The first 8 peaks are 8 100mW Sennheiser SR2050 channels through a AC3200 combiner and CP5000 antenna.
The last 2 big peaks are 2 channels of SR2050 at 50mW with rearmounted whip antennas. Notice the huge 2 TX 3 and 5 IM products from these.
This is why you should use combiners on IEM transmitters
- Thread starter Jens Bacher
- Start date
The first 8 peaks are 8 100mW Sennheiser SR2050 channels through a AC3200 combiner and CP5000 antenna.
The last 2 big peaks are 2 channels of SR2050 at 50mW with rearmounted whip antennas. Notice the huge 2 TX 3 and 5 IM products from these.
Re: This is why you should use combiners on IEM transmitters
Where was your measurement antenna placed?
Where was your measurement antenna placed?
Re: This is why you should use combiners on IEM transmitters
You would have been able to decrease the IM products by moving the two lone transmitters farther apart or using isolators between the antenna ports and the whip antennas.
However, not knowing the placement of all the antennas including the antenna for whatever scanner or spectrum analyzer produced this screen shot, under the right conditions, the IM products shown could have been produced in the scanner/"spectrum analyzer" front end.
You would have been able to decrease the IM products by moving the two lone transmitters farther apart or using isolators between the antenna ports and the whip antennas.
However, not knowing the placement of all the antennas including the antenna for whatever scanner or spectrum analyzer produced this screen shot, under the right conditions, the IM products shown could have been produced in the scanner/"spectrum analyzer" front end.
Re: This is why you should use combiners on IEM transmitters
I did put isolators on the last transmitters and saw the IM disapperar, so it was happening in the transmitter, not the SA. This was only to show why there really is a reason everybody recommend using those expensive combiners.
Re: This is why you should use combiners on IEM transmitters
henry, what's an isolator? in context my guess is some kind of filter for out of range RF? but i've not heard of this thing.
and given that it seems to help in situations with multiple antennas, does it make sense to use when using an antenna combiner? or does the combiner do this as part of it's functionality?
last question. sorry. as you may remember from previous posts, i'm using a telex SC-600 as an IEM antenna combiner. it seems to work well, but i put my new RF Explorer on the output [with -30dB RF pad] with all 6 of my sennheiser SR 350 G2 transmitters set to 100mW. i could get a screenshot to post, but suffice to say that even though i did the freq coordination with Wireless Workbench i had all kinds of IMs in there. they were low enough to not be an issue, but they certainly didn't look as clean as the OP's screenshot. any thoughts as to why this might be?
henry, what's an isolator? in context my guess is some kind of filter for out of range RF? but i've not heard of this thing.
and given that it seems to help in situations with multiple antennas, does it make sense to use when using an antenna combiner? or does the combiner do this as part of it's functionality?
last question. sorry. as you may remember from previous posts, i'm using a telex SC-600 as an IEM antenna combiner. it seems to work well, but i put my new RF Explorer on the output [with -30dB RF pad] with all 6 of my sennheiser SR 350 G2 transmitters set to 100mW. i could get a screenshot to post, but suffice to say that even though i did the freq coordination with Wireless Workbench i had all kinds of IMs in there. they were low enough to not be an issue, but they certainly didn't look as clean as the OP's screenshot. any thoughts as to why this might be?
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). Here's a very good paper on isolators.
Two points to remember: 1) Not all 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.
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). Here's a very good paper on isolators.
Two points to remember: 1) Not all 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.
Re: This is why you should use combiners on IEM transmitters
thank you henry. as usual you've answered all my questions and made me realize how little i really know without making me feel like an idiot. thanks for that.
since i got the SC600 for 100 bucks, i can live with it being not quite as good as it's more expensive counterparts. it's getting the job done, so i'm happy for now.
i didn't know that the front end of the analyzer could contribute to the IM issues shown. that's interesting. i didn't expect the RF Explorer to be anything more than it is, and it is serving me well. but i'll know not to put too much faith in what it's telling me, even if i'm running signal into it direct like i was doing....
i guess it's comparable to all the effort we put into SMAART analysis. if you don't know how to interpret what you measure or how much the tools themselves affect the measurements, the measurements aren't of much use.
thank you henry. as usual you've answered all my questions and made me realize how little i really know without making me feel like an idiot. thanks for that.
since i got the SC600 for 100 bucks, i can live with it being not quite as good as it's more expensive counterparts. it's getting the job done, so i'm happy for now.
i didn't know that the front end of the analyzer could contribute to the IM issues shown. that's interesting. i didn't expect the RF Explorer to be anything more than it is, and it is serving me well. but i'll know not to put too much faith in what it's telling me, even if i'm running signal into it direct like i was doing....
i guess it's comparable to all the effort we put into SMAART analysis. if you don't know how to interpret what you measure or how much the tools themselves affect the measurements, the measurements aren't of much use.