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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: Keith Broughton on November 27, 2019, 07:11:50 am

Title: use of 50 ohm terminators
Post by: Keith Broughton on November 27, 2019, 07:11:50 am
It has come to my attention that is good practice to terminate unused outputs of antenna distribution devices.
Would someone explain the technical reason for this?
Thanks :)
Title: Re: use of 50 ohm terminators
Post by: Steve M Smith on November 27, 2019, 08:31:49 am
They are to attenuate reflections going back down the cable which could be seen as signal.


Steve.
Title: Re: use of 50 ohm terminators
Post by: Keith Broughton on December 01, 2019, 07:34:58 am
I installed the terminators but want to follow up on the reason.
What "cable" do you refer to when it's an active splitter device?
I can see the impedance issue if using a passive divider but am less clear on the benefit in a active system.
Title: Re: use of 50 ohm terminators
Post by: Henry Cohen on December 01, 2019, 01:07:09 pm
I installed the terminators but want to follow up on the reason.
What "cable" do you refer to when it's an active splitter device?
I can see the impedance issue if using a passive divider but am less clear on the benefit in a active system.

An active multicoupler ("antenna distributor", "active splitter", etc.) is merely a passive splitter network with a low noise pre-amp feeding the splitter's input (though better designs will also incorporate bandpass filtering).

The passive splitter network itself is designed around all ports being terminated by the appropriate impedance load (50 ohms in the case of RF). When a port has improper termination, or no termination, energy is reflected back into the network to both the summing (common) port as well to the other output ports. An unterminated port causes a change in isolation between ports by changing the overall internal impedance of the network.

Reflecting energy back into the summing port will back feed into the output of the LNA possibly causing IM mixing, saturation, and occasionally even damage in cheaper units.

That said, the better wireless microphone multicouplers are rather forgiving about unterminated ports, and that alone will not cause any significant detrimental effects. But coupled with other problems the wireless system might be experiencing, unterminated ports could exacerbate problems. Given how cheap this proper RF practice is, it's not worth ignoring.

Mini-Circuits has a rather good application note (https://www.minicircuits.com/appdoc/AN10-006.html) on passive splitter networks.
Title: Re: use of 50 ohm terminators
Post by: Brian Jojade on December 01, 2019, 01:11:19 pm
RF stuff is designed to have loads on it.  Without the loads, the results can become unpredictable.  While not always necessary, it's good practice to prevent problems.
Title: Re: use of 50 ohm terminators
Post by: Keith Broughton on December 01, 2019, 02:04:42 pm
An active multicoupler ("antenna distributor", "active splitter", etc.) is merely a passive splitter network with a low noise pre-amp feeding the splitter's input (though better designs will also incorporate bandpass filtering).


Now it makes sense!
Thanks Henry. :)
Title: Re: use of 50 ohm terminators
Post by: Keith Broughton on December 08, 2019, 06:21:26 am
So I installed the terminators on the unused outputs of a Sennheiser ASA 214 and found they got very hot!
I presume this is due to the DC voltage on the connector used to power the receivers.
Everything still works OK but I have concerns the terminators may be loading the DC power supply too much.
Thoughts?
Title: Re: use of 50 ohm terminators
Post by: Pete Erskine on December 08, 2019, 11:36:28 am
So I installed the terminators on the unused outputs of a Sennheiser ASA 214 and found they got very hot!
I presume this is due to the DC voltage on the connector used to power the receivers.
Everything still works OK but I have concerns the terminators may be loading the DC power supply too much.
Thoughts?

use DC blocks before terms... or they will burn out.
Title: Re: use of 50 ohm terminators
Post by: Keith Broughton on December 08, 2019, 06:09:08 pm
use DC blocks before terms... or they will burn out.
I suspected that would be the solution but just wanted to check.
Thanks.
Title: Re: use of 50 ohm terminators
Post by: Steve-White on January 26, 2020, 01:05:41 pm
An active multicoupler ("antenna distributor", "active splitter", etc.) is merely a passive splitter network with a low noise pre-amp feeding the splitter's input (though better designs will also incorporate bandpass filtering).

The passive splitter network itself is designed around all ports being terminated by the appropriate impedance load (50 ohms in the case of RF). When a port has improper termination, or no termination, energy is reflected back into the network to both the summing (common) port as well to the other output ports. An unterminated port causes a change in isolation between ports by changing the overall internal impedance of the network.

Reflecting energy back into the summing port will back feed into the output of the LNA possibly causing IM mixing, saturation, and occasionally even damage in cheaper units.

That said, the better wireless microphone multicouplers are rather forgiving about unterminated ports, and that alone will not cause any significant detrimental effects. But coupled with other problems the wireless system might be experiencing, unterminated ports could exacerbate problems. Given how cheap this proper RF practice is, it's not worth ignoring.

Mini-Circuits has a rather good application note (https://www.minicircuits.com/appdoc/AN10-006.html) on passive splitter networks.

THIS +1

Simply put it properly loads the circuit to keep it stable.  Similar to the harmonic balancer (damper) on the opposite end of a crankshaft from the flywheel in an engine.  It deadens the vibrations in the crankshaft.  Electrical "vibrations" or resonance and reflected energy could lead to noise and overall system instability.

Same holds true for long DMX runs.  Terminating the end into a dummy load adds stability to the circuit.
Title: Re: use of 50 ohm terminators
Post by: Keith Broughton on January 26, 2020, 01:16:51 pm
The 50 ohm terminators were fairly inexpensive but now , if I have to add DC blockers, the price seems to jump up considerably!
Title: Re: use of 50 ohm terminators
Post by: Steve-White on January 26, 2020, 01:28:45 pm
use DC blocks before terms... or they will burn out.

I probably would have never considered that.  The RF riding on a DC component.  Common to all?  Or some do and some don't?
Title: Re: use of 50 ohm terminators
Post by: Russell Ault on January 26, 2020, 03:44:18 pm
I probably would have never considered that.  The RF riding on a DC component.  Common to all?  Or some do and some don't?

It's mostly a Sennheiser thing. Sennheiser's EW G3 and G4 RXs can be powered by a 12V DC bias voltage supplied on their antenna inputs (similar to how active antennas are powered, but in reverse) and a couple of their antenna distros (specifically the ASA 1 and the ASA 214) supply this voltage on the outputs (just the A outputs I believe). Most antenna distros don't do this (including other Sennheiser ones) and so shouldn't require DC blocking.

-Russ
Title: Re: use of 50 ohm terminators
Post by: Steve-White on January 26, 2020, 05:29:03 pm
^^^ Good to know.