ProSoundWeb Community

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: Andrew Broughton on February 14, 2018, 10:54:06 am

Title: When a Combiner goes bad...
Post by: Andrew Broughton on February 14, 2018, 10:54:06 am
1 P10T into PA821A

Good Combiner vs. Broken one...

(http://checkcheckonetwo.com/files/New%20Combiner%20-%20Ch%204.jpg)

(http://checkcheckonetwo.com/files/Old%20Combiner%20-%20Ch%204.jpg)
Title: Re: When a Combiner goes bad...
Post by: Scott Helmke on February 14, 2018, 11:48:05 am
Wow, haven't seen that one before.  When I've had a bad PA821A it'll be one or more ports just not getting a full signal through.
Title: Re: When a Combiner goes bad...
Post by: Andrew Broughton on February 14, 2018, 02:04:37 pm
I'm very curious what goes wrong inside them that can cause this kind of grief.
Title: Re: When a Combiner goes bad...
Post by: Henry Cohen on February 15, 2018, 09:49:28 am
I'm very curious what goes wrong inside them that can cause this kind of grief.

Presuming this is only one input channel, its amplifier stage is bad. probably overloaded at some point.
Title: Re: When a Combiner goes bad...
Post by: Andrew Broughton on February 15, 2018, 08:49:17 pm
Makes sense. Yes it was just one input.
It was tough to diagnose without a scanner.

Thanks, Henry.


Sent from my iPhone using Tapatalk Pro
Title: Re: When a Combiner goes bad...
Post by: David Pedd on February 15, 2018, 09:46:17 pm
Presuming this is only one input channel, its amplifier stage is bad. probably overloaded at some point.

Went to a Shure seminar yesterday.  Great info.

The Shure man demonstrated how you overload the receiver front end by using the paddle antenna when you don't compensate for the db overload.

I'm guessing the same type of overload could fry the combiner.

Title: Re: When a Combiner goes bad...
Post by: Lyle Williams on February 17, 2018, 07:20:53 am
An attenuator is one of the most useful RF debugging tools you can own.
Title: Re: When a Combiner goes bad...
Post by: Andrew Broughton on February 17, 2018, 11:03:47 am
An attenuator is one of the most useful RF debugging tools you can own.
I'll bite... Could you elaborate?
Title: Re: When a Combiner goes bad...
Post by: Henry Cohen on February 17, 2018, 07:36:22 pm
I'll bite... Could you elaborate?

As a quick diagnostic tool to see if any part of your signal chain is being overloaded (with saturation of the subsequent gain stage) and causing problems which can appear as co-channel interference.

They are also indispensable when gain balancing a distributed antenna system.
Title: Re: When a Combiner goes bad...
Post by: Lyle Williams on February 18, 2018, 07:09:01 am
If you add (say) 6dB of attenuation and your problems drop by more than 6dB, something is operating outside it's linear operating range.
Title: Re: When a Combiner goes bad...
Post by: Henry Cohen on February 18, 2018, 12:39:47 pm
If you add (say) 6dB of attenuation and your problems drop by more than 6dB, something is operating outside it's linear operating range.

Yes and no.

"No", in that when a gain stage (typically an amplifier) is no longer linear, it's past its 1dB compression point (P1). P1 is defined as that output level when 1dB additional input yields less than an additional dB output. Conversely, if past the P1 point, reducing the input by 1dB will result in an output level change of less than 1dB. P1 however refers only to the fundamental carrier, not to the spurs, harmonics or IM products, in which case . . .

"Yes", in that the spurs, harmonics or IM products of a non-linear stage will rise and fall at a greater rate than the input's added/reduced level. Third order intercept point (IP3) is an important spec for this phenomena; it's that point at which the third order IM product(s) are equal in level to that of the desired carrier.


edited for clarity
Title: Re: When a Combiner goes bad...
Post by: Lyle Williams on February 18, 2018, 03:13:32 pm
I agree Henry, 100%.

I was just trying to oversimplify it.

Sometimes we don't need a $2000 scope; a $10 attenuator will give the clues we need. 

The tech needs to think "If it sounds better with attenuation (better with less signal) what could be going on?"
Title: Re: When a Combiner goes bad...
Post by: Paulo Rodrigues on February 18, 2018, 04:18:26 pm
As a quick diagnostic tool to see if any part of your signal chain is being overloaded (with saturation of the subsequent gain stage) and causing problems which can appear as co-channel interference.

They are also indispensable when gain balancing a distributed antenna system.
Henry can you please explain gain balancing an antenna system
Title: Re: When a Combiner goes bad...
Post by: Andrew Broughton on February 18, 2018, 05:39:31 pm
Very interesting! Thank you.
Title: Re: When a Combiner goes bad...
Post by: Scott Holtzman on February 18, 2018, 07:08:45 pm
Henry can you please explain gain balancing an antenna system

Radio techs do this all the time without sophisticated equipment.  Often a transmitter will be made up of an exciter that makes a few watts, an intermediate PA and then the final amplifier.

Gain staging these hybrid tube/transistor beasts was not as hard as it may seem.  You would hook your wattmeter (almost always a Bird) between the stages and peak the matching circuit for max output.  This guaranteed a good impedance match between the stages.

Then you backed off the power control until you saw a loss of power on the next stage.  This was important, you only wanted to drive each stage as much as was needed to provided full output.  FM transmitters produce full power all the time as the carrier is deviated so headroom was not an issue.  If you drove a stage too hard the output would get "nasty" without a significant rise in output power.  You would, if you paid attention see the input current rise as the amplifier was producing a broader signal.

Later, when service monitors had spectrum analyzers as standard equipment you would see the noise floor rise as input power was increased.

This is the exact same principal.  With an RX multicoupler you only want to makeup losses in the distribution network.  You can't suck signal out of a line that doesn't exist.  You can however bring up the noise floor and compromise S/N ratio.

There is a serious analog here to AF gain staging.  Concepts are similar and the goal is the same.

Lastly, staging a combiner is also a similar exercise.  Increase transmitter output only to compensate for the loss of the combining network.

 
Title: Re: When a Combiner goes bad...
Post by: Henry Cohen on February 18, 2018, 07:21:15 pm
Henry can you please explain gain balancing an antenna system

In a distributed antenna system (DAS), for either RX or TX, it's important to keep the overlapping coverage areas to a minimum. Attenuators at each branch can balance the system, much the same way one balances gain levels of a multi-zone PA system.