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[Keith Broughton]

SO... I just watched the webinar on the Sennhesiser 6000 Digital system and I am quite impressed with the design and functions.
In that presentation, it was indicated that the design has eliminated the need for co-ordination for IM products within that system. A spectrum scan was shown that displayed  a 2 ch transmitter configuration with no apparent IM products. How is this possible?
However, if that system was deployed in a situation where "analogue" RF systems were in use, I imagine IM products would have to be considered and all of the 6000 TX freqs would have to be part of the co-ordination.
Thoughts or comments?

[Robert Lofgren]

Well, a scam is still a scam ;-)

[Tim McCulloch]

Henry!

My recollection is that digital modulation schemes are not impacted by RF intermod the way analog FM is.

[Pete Erskine]

SO... I just watched the webinar on the Sennhesiser 6000 Digital system and I am quite impressed with the design and functions.
In that presentation, it was indicated that the design has eliminated the need for co-ordination for IM products within that system. A spectrum scam was shown that displayed  a 2 ch transmitter configuration with no apparent IM products. How is this possible?
However, if that system was deployed in a situation where "analogue" RF systems were in use, I imagine IM products would have to be considered and all of the 6000 TX freqs would have to be part of the co-ordination.
Thoughts or comments?

This is also mostly true with other digital wireless.  for best results though they must be run in low power mode.  IM products from analog have less effect on digital signals than another analog frequency.

[Henry Cohen]

SO... I just watched the webinar on the Sennhesiser 6000 Digital system and I am quite impressed with the design and functions.
In that presentation, it was indicated that the design has eliminated the need for co-ordination for IM products within that system. A spectrum scam was shown that displayed  a 2 ch transmitter configuration with no apparent IM products. How is this possible?
However, if that system was deployed in a situation where "analogue" RF systems were in use, I imagine IM products would have to be considered and all of the 6000 TX freqs would have to be part of the co-ordination.

The virtual lack of IM products with digital transmission schemes is a long time well known phenomena; this is the reason we can now have adjacent channel digital television transmitters at relatively high power levels. Ever since the introduction of non- spread spectrum digital wireless mics (e.g. Zaxcom, ULXD, Mipro, Sennheiser S9000), we have seen, and taken advantage of, this byproduct that enables digital transmission schemes. The same Sennheiser demonstration and results can be seen with any well designed digital wireless microphone system.

In order to achieve stable and reliable somewhat wideband spectral mask, and constellation integrity (acceptable BER & FER), an extremely linear and stable RF circuit is required. Since IM products occur in non-linear gain stages (and diodic junctions), there's no meaningful IM produced in the very linear gain stages of digital RF, though there will be a rise in the overall noise floor depending on the density of wireless mic channels in operation.

As a side note, the analog Sennheiser SK5212 has a "low IM" mode which puts the transmitter into lower power and provides significantly more critical voltage control over the final PA. The result is almost no IM in an analog system, but battery life suffers.

So, if your wireless mic system is only digital equipment, you can ignore IM's and just be concerned with adjacent channel separation based on TX power levels and occupied channel bandwidth (high density vs 'standard' modes). If however you a have a mixture of analog and digital, the IM products of the analog equipment could be a potential co-channel interferer to digital channels if the analog carrier(s) is strong enough: This is where proper band planning comes into play.

[Henry Cohen]

Well, a scam is still a scam ;-)

I suggest you do some reading on digital transmission theory and design. Unless you think adjacent high power DTV stations are a scam.

[Russell Ault]

As a side note, the analog Sennheiser SK5212 has a "low IM" mode which puts the transmitter into lower power and provides significantly more critical voltage control over the final PA. The result is almost no IM in an analog system, but battery life suffers.

I always wondered how they achieved this. I just assumed it was by running the transmitter at high power but inserting a passive attenuator between the final and the antenna (which the transmitted signal only has to go through once, but any IM will be affected by at least twice). Very interesting!

-Russ

[Robert Lofgren]

He misspelled ‘scan’ and wrote ‘scam’ instead. That is my type of humor 

I suggest you do some reading on digital transmission theory and design. Unless you think adjacent high power DTV stations are a scam.

[brian maddox]

The virtual lack of IM products with digital transmission schemes is a long time well known phenomena; this is the reason we can now have adjacent channel digital television transmitters at relatively high power levels. Ever since the introduction of non- spread spectrum digital wireless mics (e.g. Zaxcom, ULXD, Mipro, Sennheiser S9000), we have seen, and taken advantage of, this byproduct that enables digital transmission schemes. The same Sennheiser demonstration and results can be seen with any well designed digital wireless microphone system.

In order to achieve stable and reliable somewhat wideband spectral mask, and constellation integrity (acceptable BER & FER), an extremely linear and stable RF circuit is required. Since IM products occur in non-linear gain stages (and diodic junctions), there's no meaningful IM produced in the very linear gain stages of digital RF, though there will be a rise in the overall noise floor depending on the density of wireless mic channels in operation.

As a side note, the analog Sennheiser SK5212 has a "low IM" mode which puts the transmitter into lower power and provides significantly more critical voltage control over the final PA. The result is almost no IM in an analog system, but battery life suffers.

So, if your wireless mic system is only digital equipment, you can ignore IM's and just be concerned with adjacent channel separation based on TX power levels and occupied channel bandwidth (high density vs 'standard' modes). If however you a have a mixture of analog and digital, the IM products of the analog equipment could be a potential co-channel interferer to digital channels if the analog carrier(s) is strong enough: This is where proper band planning comes into play.

As usual, Henry, your ability to explain things is both comprehensive and dead on accurate.  Of late i've been dabbling in some "explaining" content creation, and wow is it WAY harder than you make it look.  So thank you so much for contributions here.  I can't begin to count the number of things i've learned from you.

as to the OP, i think it was 3 or 4 years ago that i first heard someone mention that Digital Wireless systems were essentially free of the IM problems that plague us when we're using analog wireless systems.  And i distinctly recall thinking that this was clearly wrong.  But i also recall having the good sense to do some research before i waded in and "set that person straight".  Of course the person i "set straight" was me.   

The more i "know" the more i know i don't know....

[Keith Broughton]

Well, a scam is still a scam ;-)

Miss spaeling on my part. The scan was not a scam

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