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This is why you should use combiners on IEM transmitters
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<blockquote data-quote="Henry Cohen" data-source="post: 50491" data-attributes="member: 123"><p>Re: This is why you should use combiners on IEM transmitters</p><p></p><p></p><p></p><p>An RF isolator is a circulator with a load on port #3 (and port #4 if a dual design). A circulator is, as the name implies, a ferrite device that circulates the incoming RF to the adjacent port output. Thus any RF input at port #1 outputs to port #2; any RF input on port #2 outputs to port #3; input at #3 outputs to #1. Any reflected or spurious energy from the TX antenna headed back toward the transmitter will enter the isolator and be directed to the adjacent port where it will be dumped to a matched termination (50 ohm in most cases). <a href="http://www.emrcorp.com/documents/Technical%20Information/rf_isolators(1-14).pdf" target="_blank">Here's a very good paper on isolators</a>.</p><p></p><p>Two points to remember: 1) Not <em>all</em> the RF energy is directed to the next port; there is an insertion loss of usually less than a dB so 99.99% of the power is passed. 2) Some energy from the RF entering a port does go to the unintended adjacent port (1 to 3, 2 to 1, 3 to 2) and this number is given as the isolation spec, typically 18-30dB for a 3 port device and 40-70dB for a 4 port (dual) design.</p><p></p><p></p><p></p><p></p><p>A properly designed low power transmit combiner (and IEM/IFB gear in the milliwattage range is low power) has the effective result of transmitter isolation due to the forward amplifier stage and the subsequent port to port isolation of the Wilkinson combining circuit. Only in higher power systems (more than a few watts) is a discrete isolator stage required so as to protect the amplifiers.</p><p></p><p></p><p></p><p></p><p>First, remember that proper frequency coordination does not eliminate IM products in and of itself, it simply avoids selecting frequencies that conflict with those IM products.</p><p></p><p>Second, The SC-600 is not very good at IM suppression when compared with better designs such as the AC-3200 or Shure PA-821/4/8.</p><p></p><p>Lastly, I wouldn't be surprised if a lot of the IM products you're seeing with the RF Explorer are in fact being generated inside the Explorer's front end.</p></blockquote><p></p>
[QUOTE="Henry Cohen, post: 50491, member: 123"] Re: This is why you should use combiners on IEM transmitters An RF isolator is a circulator with a load on port #3 (and port #4 if a dual design). A circulator is, as the name implies, a ferrite device that circulates the incoming RF to the adjacent port output. Thus any RF input at port #1 outputs to port #2; any RF input on port #2 outputs to port #3; input at #3 outputs to #1. Any reflected or spurious energy from the TX antenna headed back toward the transmitter will enter the isolator and be directed to the adjacent port where it will be dumped to a matched termination (50 ohm in most cases). [URL="http://www.emrcorp.com/documents/Technical%20Information/rf_isolators(1-14).pdf"]Here's a very good paper on isolators[/URL]. Two points to remember: 1) Not [I]all[/I] the RF energy is directed to the next port; there is an insertion loss of usually less than a dB so 99.99% of the power is passed. 2) Some energy from the RF entering a port does go to the unintended adjacent port (1 to 3, 2 to 1, 3 to 2) and this number is given as the isolation spec, typically 18-30dB for a 3 port device and 40-70dB for a 4 port (dual) design. A properly designed low power transmit combiner (and IEM/IFB gear in the milliwattage range is low power) has the effective result of transmitter isolation due to the forward amplifier stage and the subsequent port to port isolation of the Wilkinson combining circuit. Only in higher power systems (more than a few watts) is a discrete isolator stage required so as to protect the amplifiers. First, remember that proper frequency coordination does not eliminate IM products in and of itself, it simply avoids selecting frequencies that conflict with those IM products. Second, The SC-600 is not very good at IM suppression when compared with better designs such as the AC-3200 or Shure PA-821/4/8. Lastly, I wouldn't be surprised if a lot of the IM products you're seeing with the RF Explorer are in fact being generated inside the Explorer's front end. [/QUOTE]
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