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Apr 07, 2016, 09:38 AM
Registered User
Thanks Jon.
So if you were wanting to run this for four channels, you would have to run the calculations for all possible combinations?
f1 ~ f2
f1 ~ f3
f1 ~ f4
f2 ~ f3
f2 ~ f4
f3 ~ f4
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Apr 07, 2016, 10:29 AM
Team AlienWarpSquad
Quote:
Originally Posted by andmiller View Post
Thanks Jon.
So if you were wanting to run this for four channels, you would have to run the calculations for all possible combinations?
f1 ~ f2
f1 ~ f3
f1 ~ f4
f2 ~ f3
f2 ~ f4
f3 ~ f4
Correct and with all values of n & m as per the equations I posted above.
This is tedious by hand but not to hard to do with a program or spreadsheet.
Apr 07, 2016, 02:32 PM
Registered User
Quote:
Originally Posted by waltr View Post
Correct and with all values of n & m as per the equations I posted above.
This is tedious by hand but not to hard to do with a program or spreadsheet.
Found a Whitepaper from TI:
SOME SIMPLE RELATIONSHIPS
Intermodulation distortion occurs at frequencies that are the
sum and/or difference of integer multiple of the fundamental
frequencies. For example, assume a composite signal has
fundamental frequencies ωl and ω2. Distortion products will
occur at frequencies aωlbω2 where a and b = 0, 1, 2, 3,

The following table illustrates this relationship.
2nd-Order Frequencies:
ω1 ω2
ω1 + ω2

3rd-Order Frequencies:
2ω1 + ω2
2ω2 + ω1
2ω2 + ω2
2ω1 ω2

4th order would be b = 3 etc.. So for the purposes of IMD in the 5.8ghz band, we would only to look at the 3rd-Order as the others fall outside the band, correct?
Apr 08, 2016, 07:41 AM
Team AlienWarpSquad
Quote:
Originally Posted by andmiller View Post
Found a Whitepaper from TI:
SOME SIMPLE RELATIONSHIPS
Intermodulation distortion occurs at frequencies that are the
sum and/or difference of integer multiple of the fundamental
frequencies. For example, assume a composite signal has
fundamental frequencies ωl and ω2. Distortion products will
occur at frequencies aωlbω2 where a and b = 0, 1, 2, 3,

The following table illustrates this relationship.
2nd-Order Frequencies:
ω1 ω2
ω1 + ω2

3rd-Order Frequencies:
2ω1 + ω2
2ω2 + ω1
2ω2 + ω2
2ω1 ω2

4th order would be b = 3 etc.. So for the purposes of IMD in the 5.8ghz band, we would only to look at the 3rd-Order as the others fall outside the band, correct?
Almost.
You also must calculate with a multiplier on the second frequency like this.
3ω1 + 2ω2
3ω1 - 2ω2
And it would the Difference that will fall back into the band of interest and cause IMD interference.
I did POST the equations in this thread, did you try them?
Do setup a Spreadsheet (Excel works great) to play with these equations and SEE what combinations fall back into the band.
Apr 08, 2016, 08:04 AM
Let's FPV in Middle TN!
moteasa's Avatar
Has anyone setup a spreadsheet with the raceband frequencies to see where IMD interference resides in that group of frequencies? I'm not really good at using the formulas above, but found the spreadsheets someone shared on the first page very useful.
Curious for an event running all 8 raceband frequencies if there are any more suspect to interference that I would want to try to avoid.
Apr 10, 2016, 12:04 PM
Registered User
bcc1955's Avatar
Reading through this thread I can't help but think that the current VTX/VRX system will be rejected and replaced within the next few years. It is ridiculous to think that users will be able to sort through these issues with spreadsheets. There simply needs to be a system similar to FHSS radio control that is designed intelligently to get around the problems described in this thread. The current state of the art is a joke! and we are simply suffering from decisions made by someone who lacked a basic understanding of the issues that would eventually be faced in our hobby, or simply did not care. For now I recommend buying cheap components as you will need to replace them once it has been worked out.
Last edited by bcc1955; Apr 10, 2016 at 12:11 PM.
Apr 11, 2016, 07:18 AM
Registered User
Quote:
Originally Posted by moteasa View Post
Has anyone setup a spreadsheet with the raceband frequencies to see where IMD interference resides in that group of frequencies? I'm not really good at using the formulas above, but found the spreadsheets someone shared on the first page very useful.
Curious for an event running all 8 raceband frequencies if there are any more suspect to interference that I would want to try to avoid.
The raceband frequencies aren't actually that great because they are evenly spaced which makes it prone to IMD interference. Look at some of the suggestions earlier in the thread for frequencies to use together.
Apr 11, 2016, 08:21 AM
Team AlienWarpSquad
Quote:
Originally Posted by bcc1955 View Post
Reading through this thread I can't help but think that the current VTX/VRX system will be rejected and replaced within the next few years. It is ridiculous to think that users will be able to sort through these issues with spreadsheets. There simply needs to be a system similar to FHSS radio control that is designed intelligently to get around the problems described in this thread. The current state of the art is a joke! and we are simply suffering from decisions made by someone who lacked a basic understanding of the issues that would eventually be faced in our hobby, or simply did not care. For now I recommend buying cheap components as you will need to replace them once it has been worked out.
Partly yes.
A first step is designing much better vRX front ends that have better rejection of IMD, better selectivity and sensitivity.
With a good rf front end no complex system will have much improvement.

I have lots of first hand experience with IMD and weak RX front end from Ham Radio contesting and working with Ham radio equipment, Repeaters, on crowded commercial radio towers.
Improved RX front ends helped eliminate interference issues by the greatest margin than anything else.
Apr 11, 2016, 08:47 AM
Let's FPV in Middle TN!
moteasa's Avatar
Quote:
Originally Posted by andmiller View Post
The raceband frequencies aren't actually that great because they are evenly spaced which makes it prone to IMD interference. Look at some of the suggestions earlier in the thread for frequencies to use together.
Yes, I understand this, which is why I'm not using raceband and using the ones referenced earlier in this thread.
I'd like to have a spreadsheet showing the raceband frequencies so I can show people where the issue with raceband is, and also find out what channel I don't want to be on at an event where they run raceband and I have no influence to get them to change it.

Example of a good 5, and a bad 6 that was listed on MultiGP.
I'd like to do the same for the raceband.
Apr 13, 2016, 02:52 AM
Registered User
bcc1955's Avatar
Quote:
Originally Posted by waltr View Post
Partly yes.
A first step is designing much better vRX front ends that have better rejection of IMD, better selectivity and sensitivity.
With a good rf front end no complex system will have much improvement.

I have lots of first hand experience with IMD and weak RX front end from Ham Radio contesting and working with Ham radio equipment, Repeaters, on crowded commercial radio towers.
Improved RX front ends helped eliminate interference issues by the greatest margin than anything else.
To this end I would be interested to know if the pricey Iftron Clearview Racing/Pro RX helps substantially with IMD. Anyone have one on this thread to test?
Apr 13, 2016, 07:19 AM
Team AlienWarpSquad
Quote:
Originally Posted by bcc1955 View Post
To this end I would be interested to know if the pricey Iftron Clearview Racing/Pro RX helps substantially with IMD. Anyone have one on this thread to test?
Good question and another who would like to see better testing of the Iftron Clearview Racing/Pro RX.
May 06, 2016, 11:07 PM
Registered User
Quote:
Originally Posted by moteasa
Has anyone setup a spreadsheet with the raceband frequencies to see where IMD interference resides in that group of frequencies? I'm not really good at using the formulas above, but found the spreadsheets someone shared on the first page very useful.
Curious for an event running all 8 raceband frequencies if there are any more suspect to interference that I would want to try to avoid.
I've been away from this forum for a long time. I added the "race band" frequencies to my spreadsheet. I hope it is helpful.
May 10, 2016, 09:23 AM
Let's FPV in Middle TN!
moteasa's Avatar
Quote:
Originally Posted by Rob Hair View Post
I've been away from this forum for a long time. I added the "race band" frequencies to my spreadsheet. I hope it is helpful.
interesting....
the raceband chart is full of what should be exact IMD match interference.
I ran an event a couple weekends ago running all 8 raceband, 200mw, LH/RH alternating, and it worked better than I thought.
May 10, 2016, 04:25 PM
Registered User
Yes, its not apparent to me why the Race Band frequencies were set the way they are. Perhaps the synthesizers used in our cheap VTXs can only be easily programmed to have equal spacing between channels. As was pointed out earlier in this thread, its the equal spacing that makes the current frequency choices especially vulnerable to IMD.

I adopted a strategy for choosing frequencies for our local races that I call "progressive delta". Starting with the first frequency near the bottom of the band the second frequency was far enough away so as to avoid adjacent channel interference. Then the third frequency is chosen so that its spacing from the second is a little greater. This is repeated until I got to the top end of the band. This doesn't always give you a good set of frequencies, but it beats just randomly choosing.

There are several other things that will determine if IMD is a problem in a particular race. These include VTX power, distance between VTXs and receivers, design of the receiver front end, etc. Alternating RH and LH CP antennas, as you did, can be a big help. Unfortunately I have yet to see any specifications from video receiver manufacturers that describe 3rd order intercept or anything else that would help us choose one receiver over another. Better receiver front end designs would be a big help.
May 10, 2016, 04:32 PM
Let's FPV in Middle TN!
moteasa's Avatar
Quote:
Originally Posted by Rob Hair View Post
Yes, its not apparent to me why the Race Band frequencies were set the way they are. Perhaps the synthesizers used in our cheap VTXs can only be easily programmed to have equal spacing between channels. As was pointed out earlier in this thread, its the equal spacing that makes the current frequency choices especially vulnerable to IMD.

I adopted a strategy for choosing frequencies for our local races that I call "progressive delta". Starting with the first frequency near the bottom of the band the second frequency was far enough away so as to avoid adjacent channel interference. Then the third frequency is chosen so that its spacing from the second is a little greater. This is repeated until I got to the top end of the band. This doesn't always give you a good set of frequencies, but it beats just randomly choosing.

There are several other things that will determine if IMD is a problem in a particular race. These include VTX power, distance between VTXs and receivers, design of the receiver front end, etc. Alternating RH and LH CP antennas, as you did, can be a big help. Unfortunately I have yet to see any specifications from video receiver manufacturers that describe 3rd order intercept or anything else that would help us choose one receiver over another. Better receiver front end designs would be a big help.

One hell of a ground station at the event.


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