Two-Transmitter Intermodulation Screen
Where two wireless transmitters put their intermodulation products, and how close a third channel sits to one.
Example
You enter
- Carrier 1 (MHz) 542
- Carrier 2 (MHz) 545
- Third channel to test (MHz, 0 to skip) 539
You get
- Carrier spacing 3.000 MHz
- Third low mhz 539
- Third high mhz 548
- Fifth low mhz 536
- Fifth high mhz 551
Checked against published worked examples from: Shure.
Details, formula, and sources
ScopeWhen two transmitters are near enough for one's signal to reach the other's output stage, or for both to hit a shared receiver front end, the nonlinearity mixes them and the products land at predictable frequencies. The third-order pair lands one full spacing outside each carrier, so two transmitters 3 MHz apart put products 3 MHz below the lower and 3 MHz above the upper; the fifth-order pair lands two spacings out, weaker but still able to take down a channel. The practical consequence is that an evenly spaced channel plan is the worst possible plan. Carriers at 542.000 and 545.000 MHz are 3.000 MHz apart and produce third-order products at 539.000 and 548.000 and fifth-order products at 536.000 and 551.000, so a third channel at 539.000 -- which looks like a perfectly reasonable 3 MHz step down -- sits directly on a product and is unusable whenever both other transmitters are on. Moving it to 540.100 clears every product in the list by at least a megahertz. Note also that an evenly spaced five-channel plan on 3 MHz steps collides with itself at both ends. Coordination software solves the whole set at once; this solves the pair, which is what a tech in a room needs when one channel out of twelve is dropping and the rest are fine. A screen, not a coordination: the FCC rules for the band, the licensed users in the market, and full coordination software govern a real channel plan.
Formula or tablespacing = |f2 - f1|; third order = 2 f1 - f2 and 2 f2 - f1; fifth order = 3 f1 - 2 f2 and 3 f2 - 2 f1.
Source and editionThird- and fifth-order two-tone intermodulation products, by name -- public RF theory, and the frequency-coordination practice published by the wireless-microphone manufacturers. A pair screen, not a coordination: the FCC rules for the band, the licensed users in the market, and full coordination software govern a real channel plan.
Free accessThe mixing products are arithmetic on the two carrier frequencies the user enters; nothing is reproduced from a coordination database.
What governsEstimate. AHJ and licensed professional govern.
API field namesf1_mhz, f2_mhz, test_freq_mhz, spacing_mhz, third_low_mhz, third_high_mhz, fifth_low_mhz, fifth_high_mhz
- Third order 2f1 - f2 and 2f2 - f1: one full spacing outside each carrierpublic RF intermodulation theory
- Fifth order 3f1 - 2f2 and 3f2 - 2f1: two spacings out, weaker but still capable of killing a channelpublic RF intermodulation theory
- Pair screen only the FCC rules for the band, the licensed users in the market, and full coordination software govern a real planwireless-microphone coordination practice