how create custom field based on SPField

I want to create custom field base on SPField.

I write i my xml definition of field:

...

<Field Name="ParentType"></Field>

...

such xml definition used in FLDTYPES.xml for sharepoint base field types.

but when i create field with my field type, sharepoint throws error: One or many field types install incorrect (translate from russian

[511 byte] By [RailSabirov] at [2008-1-1]