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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => Wireless and Communications => Topic started by: Michael T Casey on October 31, 2019, 12:24:05 pm

Title: PSA: Daisy-chaining active combiners (hint: don't do it)
Post by: Michael T Casey on October 31, 2019, 12:24:05 pm
Did a site visit the other day to troubleshoot IEM dropouts.  Won't name any brand names, but it was all decent gear just deployed in an unfortunate configuration. 

12 IEM transmitters, 30mW each,  into three 4:1 active combiners with nominal max input of +20dBm/100mW.  Those three combiners daisy-chained into a fourth 4:1 combiner, same model.  All frequencies were coordinated in a hypothetically clear spectrum with default WWB intermod spacing.

I've attached a screenshot to show how detrimental it was to the spectrum.  Blue markers are the transmitter freqs, everything else is intermod created by the fourth combiner!  Wish I would have saved the next scan but when we removed the second stage combiner and deployed three antennas the difference was night and day.

Title: Re: PSA: Daisy-chaining active combiners (hint: don't do it)
Post by: Scott Holtzman on October 31, 2019, 12:58:21 pm
Did a site visit the other day to troubleshoot IEM dropouts.  Won't name any brand names, but it was all decent gear just deployed in an unfortunate configuration. 

12 IEM transmitters, 30mW each,  into three 4:1 active combiners with nominal max input of +20dBm/100mW.  Those three combiners daisy-chained into a fourth 4:1 combiner, same model.  All frequencies were coordinated in a hypothetically clear spectrum with default WWB intermod spacing.

I've attached a screenshot to show how detrimental it was to the spectrum.  Blue markers are the transmitter freqs, everything else is intermod created by the fourth combiner!  Wish I would have saved the next scan but when we removed the second stage combiner and deployed three antennas the difference was night and day.

Circulators on the inputs to combiner 4 probably would have cleared that up.
Title: Re: PSA: Daisy-chaining active combiners (hint: don't do it)
Post by: Don Boomer on October 31, 2019, 01:12:32 pm
My experience agrees 100% with you. The output of a combiner is an awfully complex signal and can easily overwhelm the inputs of a summing combiner. However another factor is how tightly you can cluster your transmitter frequencies. The tighter the better under normal circumstances.

As we manufacture combiner units I get asked all the time how to tie them together. In many ways making things work is a matter of what you can get away with.So a better way if you absolutely must tie three combiners together would be with a passive combiner. However thatís still not very good.  We generally recommend not to go this way however.

At lot of users are successful tying a pair of combiners with 2x1 passive splitter and then using two antennas to get to 12.

But when the lowest IMD is necessary one antenna to each combiner is your best bet. 

Title: Re: PSA: Daisy-chaining active combiners (hint: don't do it)
Post by: Henry Cohen on October 31, 2019, 01:55:26 pm
Circulators on the inputs to combiner 4 probably would have cleared that up.

Nope. The problem is not the back feeding of other carriers into the prior combiner stage, the problems are:
1) Saturation of the final combiner by overloading the input stages (which produces all sorts of harmonic signals and unpredictable spurious emissions);
2) Amplifying multiple carriers produces IM products.

And on a pedantic note, it's isolators that are used on the outputs of transmitters and combiners, which is a circulator with port 3 [and port 4 if a dual stage design] terminated.
Title: Re: PSA: Daisy-chaining active combiners (hint: don't do it)
Post by: Jordan Wolf on November 01, 2019, 01:57:03 pm
Wow, what a visual! And what a low noise floor, lol.

I am glad to know a little bit better than to combine active outputs with another active device, but Iím still learning.  :D (Primarily thanks to this forum)