All About Wireless: Noise in RF Systems
Welcome to the eighth installment of All About Wireless. In this issue, we will identify several common sources of noise that affect wireless microphone and IEM systems.
Electromagnetic Interference and Radio Frequency Interference
Noise in RF systems can generally be regarded as any RF energy that is not the desired signal. Two terms commonly used to describe RF noise are Electromagnetic Interference (EMI) and Radio Frequency Interference (RFI). EMI is random, broadband noise whereas RFI is narrowband noise broadcast at specific frequencies.
EMI is generated by non-broadcast electronic devices and electric motors. The noise emitted by these devices is a by-product of their operation. EMI may enter a wireless microphone or IEM system via the antenna, transmission line, or power connection, and the effect is typically high-frequency noise or distortion. Interestingly, the presence of EMI can be seen on a CRT monitor where it often presents as vertical bands of dots moving across the screen.
All electronic devices emit some amount of EMI, but LED walls are of particular concern. Whilst individual LED modules may conform to relevant emissions standards, the EMI generated by a fully formed video wall can be considerable. Spectrum scans are essential to capture the impact a video wall has on the RF noise floor.
The scan presented below reveals the EMI generated by an energized, but blank, video wall. Without an image displayed, average power in the noise floor remains below -85dBm. However, multiple spikes of energy extend well above this threshold at power levels strong enough to interfere with wireless microphone and IEM systems.
The situation is markedly worse once an image is displayed. The average power of the noise floor is raised by approximately 6dB, with many strong peaks of EMI presenting at up to 50dBm above the noise floor. To make matters worse, the distribution of energy is constantly changing according to the video signal. So, in practice, the frequencies at which the EMI peaks exist fluctuate randomly.
This can pose a very serious problem for the coordination of wireless microphone and IEM systems as the peak EMI energy radiated from a large video wall may be stronger than that of the microphone and IEM transmitters. In difficult RF environments like this, antenna choice and positioning are very important. Directional antennas may be positioned such that the null point is directed at the source of EMI. Boosting transmitter output power and inserting an equivalent pad at the input to the receiver can also be an effective way to increase the signal-to-noise ratio without increasing the chances of overloading the receiver front end. In any case, the identification and monitoring of potential sources of EMI are critical to ensuring RF systems are configured for optimal performance.
RFI differs to EMI in that it is not unintentionally radiated energy at random power levels, but rather, is simply the presence of unwanted RF signals broadcast by RF transmitters. Sources of RFI may include other wireless microphones and IEMs, radio and television broadcast, wireless communications systems, or consumer electronic devices with wireless functionality. The presence of RFI can also be seen on a CRT monitor where it often presents as several horizontal bars or wavy lines on the screen.
The good thing about RFI, as opposed to EMI, is that it can often be accommodated as the interfering frequencies usually remain constant. Local television stations, for example, are reasonably easy to identify and avoid. Rogue mobile transmitters present a greater challenge as they can be difficult to locate physically. RF Engineers at broadcast events, for example, are often required to track down ENG teams on-site that are transmitting on interfering frequencies and assign them coordinated frequencies to use instead. Intermodulation Distortion (IMD) products are another primary source of RFI, which will be the topic of next month article.
Local Oscillator and Intermediate Frequencies
One less commonly recognized source of interfering noise can be attributed to the receiver demodulation circuitry. Local Oscillator (LO) and Intermediate Frequencies (IF) generated within the receiver can cause distortion, both within the receiver itself and in other receivers in the system. If the RF inputs of two receivers are able to interact electronically, one receiver may interfere with the other if the operating frequency of one is equal to the LO frequency of the other.
For example, a standard single conversion superheterodyne FM receiver tuned to an operating frequency of 600.7MHz would have its LO operating at 590.0MHz. If not electronically isolated, a second receiver tuned to an operating frequency of 590MHz could experience interference from the LO of the first, especially if the power level received from the 590MHz microphone transmitter is low. An Antenna Distribution Unit (ADU) electronically isolates receiver RF inputs, minimizing the risk of LO induced interference. Even with an ADU in place, it is still recommended that carrier frequencies are coordinated to avoid LO frequencies by at least 250kHz.
Image frequency is another potential source of internally generated interference. In a single conversion FM receiver, depending on the design, the LO may track above or below the tuned operating frequency by 10.7MHz. When the tuned operating and LO frequencies are applied to the receiver mixer section, one of the outputs of the mixer is the 10.7MHz. If another carrier existing 10.7MHz away from the LO and 21.4MHz away from tuned operating frequency is allowed to enter the receiver demodulation circuitry, the result will be a second interfering output from the mixer stage at 10.7MHz. This is referred to as an Image of the tuned operating frequency. Although professional receivers incorporate filters to suppress this, it is recommended that operating frequencies are coordinated at least 250kHz away from any potential Image frequency.
It is important to understand the effect noise producing devices will have on the RF noise floor so that an informed assessment can be made regarding their impact on our wireless microphone and IEM systems. As depicted above, a higher RF noise floor typically results in a reduction of operating range. In analog systems, an increasing noise floor will produce increasing amounts of audible noise in the demodulated signal until the squelch point is reached. Digital systems do not exhibit this behavior, typically demodulating clean audio to the point that noise-induced errors cause the system to mute. This is advantageous as the impact of a raised RF noise floor is not immediately audible. However, if a high noise floor is not identified, the resulting decrease in operating range may come as an unwelcome surprise to the RF engineer. Spectrum scanning is the key to identifying and managing noise in RF systems.
Next month, we will focus specifically on one form of RFI - Intermodulation Distortion. We will learn how and why IMD products are generated, and how to manage IMD in multi-channel systems.
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