Javascript - $(document).ready(function() Does Not Work Perfectly In Content Pages / Telerik Controls

The pages on my project are base on master and content pages...

I want to do something with javascript(rather than jquery) in one of content pages after ALL OF MASTER AND CONTENT ELEMENTS ARE LOADED COMPLETELY.(for example set focus on a RadComboBox Control)

For doing that I used the below code :

<asp:Content ID="Content1" ContentPlaceHolderID="head" runat="server">
<script src="../JQuery/jquery-1.4.1.js" language="javascript" type="text/javascript"></script>
<script type="text/javascript"> [code]...

But alert(combo); always returns null.(the $find code is for telerik controls and the upper codes about telerik controls are completely true)To solve this null problem I test the ways shown below:

1-I Removed all of controls from master and content page except RadComboBox Control and null problem disappeared , so i derived the null peoblem is about all of elements of master and content page have not been loaded when

$find("<%= RadcbPersonelCompleteNameInvwNoskhehEdit.ClientID %>"); is fired.

2-so i used $(document).ready(function() { my codes }); instead of onload = onloadOfDocument;

3-at last i test the below code and it works perfectly :
<asp:Content ID="Content1" ContentPlaceHolderID="head" runat="server">
<script src="../JQuery/jquery-1.4.1.js" language="javascript" type="text/javascript"></script> [code]...

What function of document should i use for doing some javascript codes after all Of MASTER AND CONTENT ELEMENTS are loaded completely?