Adapted: from Broadcast Engineering 9/87 & Recording Engineer/Producer 11/87 Multi-Wireless Mic Compatibility Advantages of wireless microphones make them popular and occasionally mandatory for various broadcast and recording applications. Freedom of movement and benefit of concealment allow production options made impossible with wired units. The performance is no longer restricted to a stage, extending the creative opportunities. The problem with wireless microphones is that they are radio systems and are subject to the laws of radio frequency physics and regulations of the FCC. When two or more wireless mics are operating simultaneously, a complex set of restrictions apply to the equipment design and the selection of operating frequencies. MULTIPLE SYSTEM FREQUENCY COMPATIBILITY is defined as the condition of two or more wireless mic systems operating simultaneously with no degradation in the performance of one due to the presence of the other systems. A SYSTEM is a transmitter and companion receiver. Suppose that five wireless systems are operating simultaneously. If each system functions equally well with the other four systems turned on, as well as with them off, then the five are said to be compatible. If, however, the presence of any of the other four systems degrades the performance of the fifth, they are incompatible. Only one of the other systems may be responsible for the interference. On the other hand, a combination of the other systems could be responsible. FREQUENCY INCOMPATIBILITY Restrictions on operating frequency selections are imposed by the FCC and by transmitter and receiver circuitry limitations. According to FCC Part 90.265(b), there are eight specific frequencies for wireless mic operation. Because of the channel spacing selected, however, only combinations of two of these frequencies are truly compatible in spite of the eight available frequencies. Selecting frequencies in the TV-channel spectrum allows larger compatible systems to be built. Six categories can be identified wherein limitations exist. For complete compatibility, all six restrictions must be met simultaneously. They include: * Separation between operating frequencies, * Transmitter spurious signals, * 2-signal intermodulation, * 3-signal intermodulation, * Receiver local oscillator radiation and * Receiver image-frequency sensitivity. SEPARATION BETWEEN OPERATING FREQUENCIES Separation between operating frequencies is simply how close together in frequency the systems (carriers) are spaced. The limiting factor in receiver design is the selectivity of the intermediate frequency (IF) filter and the dynamic range of the RF pre-amplifier and mixer circuits. The more selective the filter and the higher the dynamic range, the closer the operating frequencies can be. A basic guideline is that all operating frequencies should be separated from one another by at least 400kHz. Selectivity problems can be identified by turning on all receivers, then turning on only one transmitter at a time. If any receiver, other than the companion receiver, unsquelches, the operating frequencies may be too close together. Calculating differences in operating frequencies can confirm this possibility. The best solution is to change system frequencies. Calculate the differences between system frequencies. If any are less than 400kHz apart, they must be changed. Suppose the following carriers are being used: F1 = 174.8MHz F2 = 175.4MHz F3 = 175.7MHz The difference between F2 and F3 is less than 400kHz, so either F2 or F3 must be changed. An acceptable frequency for F3 would be 178MHz to place the required 400kHz between each carrier. An alternative is to turn on the transmitter companion to the affected receiver in an attempt to capture the receiver and reject the interfering signal. Capture is an FM receiver phenomenon whereby the stronger of two co-channel signals suppresses the weaker one. However, this alternative may or may not be successful. SPURIOUS SIGNALS It is not uncommon for transmitters to emit energy on frequencies other than the desired carrier frequency. Most wireless mics use a quartz crystal as a frequency determining element. The signal generated by the crystal is then multiplied to the operating frequency. Consider a transmitter operating at 160MHz, starting from a 20MHz crystal. The first multiplication (X2) produces 40MHz, a second doubling yields 80MHz, while a third results in the 160MHz operating carrier. Unfortunately, signals are radiated at 80MHz (X4), 140MHz (X7), 180MHz (X9), 200MHz (X10) and others. Any receiver operating on one of these undesired output frequencies may receive the spurious signal with a resulting degradation in the audio. Assume that a spurious signal is transmitted at a level 70dB less than the desired signal. Its transmitted power might be -53dB, which is 57dB above the threshold sensitivity of the receiver and will unsquelch the receiver. The solution is to select operating frequencies such that the spurious signals do not fall on or near other operating frequencies. Ideally the undesirable signals should be at least 250kHz away from any operating frequency. To determine this, multiples of 1 through 16 times the crystal frequency of a transmitter should be computed and compared with all other operating frequencies. To check for spurious signals, turn on all receivers, then turn on one transmitter at a time. If any transmitter unsquelches a receiver other than its companion receiver, you may have a spurious problem. Calculation of undesirable crystal harmonics will determine if this is the case. The best solution is to change system frequencies. First, calculate the offending crystal harmonic, then change the offending transmitter or the offended receiver. Suppose you have the following system frequencies: F1 = 190.8MHz F2 = 214.7MHz The ninth harmonic of the crystal for No 1 occurs on 214.65MHz, within 150kHz of F2. The calculations follow. 1- 190.8MHz/8 = 23.85MHz (crystal freq) 2- 23.85MHzx9 = 214.65MHz (9th har) 3- 214.8MHz - 214.65MHz = 150kHz. An acceptable frequency for F2 would be 214MHz, because that is 650kHz away from the 9th harmonic of system 1. Alternatively, you might keep the F2 transmitter turned on in an attempt to mask the undesired signal from transmitter F1. Results of this approach vary. 2-SIGNAL MODULATION Two signals applied to a non-linear circuit create additional signals or intermodulation (IM). These signals include sum-and-difference products of each of the fundamental input signals and their associated harmonics. The following components are produced: * Fundamental: F1, F2 * Second order: 2F1, 2F2, F1+/-F2, F2-F1 * Third order: 3F1, 3F2, 2F1+/-F2, 2F2+/-F1 * Fourth order: 4F1, 4F2, 2F1+/-2F2, 2F2+/-2F1 * Fifth order: 5F1, 5F2, 3F1+/-2F2, 3F2+/-2F1 * and additional higher orders. Order is the sum of the numerical coefficients that multiply F2 and F2 terms. Note that even-order products usually occur far removed in frequency from F1 and F2 and, therefore, are omitted here for simplicity. If F1 and F2 are close to each other in frequency, the 2F1-F2 and 2F2-F1 terms also fall close together. If F1 and F2 are separated by 1MHz, those products will be separated from F1 and F2 by 1MHz. For example, if F1 = 160MHz and F2 = 161MHz, the following IM signals occur: * Third order: 159MHz (2F1-F2) and 162MHz (2F2-F1). * Fifth order: 158MHz (3F1-2F2) and 163MHz (3F2-2F1). * Other higher orders. If any of these products fall on or near any system frequency, interference and incompatibility will result. The guideline is that IM products should be at least 250kHz away. Note that 2-signal IM will occur only when two systems are operated simultaneously. However, if the two transmitters are separated by the required minimum of 400kHz, the interference is not a problem when only two systems are in operation. This is because the close-in IM products (2F2-F1 and 2F1-F2) will be at least 400kHz away from either system frequency. However, 2+signal IM can cause interference when three or more systems are in use. IM can occur in output stages of two closely located transmitters. If this happens, the interference is actually retransmitted by both transmitters. The IM also may occur in the receiver RF circuitry due to close proximity of the transmitter to the receiver antenna. In any case, a signal produced on or near a system frequency may unsquelch an undesired receiver. The problem can be identified in two ways. First, the interference occurs only when two transmitters are on. Turning either one off removes the interference. Second the interference is more severe when two transmitters are in close proximity with one another or are close to the receiver antenna. The problem may disappear when the transmitters and receiver antenna are separated from one another. Again, the best solution is to change frequencies. Calculate IM products to see if any fall within 250kHz of any system frequency. For example, the following carriers are in use: F1 = 174.8MHz F2 = 175.4MHz F3 = 176.6MHz When tested, with F1 and F2 transmitters on and close to one another, the receiver on F3 unsquelches and receives the audio from F1 and F2. Use the following formulas to identify the cause: 1 2F1 - F2 2 2F2 - F1 3 3F1 - 2F2 4 3F2 - 2F1 Calculations show that 3F2-2F1 = 176.6MHz, which is F3. You must select a new F3. Recalculate IM products using the new F3 to be sure no IM products fall within 250kHz of the new F3 frequency. In addition, take into account all of the combinations of any two of the three system frequencies. In this case, 178MHz would be suitable for F3. An option is to keep the transmitters separated from each other and from the receiving antenna by at least 10 feet. Also, turning on the transmitter for the offended receiver may help to mask the problem. 3-SIGNAL INTERMODULATION Logically, if problems can occur with two systems, the same kind of sum-difference signal products exist with more than two signals. Although not as severe, 3-signal IM also can be a problem. The following signals are produced by 3-signal IM. * Fundamental: F1, F2, F3 * Third order: 2F1+/-F2, F1+/-F2+/-F3 The even-order and higher-order products usually are far removed in frequency and are seldom problematic. Consider a system with microphones operating on F1 = 159MHz F2 = 160MHz F3 = 161MHz which produce the following third order products: Frequency IM Product Formula 157MHz 2F1-F3 158MHz F1+F2-F3, 2F1-F2 159MHz 2F2-F3 160MHz F1+F3-F2 161MHz 2F2-F1 162MHz F3+F2-F1, 2F3-F2 163MHz 2F3-F1 In the example, third-order IM products fall exactly on the system frequencies themselves. Hence, equal spacing of system frequencies results in 2-signal and 3-signal IM. Characteristics of 3-signal problems and the solutions are the same as for 2-signal interference. Adding more systems creates even more possibilities of IM interference and must be handled in the same manner for avoidance. RECEIVER LOCAL OSCILLATOR RADIATION In a superheterodyne receiver, a local oscillator (LO) generates a carrier, which is in turn mixed with the received signal from the antenna. The process generates a new signal at an intermediate IF frequency. If the local oscillator signal should be coupled to the receiving antenna, interference may result. For a receiver operating on 160MHz with an IF frequency of 10.7MHz and low side injection, the local oscillator operates on 149.3MHz. If the receiver LO signal falls on or near a system frequency, interference and incompatibility result. As a guide, no receiver LO should be closer than 250kHz to any system frequency. LO radiation is identified by turning off the offending receiver and noting if the interference disappears. This effect is most obvious with all transmitters turned off. Consider the following system frequencies: F1 = 203.3MHz F2 = 211.4MHz F3 = 214.0MHz When the receiver F3 is on, receiver F1 unsquelches. You determine that the LO frequency of F3 is 203.3MHz. Select a different carrier for F1 or F3, to be at least 250kHz away from the LO of the other receivers. An acceptable frequency for F1 would be 210.8MHz. Physical separation of the offending receiver and its antenna from other receivers and turning on other transmitters should solve the problem. RECEIVER IMAGE-FREQUENCY SENSITIVITY Receiver image frequencies can produce interference. An image frequency is equal to the LO frequency minus the IF frequency. If a receiver operates on 160MHz with an IF of 10.7MHz and an LO of 149.3MHz, the image frequency is 138.6MHz. The typical receiver is 70dB less sensitive at its image frequency than at its operating frequency. This provides an image frequency threshold sensitivity of about -40dBm. A transmitter with an output level of +17dBm will generate a signal 57dB above the receiver's image-frequency sensitivity threshold. Interference easily results. Identify the problem by noting if the offending transmitter is 21.4MHz lower than the offended receiver. Although these calculations are based on low-side injection and an IF of 10.7MHz, the principle is value regardless of the actual system parameters. Also, the 250kHz separation of operating frequencies is recommended here. DEGREES OF COMPATIBILITY The severity of the interference may aid in determining the solution to IM problems. Mild incompatibility often can be masked by keeping all transmitters turned on. The transmitter's signal strength is sufficient to capture its companion receiver. If a transmitter is turned off, any signal stronger than 1 microvolt may be heard in its companion receiver. If a transmitter is on, an interfering signal may have to be as strong as 1mV to be heard in the receiver (a 60dB difference). MISDIAGNOSIS Other problems may appear to be caused by incompatible frequencies. Outside RF interference or dropouts are sometimes responsible for poor microphone performance. It is important to be aware of this possibility to avoid wasting time looking for a cause in the wrong place. Be sure that the entire wireless system is operating on compatible frequencies. For the record, with a 4-microphone system, more than 14,500 calculations are involved! Fortunately, computer programs can be written to search for a desired number of compatible frequencies when given lower and upper frequency boundaries. Such programs may be available through software companies and various database/bulletin boards. If purchasing multiple wireless systems, request the manufacturer to make the compatibility analysis for you. The company should be able to suggest alternative frequencies if compatibility problems appear likely.