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Author Topic: Avoiding intermod products based on anticipated strength?  (Read 3272 times)

Pete Erskine

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Re: Avoiding intermod products based on anticipated strength?
« Reply #10 on: March 01, 2018, 01:17:11 pm »

Another, more complicated, example, would be the case of an actor wearing a primary and secondary microphone. In addition to wanting other microphones to avoid the 2T5O products from this pair of transmitters (since they're guaranteed to be too close together without any kind of isolation), it would be great to avoid 3T3O products from this pair plus all of the rest of the microphones in the zone. Of course I could try to avoid all 2T5O and 3T3O, but the likelihood of getting 30+ usable frequencies post-repack is basically nil.
-Russ

IM due to adjacent TX and NOT through a combiner only is a local effect not extending beyond a few feet and probably not to the rx
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Russell Ault

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Re: Avoiding intermod products based on anticipated strength?
« Reply #11 on: March 01, 2018, 05:20:56 pm »

Again, thanks to everyone for their help!

In IAS and WWB all devices are treated the same in the generation of IMDs and all devices in the same zone generate 2T3O 2T5O and 3T3O between devices of the same profile and between devices of other profiles. The only option is with each devise you can decide what IMDs you want to avoid. This method works very well when coordinating devices of the same type i.e. analogue wireless microphones. There are some issues with this rationale when you have types of devices that have IMD generation characteristics that are vastly different.

[...]

When the devices are coordinated with other devices in the same category the software produces very reliable results But when you coordinate between categories you will have some issues. For example if you have 2T5O on for the calculation of your IEMs and BTR TX, when you calculate Axient Digital microphones with 2T5O off the will not avoid the 2T5O IMDs from the IEMs and BTR TX. Also to consider if you have 2T5O turned on when calculating other devices they will ovoid the 2T5O IMDs that the Digital microphones will most likely never generate.

This is exactly the problem I was wondering about (and seeking a solution for).

Features that would allow the device profiles to determine if a devices produce 2T5O, 3T3O... wold be nice but is not available at this time and may be to confusing for some users.

Thank you for confirming this; this is what I thought, but I wanted to be sure.

5. In a excel spreadsheet calculate all the IMD you want the other devices to avoid. Add these frequencies in a separate zone with a generic profile so they are treated as devices and not IMDs. (if anyone has a spreadsheet already made for this please share, if not I will share one when I get a chance)

That's basically the conclusion I was coming to as well; I would really appreciate a copy of that spreadsheet if you have a moment!

These are the methods that I use most, but I also create guard bands around likely IMD sources after assigning their freqs. In IAS it can be done by using the "edit active spectrum" function.

FWIW, this is why strict band planning can be so helpful.

That's something I'll remember to keep in mind as well. I (very fortunately) seem to have done this entirely by accident on my most recent coordination; next time I'll try to make it intentional.

IM due to adjacent TX and NOT through a combiner only is a local effect not extending beyond a few feet and probably not to the rx

This confuses me: if IM due to adjacent TX aren't a big deal, why do we bother putting our TX in metal pans (or combining our IEM antennas)?

Thanks!

-Russ
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Mac Kerr

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Re: Avoiding intermod products based on anticipated strength?
« Reply #12 on: March 01, 2018, 06:45:19 pm »

This confuses me: if IM due to adjacent TX aren't a big deal, why do we bother putting our TX in metal pans (or combining our IEM antennas)?

Reason 1, we are often talking about dozens of TX 3rd order and 5th order, all of which may be right next to our receive antennas.

Reason 2, the combination of all those IM products on top of one another can create some pretty high levels of RF interference.

If you have a spectrum analyzer take a look at what happens when you put a bunch of transmitters next to each other without pans, and then with pans. Then take a look at your double mic'd performer when they're out on stage. How does the RF scan look in each case?

Mac
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Henry Cohen

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Re: Avoiding intermod products based on anticipated strength?
« Reply #13 on: March 01, 2018, 06:52:03 pm »

This confuses me: if IM due to adjacent TX aren't a big deal, why do we bother putting our TX in metal pans (or combining our IEM antennas)?

Because two beltpacks separated by some fatty salt water will not produce the massive rise in the RF noise floor created by the thousands of IM products that a pile of transmitters will.
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Pete Erskine

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Re: Avoiding intermod products based on anticipated strength?
« Reply #14 on: March 01, 2018, 07:58:23 pm »

This confuses me: if IM due to adjacent TX aren't a big deal, why do we bother putting our TX in metal pans (or combining our IEM antennas)?


It is a big dela near the table full of wireless.  Try wearing a BTR and walk to an RF table with no pans and it crashes...   Better not to make dirty rf areas with the pans.
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Jason Glass

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Re: Avoiding intermod products based on anticipated strength?
« Reply #15 on: March 01, 2018, 08:10:47 pm »

IM due to adjacent TX and NOT through a combiner only is a local effect not extending beyond a few feet and probably not to the rx
To elaborate on what Pete is describing here, the IMD products generated by densely located TX are often only strong enough to make the noise floor intolerable to devices within a local radius. However, these products may be so weak as to be tolerable at a distance due to the inverse square nature of RF attenuation through free space. A great example is all IEMs in Monitor World taking hits from a pile of RF mics on the MON drive rack while they sound fine everywhere else. This is one of many reasons that mounting remote, directional RX antennas high above the performance space can be desirable. It helps to achieve an appropriate balance of gain vs. distance from TX sources. It's also a GREAT reason to locate the RF table well away from critical user positions.

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« Last Edit: March 01, 2018, 08:13:21 pm by Jason Glass »
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Russell Ault

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Re: Avoiding intermod products based on anticipated strength?
« Reply #16 on: March 01, 2018, 10:42:41 pm »

If you have a spectrum analyzer take a look at what happens when you put a bunch of transmitters next to each other without pans, and then with pans. Then take a look at your double mic'd performer when they're out on stage. How does the RF scan look in each case?

Well, that's certainly educational. I had done the pans vs. no pans test before, but I was operating under the assumption that "double-packed actor" would act about the same as "no pans". Of course, now that you (and everyone else) mention it, it makes sense that the 10-25 dB (or more) of absorption caused by a big bag of water (etc.) would have a huge impact on IMD production as well. Sure enough, pulling out my RF Explorer, I see that the 2T3O of my double-packed leads are at least 50 dB below the TXs fundamentals (I say "at least" because from where I'm sitting backstage I can't actually see the 2T3O above the noise floor).

Thanks everyone for answering my questions. One of things I love about this line of work is that there's always another layer, and always something more to learn.  :)

-Russ
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Paul McDermott

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Re: Avoiding intermod products based on anticipated strength?
« Reply #17 on: May 08, 2018, 09:17:50 am »


5. In a excel spreadsheet calculate all the IMD you want the other devices to avoid. Add these frequencies in a separate zone with a generic profile so they are treated as devices and not IMDs. (if anyone has a spreadsheet already made for this please share, if not I will share one when I get a chance)


Here is a link for an Excel Spreadsheet I made that calculates 2T30, 2T50, and 3T30 IMDís for up to 16 device frequencies. A little late but I hope this is useful to you.

http://rfcoordinationnyc.com/downloads.html
« Last Edit: August 09, 2019, 12:23:49 pm by Paul McDermott »
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Terry Martin

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Re: Avoiding intermod products based on anticipated strength?
« Reply #18 on: May 08, 2018, 10:40:30 am »

Here is a link for an Excel Spreadsheet I made that calculates 2T30, 2T50, and 3T30 IMDís for up to 16 device frequencies. A little late but I hope this is useful to you.

http://rfcoordinationnyc.com/download/imd_calc_16_v_1_03%20(1).xlsm
Link didnít work on my end. 


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Terry Martin

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Re: Avoiding intermod products based on anticipated strength?
« Reply #19 on: May 08, 2018, 10:49:43 am »

This one worked....

http://rfcoordinationnyc.com/download/imd_calc_16_v_1_03%20(1).xlsm

Link didnít work on my end. 


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Re: Avoiding intermod products based on anticipated strength?
¬ę Reply #19 on: May 08, 2018, 10:49:43 am ¬Ľ


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