Wireless Frequency Coordination

[Chris15]

So this is a higher level engineering question...

If one looks at the various frequency coordination software packages, Workbench, IAS, SIFM, etc. you find that different models of wireless have different spacing parameters. As a broad sweeping statement, it is accurate to say that the more expensive the product, the tighter you can pack them in.

What underlying engineering determines these spacing parameters?

To use an example, in Sennheiser G3, the EM300 and the EM500 have the same sensitivity, the same adjacent channel rejection, the same intermod attenuation and the same blocking according to the numbers in the brochures. Why then can I fit 32 500 systems in a 42 MHz bank and only 24 300 systems?

My gut reaction is something related to frequency stability keeping everything tighter, but I'm hoping others might be able to shed some light on what quantifiable parameters make the differences...

 

[mbenonis]

Generally speaking, yes--the more expensive the system, the tighter you can pack them in. I'll edit this post with a much more detailed reply later today (I'm in class right now).

 

[zmb]

Most likley that the EM 500 system uses less bandwidth than the EM 300, therefore, more transmitters can be used within the 42mHz.

 

[Chris15]

Most likley that the EM 500 system uses less bandwidth than the EM 300, therefore, more transmitters can be used within the 42mHz.

While normally I'd tend to agree, I think in this case it's not the factor. Because both systems are using analogue FM modulation, the bandwidth is pretty much determined by the dynamic range of the system, which is the same in both cases, 25-18k Hz. It would also tend to suggest that lower end products having a smaller dynamic range can pack more tightly, which we know to not be true...

 

[mbenonis]

Most likley that the EM 500 system uses less bandwidth than the EM 300, therefore, more transmitters can be used within the 42mHz.

Nope, they use the same wideband FM modulation technique.

What it comes down to is the RF front end. More expensive systems come with much better filtering and amplifiers that will not duffer from various types of interference, including adjacent channel interference (interference from a transmitter close in frequency to the one you are using), intermodulation distortion (interference caused by nonlinear active devices, such as preamplifiers), and noise in general. How manufacturers come up with the maximum number of channels number is rarely stated, but I would say that for most pro quality systems if you maintain a minimum spacing of 600 kHz between systems, with 1 MHz being better, you will be fine. Run the numbers through an Intermod checker and make sure they all work. From here, the number of system you can fit into a passband is strictly limited by (a) the number of free TV channels and (b) the bandwidth of the receiver in MHz.

BTW, as a rule tighter is better. Lectrosonics' system are only 25 MHz per bandsplit. This means that the odds of a really strong signal getting into their receivers is very small compared to a system. Systems with wider front ends are much more susceptable to very hot signals causing trouble unless they are equipped with tracking front end filters that narrow that passband significantly. Those are costly, unfortunately.

 

[museav]

It was all much easier when you only had fixed frequency systems but making a front end that can be both highly selective and frequency agile over a wide range is much more difficult.

 

[lakota651]

Hi Chris,

The answer to your specific example is that the difference is in the price point, sort of speaking. Sennheiser pre programs more compatible frequencies in each group of the the 500's than the 300's or 100's, but all three can theoretically fit (assuming you are clear of TV or other transmissions) the same number of channels into their given bandwidth. You just need to use additional software or math to figure out the frequencies and then set them manually.

Something else that has not been mentioned yet is the output strength of the transmitter. With the 300 and 500 G3s (and other higher end wireless systems), you can drop down the RF strength. This only advisable if you have a good antenna systems to help you reject/ignore outside signals, but the advantage is you can use more transmitters in a confined space without them stepping on each other.