what the requirements are for the invitation letter are? eg can it be an email and does it need to be signed ect?

Thanks