Frequency planning in depth: 3rd- and 5th-order intermodulation

The RF coordination guide explains why two transmitters create phantom signals. This one goes a step further: which products need to be calculated, how many appear as the channel count grows, and how to build, step by step, a frequency plan that works with everything switched on.

Intermodulation orders

A product's order is the sum of the coefficients in its formula. 2·f1 − f2 is 3rd order (2 + 1), and 3·f1 − 2·f2 is 5th order (3 + 2). The higher the order, the weaker the product tends to be: Shure notes that beyond the 5th order products can usually be ignored, even when they fall in-band.

How many products there are

Each pair of transmitters creates two 3rd-order products (2·fa − fb and 2·fb − fa), and each trio creates three combinations of the form fa + fb − fc. The count grows far faster than the number of channels:

Some products coincide or fall outside the band, but the order of magnitude doesn't change. Beyond a handful of channels, checking by hand stops being practical: manufacturers' compatible groups or coordination software such as Shure Wireless Workbench are used instead.

Margins: how far to stay away

• Between a carrier and any 3rd-order product: most manufacturers recommend at least 250 kHz, according to Shure.
• Between carriers: it depends on receiver selectivity; the minimum is in each unit's manual and tends to be larger on budget equipment.
• 5th-order products: coordination software lets you set a separate margin, usually smaller than for 3rd order. In a crowded spectrum it is sometimes relaxed; in a clean one it's worth keeping.
• Manufacturer groups only guarantee compatibility between units of the same series, with no outside signals. When brands or series are mixed, the whole set has to be calculated.

Worked example with three transmitters

Three mics at 600.000 · 600.400 · 601.300 MHz. These are their nine 3rd-order products and the nearest carrier to each:

The smallest margin is 400 kHz, so the plan meets the 250 kHz rule. Now add a fourth mic at 600.800 MHz, which looked clear on the scan: it lands exactly on 2·f2 − f1. That channel will get noise as soon as mics 1 and 2 are on, even though no other device uses that frequency.

Building the plan, step by step

1. Inventory of everything that transmits on set: mics, in-ears, IFB, intercoms and video links, with the tuning range of each. A 626–668 MHz system can't go to 600 MHz, however compatible that frequency is.
2. Scan of the location, excluding active TV channels and any steady signal.
3. Splitting into blocks: it's common practice to reserve a section for in-ears and IFB, apart from the mics, since they transmit continuously and from close to the sound cart.
4. Calculation in software: equipment, ranges and margins are loaded, and the program proposes a compatible set.
5. Backup frequencies: calculated together with the plan, so a channel can be changed without breaking the compatibility of the rest.
6. Verification: everything on, a new scan and a walk around the location with each transmitter while listening.