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[Įrni F. Siguršsson]

New Sennheiser EW-DX line claims that it causes no intermods.

From product website:

Extended 88 MHz switching bandwidth, intermodulation-free operation [intermodulation-free equidistant channel spacing] for up to 293 channels with secure AES 256 encryption and four modular network ports.

Equidistant channel spacing to easily allocate intermodulation-free channels: Standard Mode with 600 kHz spacing for up to 146 channels; Link Density (LD) Mode with 300 kHz spacing for up to 293 channels

How are sennheiser bending the laws of physics? And has anyone had their hands on these units and measured them out ?

[Brad Harris]

New Sennheiser EW-DX line claims that it causes no intermods.

From product website:

Extended 88 MHz switching bandwidth, intermodulation-free operation [intermodulation-free equidistant channel spacing] for up to 293 channels with secure AES 256 encryption and four modular network ports.

Equidistant channel spacing to easily allocate intermodulation-free channels: Standard Mode with 600 kHz spacing for up to 146 channels; Link Density (LD) Mode with 300 kHz spacing for up to 293 channels

How are sennheiser bending the laws of physics? And has anyone had their hands on these units and measured them out ?

Similar to other modern digital transmission units from the same (6000) and other companies...


Brad

[Russell Ault]

New Sennheiser EW-DX line claims that it causes no intermods.
{...}
How are sennheiser bending the laws of physics? {...}

By only telling part of the truth: digital wireless microphones, like all RF TXs, are perfectly capable of creating IMD, but the nature of those IMD products means they don't have nearly the same practical impact as FM IMD.

An FM TX works by creating a sine wave at a designated frequency, and then modulating the frequency of that sine wave in relation to the amplitude of the audio being transmitted. For UHF wireless microphones the frequency of this sine wave is allowed to deviate from its designated frequency by (IIRC) 100 kHz in either direction (with louder audio producing greater deviation), but the output frequency of a TX will always average back to its designated frequency. In practice, most FM TXs spend a lot of their time outputting at (or very near) their designated frequencies, which is what produces the pronounced "carrier" seen on spectrum analyzers. The stability of these carriers means that any IMD products resulting from interactions between FM TXs will be similarly stable, which (a) allows them to cause reliable interference with other microphones and (b) allows us to predict where they'll be so we can avoid them.

Digital TXs, on the other hand, typically use modulation schemes that distribute the RF energy evenly across a given bandwidth and don't produce a pronounced carrier. Put another way, if an AM signal can be thought of as a carrier with two side-bands, and an FM signal as a carrier with an infinite number of side-bands, then a lot of digital signals are effectively just an infinite number of indistinct, overlapping carriers; on a spectrum analyzer this can look a lot like band-limited noise. At a physics level the outputs of these TXs interact in exactly the same way and produce just as much IMD, but instead of two (or more) distinct sine-wave carriers producing multiple distinct sine-wave IMD products, an infinite number of indistinct, overlapping carriers are now interacting with an infinite number of other indistinct, overlapping carriers to produce even more indistinct, overlapping carriers; on a spectrum analyzer these IMD products will also look like noise, albeit lower in level and more spread out (since the IMD math itself doesn't change).

The reason Sennheiser (and others) can make claims about "IMD-free operation" is because a lot of the time the IMD products of digital TXs have so little power that they actually fall below the ambient noise floor, so even though they technically still exist they practically don't. Of course, more TXs means more IMD products, which can ultimately result in an increased noise floor, although even that has to be pretty extreme (or the use-case pretty nominal) before it starts causing problems.

(That said, digital RXs do tend to need a higher carrier-to-noise ratio to work reliably than FM RXs despite also operating at lower output powers, so the additional IMD from adding more digital wireless microphones to a rig will probably start causing issues for the mics long before it starts taking out the FM IEMs.)

-Russ

[Henry Cohen]

By only telling part of the truth: digital wireless microphones, like all RF TXs, are perfectly capable of creating IMD, but the nature of those IMD products means they don't have nearly the same practical impact as FM IMD.

. . .

-Russ

So Russ has told only part II of the explanation. Part I is that IM only occurs in non-linear circuits. Non-linearity occurs when a component in the RF signal chain, or in nearby external radiators (intentional or not), are overloaded and go into saturation: The final PA in a transmitter; the preamp stage in a receiver or multicoupler (antenna distribution); a mixer; a diodic junction such as the infamous rusty fence. Digital transmission schemes, with their relatively wideband emissions mask, require highly linear components and designs (high IP3) in order to remain stable. A byproduct of that high linearity is significant mitigation of IM production to start.

[Russell Ault]

So Russ has told only part II of the explanation. {...}

Ah, the irony of only telling part of the truth about "only telling part of the truth"; next time I'll remember to add the usual "someone who knows more than I do will be along shortly to correct my omissions" disclaimer.

Thanks, Henry!

-Russ

[Henry Cohen]

Ah, the irony of only telling part of the truth about "only telling part of the truth"; next time I'll remember to add the usual "someone who knows more than I do will be along shortly to correct my omissions" disclaimer.

Thanks, Henry!

-Russ

To be fair, I omitted the example of designed in IM: Mixer stages.

[Russell Ault]

To be fair, I omitted the example of designed in IM: Mixer stages.

Right, of course; am I correct in thinking, though, that intermediate stages are inherently less susceptible to producing IMD from external signals because of their relative isolation?

-Russ

[Henry Cohen]

Right, of course; am I correct in thinking, though, that intermediate stages are inherently less susceptible to producing IMD from external signals because of their relative isolation?

Isolation in the sense that the mixer stage needs to be well shielded and impedance matched at all ports.

[ProSoundWeb Community]