Microsoft Word AM Noise Article.doc


1 AM NOISE: THE QC STANDARD FOR FM BROADCAST As read in: By Joel Bump PART 1 It has been slightly more than 16 years since I first publis hed a series of detailed technical articles in RW on the subject of AM noise in FM transmission systems. Over the years, a number of engineers I knew in the Southern California area have re tired or moved on to other markets and positions. More solid state transmitters are in service and AM noise has been routinely monitored and controlled in hundreds of demands on FM transmission system s and reception quality. Yet stations improving service area bandwidth persist and questions often are asked about proper coupling methods for AM noise monitoring in both new installations and existing transmitter plant upgrades. AM NOISE DEFINED e modulation of an FM carrie r. There are two types A simple definition of AM noise is: unwanted amplitud chronous noise consists of amplitude modulation of AM Noise, synchronous and asynchronous. Asyn y caused by power supply hum or vibration. Unless unrelated to the FM modulation of the carrier, typicall there is a serious problem with the transmitter, asyn chronous AM is far less significant than synchronous AM. Unwanted AM modulation produced by normal FM carrier modulation from baseband audio and all ed to as incidental AM. Consistent control of subcarriers is synchronous AM, sometimes referr synchronous AM noise can result in improved audio clar ity, better stereo separation, lower crosstalk into subcarriers and extended service area. EFFECTS OF UNCONTROLLED AM NOISE FM receivers ultimately are required to produce a nalog output signals. This is accomplished by converting the frequency swing of the incoming carrier to amplitude values that feed the receiver output or are further decoded to produce stereo audio outputs. Receiver designs seek to minimize the effect of carrier amplitude variations during this process, but rece ived RF levels in weak signal areas or in mobile receivers vary drastically over short distances. Wh en the received signal instantaneously drops below a ude variations are directly detected and combined with certain threshold in the radio, the received amplit the stereo audio, reduced separation, poor subcarrier the baseband audio. The result is audible noise in performance and a reduction in the stat ion’s effective “solid” service area. SOURCES OF AMPLITUDE VAR IATIONS AT THE RECEIVER iver may result from multipath distortion of the Amplitude variations that can be detected by the rece carrier. At the relatively high frequencies in the FM band, transmitted signals reflect off many surfaces, from hills to buildings to power lines. When a direct and a reflected signal reaches the receiving antenna at the same time, they will add or subtract resulting in significant variations in level. I have measured multipath variations as great as 30 dB over distances of 50 feet. Amplitude variations from AM noise are also inhere nt in the actual transmitted carrier. AM noise produces an effect similar to multipath at the receiver in weaker signal areas. Large and often important portions of a station’s coverage area may be located in these regions. AM noise in the transmitted signal tends to significantly multiply the effects of mult ipath. A moderate level of AM noise together with moderate, otherwise unobjectionable, multipath c an produce highly objectionable noise in many receivers. This effective reduction in coverage area can be substantially controlled by insuring that low AM noise is being transmitted at all times. HOW SYNCHRONOUS AM NOISE IS PRODUCED Synchronous AM noise results from tuned circuits. Coupling between rf amplification stages in a transmitter, tuned output circuits, low pass filters, antenna tuning and even transmission line bullets contribute to AM noise. Ideally, all power is transf erred equally across the frequency deviations of the band attenuation is never perfectly equal and many transmission system. In practice, however, side system elements vary with time and temperature. 1

2 produced as a result of modulation. The FREQUENCY MODULATION These graphs show the AM Noise waveform and relative amplitude w the center frequency. The PASS BAND of the transmission system at each frequency pr oduces waveform drives the carrier above and belo variations in the carrier amplitude producing AM NOISE. Time is indicated in 30 degree increments relative to the modulation frequency. The vertical lines drawn from the modulation waveform represent in stantaneous carrier frequencies above and below the station’s assigned carrier frequency. The intersection of the ope is carried to the left side of the graph where the instantaneous frequency with the pass band sl resulting AM noise is plotted ag ainst the same time increments. The fundamental production of synchronous AM noise is shown in Figure 1. As the FM carrier frequency shifts with modulation, shown as a sine wave, the pass band slopes produce a direct variation in the carrier amplitude. These variations are defined as AM noise. Because the pass band is symmetrical, lower frequency slopes. The resulting AM is twice amplitude variations result from both the higher and the frequency of the FM modulation. center frequency, the resulting AM will be the same If the center of the pass band is shifted below the frequency as the FM modulation. As seen in Figure 2, the amplitude of the AM noise also increases as the tuning shifts off center. These diagrams visually indicate how AM noise is pr oduced. Multiple amplific ation and tuning stages are designed to maintain the flattest pass band and wides t bandwidth. Interstage coupling, however, results in actual pass bands that are less uniform than in these basic examples. Coupling may be increased to produce flatter response over a broader range of frequencies while simultaneously producing steeper skirts. 2

3 stage overcoupling and is more nearly representative The pass band in Figure 3 results from some inter- of a multistage transmission system. This example onl y shows one-half cycle of modulation applied to the carrier, yet the resulting AM noise is now four time s the modulation frequency. The waveform of the AM noise does not exhibit uniform positive and negative am plitudes but it does produce four AM cycles for each FM cycle applied to the carrier. This graph clearl y shows the effect of t he skirts on the amplitude of the AM noise. If the pass band in Figure 3 was slightly wider, the same frequency deviation would produce a significantly lower AM peak amplitude. Figure 4 shows the AM noise produced by the pass band from Figure 3. This waveform exhibits a high peak excursion relative to the corresponding RMS voltage. It is particularly important to note the ratio of RMS to peak energy in the AM component. It is the peak AM that is subject to detection in the receiver. If the slope of the pass band skirts in Figures 3 and 4 was tightened, the peak AM excursion would become narrower while still producing the sa me objectionable amplitude at the receiver. The sine wave modulation used in these examples clarifies the generation of AM noise and its relationship to the system pass band. The nature of program audio and subcarrier modulation consists of narrower waveforms contai ning even less RMS energy while still producing the same peak amplitudes. ill examine sampling methods, planning for AM noise The second and final installment of this series w measuring, typical waveforms and practical considerations. 3

4 PART 2 Making accurate measurements and monitoring the AM noi se levels at all critical times are essential to optimum FM transmission system performance. Part one of this series explored the theory of AM noise generation and the effects it has on receivers. In this final installment we will examine typical waveforms and practical considerations for planning successful AM noise monitoring. WHERE AM NOISE SHOULD BE MEASURED The best measurement would include the effects of every bandwidth limiting factor in the transmission system. Ideally, AM noise should be measured at the output of the transmitting antenna without the practical, the optimum sample is immediately prior to effects of any external reflections. Since that is not the transmission line feeding the antenna. It is imper ative that the sample be taken from a directional sample of the forward carrier wave. The sample should be as close to the antenna in the signal path as possible. It should be after the harmonic filter and afte r any other notch filters or coaxial switches. The location of the monitoring sample is very important a nd can be useful in verifying the mechanical integrity of the plumbing prior to the sample. In addition to verifying transmitter performance, I have found AM noise readings to be instrumental in identifying burned bullets in rigid line or RF switches before they became off-air critical. Many older transmitters and some line sections provi de monitor outputs that are capacitively coupled to the carrier. Such samples must be avoided for AM noise measurements because they contain harmonic and reflected components that produce erroneous AM. When unwanted signals combine with the forward signal, spurious AM is produced that would ca use the engineer to mistune the transmitter, degrading rather than improving performance. Certain sampling sl ugs that fit line section ports are capacitive as evidenced by a coupling adjustment screw adjacent to the output jack. These samplers must not be used for AM noise. SAMPLING RF CARRIERS FOR AM NOISE MEASUREMENT A transmission line section with an available sampli ng port is normally installed just prior to the transmission line. A directional sampling slug is used for proper AM noise measurement. The slug should be oriented toward the forward carrier wave and the AM measurement detector must be connected directly to the output of the slug. Two characteristics of the slug are most important. First, it must have on over the wide dynamic range to be measured. sufficient RF output level to produce linear detecti ssipation to deliver its output continuously into a 50 Second, it must have sufficient 50 Ohm internal load di Ohm detector load. Some common sampling slugs do not meet these requirements. Available samplers meeting the requirements are listed at: in a properly operating facility The AM noise levels being measured can be as low as 70 dB below the carrier level. Clearly, if the sample is to be accura te, it cannot contain any spurious material. For that reason, the detector must be directly connected to the sample. If the detector is connected to the sampler using a coaxial cable, even minor reflections in that cable w ill produce serious errors in the detected AM. The output from an AM detector cont ains dc plus detected amplitude modulation with a bandwidth less than 100 kHz, therefore standard coax can carry the detector output a long distance to the monitor without any compromise in the reading accu racy. Modulation monitors connected to a sample oduce meaningful synchronous AM readings. using coaxial cable cannot be expected to pr IMPORTANCE OF SETTING UP AN ACCURATE SAMPLE It may seem that I am over stressing the importanc e of an accurate sample position and level. It is imperative to plan sampling and detection properly bec ause an inaccurate sample can prompt severe mistuning of the transmitter and yield incorrect overall indicated levels of AM noise. When the sample is established properly, it can be relied on for real-time monitoring of transmission system integrity for many years. The linearity of the detection circuit is equally as important as the RF sample. The optimum detector employs full wave carrier rectification with LC filt ering to produce an AM level that can be calibrated against the detected carrier level. The RDL DCF- 100MB detector used with the ACM-1, ACM-2 and Engineers have used half-wave diode rectifiers and ACM-3 AM Noise Monitors relies on this method. 4

5 suitable filter capacitors to produce samples that can be used as a relative tuning indicator when a quantitative value of AM is not desired. HOW LITTLE IS ENOUGH? Each station may establish its own threshold for maxi mum AM noise based on terrain, multipath in critical coverage areas and transmitter capabilities. In general, simply tuning for minimum AM without taking a 25 dB to -50 dB or better. Readings of -40 may be calibrated reading can result in actual levels from - acceptable if there is minimal multi path in critical listening areas and no subcarriers are in use. The same value may be unacceptable in a competitive market wi th moderate to severe terrain or other reflection producing obstructions. Most systems can attain a sustained level of at least -50 dB with good maintenance. Levels of station performance in any environment. -55 dB or better will produce optimum It is best if the station can constantly monitor the le vel, allowing a predetermined threshold to trigger an alarm. Changes in the bandwidth caused by any sour ce such as tube aging, variations in power service voltages, temperature, and transmi ssion line or switch heating, ca n alert the station of impending maintenance before the station’s se rvice area or subcarrier performance is noticeably affected. In selecting the proper threshold, it is normal to allow for a variation of 5 dB or more during each broadcast day. e or practical. Today’s monitoring system costs Monitoring is always desired though not always possibl are within most expense b udgets, but some facilities prefer periodi c measurements by engineering staff. can not sufficiently reject neighboring carriers to Other sites share common antennas with combiners that allow constant monitoring. At such sites, AM noise should be checked individually on each transmitter during a coordinated maintenance period. That period can also be used to check the antenna’s effect by measuring and logging the AM noise on a reflected carrier sample. The low amplitude of the reflected signal should require a more sensitive sampling slug. Such a maintenance check is equally beneficial to stations that do not share a common antenna. PROPERLY TUNED AND OPERATING PASS BANDS When a transmitter and associated components are pr operly tuned and operating, a waveform similar to Figure 1 will be produced by a detector or at the output of an AM noise monitor. With this display of or better, the station is assured of optimum centered tuning and a measured AM level of -55 dB performance. 5

6 band that is not perfectly centered, provided the Good performance may also be possible with a pass measured AM level is sufficiently low. Frequently, offset pass bands as indicated in Figure 2 produce excessive AM noise levels. QUALIFYING AND INTERP RETING THE READINGS does not produce a calibrated level, care must be If an AM noise measurement method is being used that exercised in qualifying the results. An important charac teristic to understand is that typically as AM noise becomes worse, the peak content increases while the RMS energy remains the same or decreases. Therefore, metering that produces RMS or averaged results will become more inaccurate as AM noise increases. Receiver perf ormance, however, will degrade as peak AM in creases. This makes it critical to monitor the peak AM excursions. AM indicated by pass band A was measured at -39 dB, Figure 3 details the problem. If the peak value of an RMS meter would indicate the same waveform at a level of -45 dB. If the pass band was then tuned for an RMS null, waveform B would result in an RMS reading of -46 dB. However, the peak value 6

7 produced from pass band B would actually be -32 dB. Nulling the RMS reading would result in an actual performance degradation in AM noise from -39 dB to -32 dB! ctifier, equally erroneous results can easily be Similarly, if a common diode was used as an RF re th 10% applied AM modulation will produce a dc/ac produced. A 1N4148 fed with 10 V p-p at 100 MHz wi ratio of -20 dB, which is an accurate indication of AM. The same diode fed the same signal at 4 V p-p would result in a reading of -17.9 dB, while with a carri er sample of 1 V p-p it produces a reading of -9.8 dB. This is an error of more than 50%. Accura te, repeatable AM noise measurements are improved through the use of low capacitance high frequency diodes. BENEFITS OF MONITORING AND CONTROLLING AM NOISE Nearly any anomaly in the performance of the tr ansmitter and associated plumbing has an immediate and measurable effect on AM noise. When the accurate AM level is monitored continuously against a oper performance. Control of AM noise optimizes threshold standard, the station can be assured of pr relying on signals in the upper spectrum of the subcarrier operation and any broadcast services rrier, and is materially worsened by increased baseband. Stereo separation relies on the 38 kHz subca AM levels. Proper sampling and monitoring of AM noise levels produces coverage consistency, maximum service radius, improved stereo separation and satisfied subcarri er tenants. It provides the final “QC” for the station’s carrier signal, program content excepted. Joel Bump is President and Director of Engineering at RDL, and was formerly a radi o engineering consultant in southern Californ ia. He is responsible for the design of the RDL ACM-1, ACM-2 and ACM-3 AM noise monitors. at: Article available in .pdf format RDL • 659 N. 6th Street, Prescott, AZ USA 86301 • (928) 778-9678 • Fax: (928) 778-9430 • Sales (928) 443-9391 • (800) 281-2683 • Fax: (928) 443-9392 • [email protected][email protected] • Technical Service (92 8) 778-3554 • (800) 933-1780 • Fax: (928) 778-3506 • [email protected] RDL EUROPE, BV Banking: INGBNL.2A • Account • VAT nr: NL 99.30.553.B01 • Chamber of Commerce: Amsterdam 25746 0-6225 287 • [email protected][email protected] Sales and Service • Gebouw Y-Tech, Van Diemenstraat 36, 1013 NH Amsterdam, The Netherlands • (++31) 20-6238 983 • Fax: (++31) 2 7

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