Lectrosonics Wireless Designer question

[Matthew Steel]

I have encountered a situation in my RF coordination that seems just at the edge of my knowledge and I'm hoping some of the Lectrosonics RF gurus that frequent the forum can chime in on whether I'm understanding the situation correctly.  And whether I should be concerned.  I'll try to be succinct but this might be a bit long, sorry.

 

What sparked the question is this:  I use Shure's Wireless Workbench software to calculate coordinated frequency sets for the various wireless systems used on our campus.  A portion of that equipment is Lectrosonics, and I can connect to that equipment for monitoring with Lectrosonics' Wireless Designer software.  I ran into a situation where the Lectrosonics software is complaining about frequencies that the Shure software says is fine.

 

The specific complaint Lectrosonics Wireless Designer has is: "The System at 475.300 MHz is subject to interference from 3rd Order IMD at 475.175 MHz caused by the mixing of carriers from transmitters at 532.700 and 590.225 MHz. The offending carriers do not all fall within the passband of the affected receiver."  On the other hand, the Shure software is not complaining.  Both programs are configured to flag 2Tx 3rd Order intermod products that fall within 149kHz of carriers - and I can do the math myself to see that this potential intermod product fails to meet the spec.  So I am wondering why Shure Wireless Workbench would not flag it.

 

The only reason I can come up with why the Shure Wireless Workbench software might not be flagging this as a problem is that it has a "Filter Selectivity" parameter (which I have left set to its default of +-50MHz outside the tuning range of the receiver).  It seems that the Shure software is predicting that the 532.700 and 590.225 carriers will be filtered out by the receiver front end before they can intermodulate and create a problem.  This seems to be the basic idea of the Lectrosonics software's message about the carriers not falling within the passband of the receiver.

 

But that's not the only way this intermod product can be a problem, is it?  For example couldn't it be created by transmitters in close proximity to each other or by strong signals in a multicoupler upstream of the receiver?  Am I being paranoid or should I legitimately distrust the Shure software because of this?

[LarryF]

Hi Matthew,

From what you posted, you've got a good grasp on what the two different programs are doing. The Lectrosonics' program is very conservative. As it said, the carriers are outside the passband of the receiver and probably won't cause much intermod at the receiver, particularly if the talent is more than a few feet away from the receiver antennas. The receiver front end filters will reduce the out of band carriers while leaving the desired signal at full strength. The "offending" transmitters output stages, however, don't have that filtering and if very close to one another (a foot or so) may generate an intermod product while they are physically close together.  If the transmitters are always farther than that apart (not in a bag) or the talent transmitters are not very near each other, the intermod will not be a problem. If you have weak amplifiers in the multicouplers and the talent gets close to the antennas simultaneously (less than 10 feet), you might have a problem. With robust amps in the distribution system (Lectros) it probably isn't a problem.

 

So the Lectro program is telling you, don't worry about receiver intermod but keep an eye on the transmitters being physically together (in a bag) or close to antennas tied to distribution amps.

 

The Shure program is saying, Lectro is a bunch of worry warts, get on with the show.

Best Regards,

Larry Fisher

[Matthew Steel]

Thanks very much @LarryF for your comments.  I will try not to be a worry wart.

 

On the receiver side it (almost) impossible for talent to get within 10 feet of any of our antennas, and anyway the multicouplers are all Lectrosonics VRMWB either in a short chain or fed from a UMC16b.

 

On the transmitter side it sounds like we should be fine under almost all circumstances.  We are not operating out of a bag.  In the theater-like scenarios where we see our largest channel counts it does seem like there is a chance talent could get that close.  I will have to keep that in mind then.

 

Thanks again for sharing your knowledge.  Most of the information I have found on this topic gets to the point of saying "you need to use coordination software" but to get past that point it seems to be a lot harder to find specific recommendations about what settings are most important and how to most reliably get the channel counts higher.

[LarryF]

Hi Matthew,

One other factor that comes into play is that the intermod products are always at lower levels than the carriers of the two off frequency offending transmitters. This means that the desired third carrier is almost guaranteed to be much stronger at the receiver than the intermod from the two offending carriers. Remember it takes at least three transmitters to create a problem. You must have two offending transmitters interfering with a third desired transmitter signal. If all three transmitters are at roughly the same distances from the receiver, the desired signal will overcome the relatively weak generated intermod. When intermod does become a concern, is when the offending transmitters are close to the receiver (or each other) and the desired transmitter is at a long distance or the desired signal is momentarily weak due to a drop out, i.e.,  multipath (null).

 

As I said, the Lectro program is a worry wart.

Best Regards,

Larry Fisher