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Author Topic: Avoiding intermod products based on anticipated strength?  (Read 3285 times)

Russell Ault

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Avoiding intermod products based on anticipated strength?
« on: February 28, 2018, 05:22:18 pm »

This may seem like a strange question, but I'm wondering if there's a way to have WWB (or IAS, for that matter) factor the anticipated strength of intermodulation-produced signals into its suggestions?

For example, let's say I have a coordination that includes an unmodified BTR-800 base station running at high power (I know, I know) and two press-to-transmit beltpacks. I go to add a wireless microphone, and I ask WWB to avoid 2T3O and 2T5O intermodulation products. When WWB does the calculations, it will try to avoid all of the possible 2T3O and 2T5O products, despite the fact that the 2T5O products coming from the BTR-800 base station TX will likely be quite a bit stronger than any other paring's 2T3O. In reality, I can probably get away with avoiding the 2T5O coming only from the base station, and simply avoiding 2T3O is likely fine for the rest, so avoiding all possible 2T5O is spectrum inefficient.

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.

Is there some way to do this with existing software?

Thanks!

-Russ
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Mac Kerr

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Re: Avoiding intermod products based on anticipated strength?
« Reply #1 on: February 28, 2018, 05:37:27 pm »

This may seem like a strange question, but I'm wondering if there's a way to have WWB (or IAS, for that matter) factor the anticipated strength of intermodulation-produced signals into its suggestions?

For example, let's say I have a coordination that includes an unmodified BTR-800 base station running at high power (I know, I know) and two press-to-transmit beltpacks. I go to add a wireless microphone, and I ask WWB to avoid 2T3O and 2T5O intermodulation products. When WWB does the calculations, it will try to avoid all of the possible 2T3O and 2T5O products, despite the fact that the 2T5O products coming from the BTR-800 base station TX will likely be quite a bit stronger than any other paring's 2T3O. In reality, I can probably get away with avoiding the 2T5O coming only from the base station, and simply avoiding 2T3O is likely fine for the rest, so avoiding all possible 2T5O is spectrum inefficient.

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.

Is there some way to do this with existing software?

Thanks!

-Russ

In IAS it is easy to set turn on and off checking for any of the IM products. Turn off what you can get away with and do the bulk of your co-ord, then turn it back on and check against the mics you added to your list previously. The transmit from the BTR base should be kept as far from your mics as possible. An issue with this method will be that the BTR is you least frequency agile device, and you are adding to an already full spectrum. It is easy to do the BTR first and add your mics, but there is not much point in gaming the 5th order harmonics on the first device you add, there won't be many because there is only the BTR to check against.

Mac
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John Sulek

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Re: Avoiding intermod products based on anticipated strength?
« Reply #2 on: February 28, 2018, 05:50:24 pm »

This may seem like a strange question, but I'm wondering if there's a way to have WWB (or IAS, for that matter) factor the anticipated strength of intermodulation-produced signals into its suggestions?

For example, let's say I have a coordination that includes an unmodified BTR-800 base station running at high power (I know, I know) and two press-to-transmit beltpacks. I go to add a wireless microphone, and I ask WWB to avoid 2T3O and 2T5O intermodulation products. When WWB does the calculations, it will try to avoid all of the possible 2T3O and 2T5O products, despite the fact that the 2T5O products coming from the BTR-800 base station TX will likely be quite a bit stronger than any other paring's 2T3O. In reality, I can probably get away with avoiding the 2T5O coming only from the base station, and simply avoiding 2T3O is likely fine for the rest, so avoiding all possible 2T5O is spectrum inefficient.

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.

Is there some way to do this with existing software?

Thanks!

-Russ
Not that I'm aware of, but there are many folks smarter than me here. You can always change the parameters as you work through your coordination.
You probably already know all this but here goes...


In the options tab of IAS, you can turn on/off the 2T30, 3T3O, 2T5O avoidance and also tell it to ignore 3 Tx intermods above a certain spacing which is adjustable.
You can change these options depending on what you feel will work in your situation.
I change these settings depending on which devices I'm coordinating with other devices.

I'm not as familiar with WWB, but there is a "compatability" tab beside the coordination workspace where you can change the intermod options for each device header (ie, all the psm1000's or all the BTR800)

Mac beat me to it.
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Scott Helmke

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Re: Avoiding intermod products based on anticipated strength?
« Reply #3 on: February 28, 2018, 06:25:45 pm »

I'm not sure how relevant this might be to your situation, but both IAS and WWB allow splitting things up into zones, with little or no intermod calculation between zones. I've used this feature sometimes on events like football games where all the gear is spaced out over a large area and I can make reasonable assumptions about what might get close to what.
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Henry Cohen

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Re: Avoiding intermod products based on anticipated strength?
« Reply #4 on: February 28, 2018, 07:56:55 pm »

WWB does provide means to turn off the individual orders and number of TX's for each group of equipment. In the Frequency Coordination tab, populate the coordination workspace from inventory or a prior file. To the right of the workspace, select the tab "Compatibility" and turn xTyO on/off as desired/needed. You can break out individual devices by creating a new equipment profile and bring that into the inventory and then the workspace.

As for WWB, or any other IM program, attempting to calculate the signal strengths of the IM product(s), the answer is no. The software would have to have fields to input the signal strength values for every carrier, and that would add a tremendous amount of calculations and time. And what do you do about moving transmitters, where the signal level can vary by 10's of dB as the talent moves around?  If a carrier is within the RF space of concern, then any IM product, for which the carrier is a constituent, should be considered a potential interferer.
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Russell Ault

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Re: Avoiding intermod products based on anticipated strength?
« Reply #5 on: February 28, 2018, 10:13:03 pm »

Thanks for the responses, everyone!

WWB does provide means to turn off the individual orders and number of TX's for each group of equipment.

Correct me if I'm wrong, but my understanding of the Compatibility tab in WWB is that it allows you to select whether a particular type of device (or an inclusion group) should avoid the 2T5O (etc.) of all other transmitters in that zone; what I'm looking for is a way to make sure that all other transmitters in that zone avoid the 2T5O (etc.) of a particular subset of devices. The unmodified BTR-800 makes for a good example: I know it's going to throw serious 2T5O noise, so I want everything else to avoid its 2T5O, but not at the spectral expense of making everything else avoid all of each other's possible 2T5Os.

As for WWB, or any other IM program, attempting to calculate the signal strengths of the IM product(s), the answer is no. [...] If a carrier is within the RF space of concern, then any IM product, for which the carrier is a constituent, should be considered a potential interferer.

Understood; what I'm looking for is a way to deal with situations where I can reasonably assume that some IMD products are much more likely than others of the same order to be strong enough to cause interference. For example, I'm much less worried about the 2T5O from two ASMs with push-to-transmit beltpacks that stand 50' apart than I am about the 2T5O from the primary and backup 5212s straped 4" apart on the back of one of my leads. For a more complicated example, I'm a little worried about all 3T3O, but I'm particularly worried about the 3T3O produced by my double-packed lead having an intimate moment with her love interest. I'm not trying to predict the actual levels of IMD produced, which, as you say, are constantly in flux. What I'm looking for is a more granular way to indicate which sources of IMD interference I'm trying to avoid beyond "all possible 3T3O (etc.) in this zone".

I'm not sure how relevant this might be to your situation, but both IAS and WWB allow splitting things up into zones, with little or no intermod calculation between zones.

I'm not sure about in IAS, but in WWB a device in one zone will not avoid the IMD products from devices in other zones (only their fundamental frequencies). Definitely useful, but not quite what I'm looking for here.

It is easy to do the BTR first and add your mics, but there is not much point in gaming the 5th order harmonics on the first device you add, there won't be many because there is only the BTR to check against.

That's more along the lines of what I'm trying achieve. Not 2T5O mic-to-BTR, but just trying to avoid the BTR's 2T5O.

Thanks!

-Russ
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Scott Helmke

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Re: Avoiding intermod products based on anticipated strength?
« Reply #6 on: February 28, 2018, 11:15:50 pm »

You can break out individual devices by creating a new equipment profile and bring that into the inventory and then the workspace.

For that matter, couldn't you create new equipment profiles with some of the intermod fields blank to selectively turn off certain calculations?
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Russell Ault

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Re: Avoiding intermod products based on anticipated strength?
« Reply #7 on: February 28, 2018, 11:52:59 pm »

For that matter, couldn't you create new equipment profiles with some of the intermod fields blank to selectively turn off certain calculations?

My understanding is that this would turn off those calculations when calculating the available frequencies for that equipment (i.e. same thing as adjusting settings in the Compatibility tab), not turning off those calculations when calculating available frequencies for other equipment (which is what I'm looking for).

-Russ
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Paul McDermott

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Re: Avoiding intermod products based on anticipated strength?
« Reply #8 on: March 01, 2018, 09:55:33 am »

My understanding is that this would turn off those calculations when calculating the available frequencies for that equipment (i.e. same thing as adjusting settings in the Compatibility tab), not turning off those calculations when calculating available frequencies for other equipment (which is what I'm looking for).

-Russ

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. I would put these in three main categories.

1. IEM and BTR TX
2. Analogue mics and belt packs
3. Digital mics and belt packs

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.

WWB and IAS are calculators with very useful calculation features but have some limitations. 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. So use these powerful tools for what they are and we use work arounds for tasks they do not perform.

So lets look at what we can do for this BTR TX problem. I have a few solutions we should use as many methods to improve stability as possible.
1. In WWB use inclusion groups to segregate the BTR TX from other devices
2. put your BTR TX frequencies in tightly packed together using as little bandwidth as possible to minimize the impact of the IMDs on other devices
3.When posible put the BTR TX as close to an adjacent active TV station as possible or between 2 active TV stations. This will eliminate the impact many of the IMDs generated by this device
4. Use inclusion groups to keep analogue and digital microphones at least 10 MHz from the BTR TX frequencies
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)

I hope this helps and good luck.
« Last Edit: March 01, 2018, 02:40:04 pm by Paul McDermott »
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Jason Glass

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Re: Avoiding intermod products based on anticipated strength?
« Reply #9 on: March 01, 2018, 12:14:12 pm »

2. put your BTR TX frequencies in tightly packed together using as little bandwidth as possible to minimize the impact of the IMDs on other devices
3.When posible put the BTR TX as close to an adjacent active TV station as possible or between 2 active TV stations. This will eliminate the impact many of the IMDs generated by this device

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.

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« Last Edit: March 01, 2018, 07:54:35 pm by Jason Glass »
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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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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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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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Russell Ault

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Re: Avoiding intermod products based on anticipated strength?
« Reply #20 on: May 08, 2018, 01:20:13 pm »

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.

That's hugely useful. In addition to calculating guard bands, it's also a great learning tool, just being able to throw in frequencies and see what happens. Thanks!

-Russ
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Paul McDermott

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Re: Avoiding intermod products based on anticipated strength?
« Reply #21 on: May 08, 2018, 04:02:44 pm »

That's hugely useful. In addition to calculating guard bands, it's also a great learning tool, just being able to throw in frequencies and see what happens. Thanks!

-Russ


Great to hear that you found it useful. I also use it to reverse engineer the acceptance criteria for device and software frequency coordination.

Feel free to contact me with suggestions to make it more useful.

http://rfcoordinationnyc.com/downloads.html
« Last Edit: August 09, 2019, 12:26:07 pm by Paul McDermott »
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Russell Ault

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Re: Avoiding intermod products based on anticipated strength?
« Reply #22 on: May 08, 2018, 05:18:30 pm »

I also use it to reverse engineer the acceptance criteria for device and software frequency coordination.

Do you mind sharing your procedure for this? It sounds like a very useful process.

Thanks!

-Russ
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Re: Avoiding intermod products based on anticipated strength?
« Reply #23 on: May 08, 2018, 06:26:41 pm »

Do you mind sharing your procedure for this? It sounds like a very useful process.

Thanks!

-Russ


For example paste all 16 of the frequencies of UHF-R H band Group 1

you can change the acceptance criteria for channel spacing and IMD spacing to find out the parameters they used to calculate the groups.

For this example Channel spacing was 399KHz and 2T3O was 199KHz
2T5O and 3T3O and not calculated for these groups

I don't get too in-depth with this, but I like to be aware of how these calculations are made so when I see a problem in the field I have some facts to work with.

One more reason to always use coordination software and not use magic buttons on devices.
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Re: Avoiding intermod products based on anticipated strength?
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