how to get control out of a frame in WPF xbap

Question

For example: if I want to get txtControl and set its Text property in the code behind of subPage.xaml.

<Page x:Class="Prototype.MainPage"
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
      xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
   Title="MainPage">
    <Grid VerticalAlignment="Top">
        <TextBlock x:name = "txtControl"/>
        <Frame Source="subPage.xaml" Width="900" ></Frame>
    </Grid>
</Page>

Thanks,