RF INTERMODULATION PRODUCTS – Application Note 18

All AM broadcast transmitters employ RF output filtering networks which are required to attenuate the carrier frequency harmonics.  The RF filter design typically attenuates RF harmonic levels to -80dBc and includes a bandpass network.  The transmitters, in the absence of other RF signals, will produce the carrier frequency free from unwanted harmonics or spurious emissions.  Unfortunately, when transmitters are co-located or multiplexed, there is a finite possibility that RF intermodulation products will be produced.  See our Intermodulation and Interference FAQs for more information.

For instance, in the United States, the permissible spurious radiation for a radio station is set out in FCC Rules, Section 73.44.  The permissible level is a function of frequency separation from carrier and carrier power, and is referred to as an “emissions mask”.

FCC Mask

This mask applies to transmitter generated spurious emissions, typically controlled in transmitter design, and those from mixing products with other stations, which are the licensees responsibility.

Most often, problems arise because of multiplex combining of multiple stations on one antenna, or from antenna pickups from nearby stations. Defining the magnitude of expected intermodulation for any transmitter is complex, and includes evaluation of losses not only through filter circuits, but also antenna coupling and external paths. Where directional antennas are involved, an additional level of complexity is added. However, proper evaluation is important so as not to over or under-specify the antenna and combining equipment.

A common example is a transmitter site where two transmitters are diplexed into one antenna. In this case, a portion of the second transmitted signal will flow back into the first transmitter.  The magnitude of this signal flowing back depends on the gain and bandwidth of the antenna, configuration of the transmitter output network and distance between the two antennas or isolation between parts of diplexer.  Transmitter 1 will produce its worst intermodulation product at a frequency equal to its second harmonic minus the second transmitter carrier frequency.

2f₁ - f₂  = F_IM

Transmitter 2 could also produce an intermodulation product at:

F_IM = 2f₂  - f₁

The magnitude of the intermodulation product will depend on the efficiency of the transmitter as a mixer with an RF filter.  This is known as the turn around loss of the transmitter and is typically 16dB for a solid state transmitter.  If the magnitude of the signal from transmitter 2 flowing into transmitter 1 is -50dB relative to transmitter 1's signal, then an intermodulation product at 2f₁ - f₂ with amplitude of -66dB relative to transmitter 1's signal would be expected.  Where this level of intermodulation product is unacceptable, a filter would have to be installed in the transmitter 1 output matching network which would pass transmitter 1's signal with minimal loss or VSWR and provide an acceptable rejection at the carrier frequency of transmitter 2.

The above notes refer to “third-order” intermodulation product, but there are many more combinations possible at higher “orders”. However, the nature of practical broadcast filters normally makes the higher orders of little importance. Where multiple strong signals are presented to a transmitter, as in a crowded antenna farm area, or when using a triplexer or quadraplexer, the number of potential combinations can be impressive. This is particularly troublesome should one of these combinations fall on a market competitor’s frequency!

A discussion often arises regarding which type of equipment is more likely to produce intermodulation products.  All transmitters will produce intermodulation products, but the bandwidth of output networks varies between solid state transmitters and tube transmitters.

A tube transmitter will have an output network which typically has a loaded Q of 2 and can be as high as 5 where solid state transmitters utilize output networks which typically have a loaded Q between 0.5 and 1.  An analysis of this situation shows that the solid state transmitter may produce intermod products 3dB-6dB higher than a typical transmitter with vacuum tube final.  In practical applications, all transmitters which are multiplexed require traps and sufficient isolation between transmitters to prevent intermodulation products.