RF Column 16 - January 1993 Copyright (c) 1992, 1993, 1995 H. Douglas Lung ALL RIGHTS RESERVED TOPICS: FCC mandated frequency tolerances and measurements New emission designators required for FCC form 313 filings Extending klystron life - filament voltage, tips from Varian Cheap Remote software notes -------------------------------------------------------------------- This month I'll start a multi-part series on frequency and modulation measurements. I've noticed over the last couple years that many engineers aren't watching their transmitter's frequency as close as they did before the FCC relaxed the requirements for logs. This month we'll look at what the FCC expects. Also, at the SBE convention in San Jose I picked up a nice package of information on klystrons from Varian. I'll pass along some of the tips and give you information how to get the package. Keeping with klystrons, I found an interesting failure mode I hadn't seen before, unfortunately on some of my klystrons! Details on that and what you can do to prevent the same problem later. Finally, the FCC has moved its broadcast auxiliary licensing to the Private Radio Branch in Gettysburg. This will affect your form 313 application! Read on. FCC frequency tolerances... To start the series on frequency measurements, I'll summarize what the FCC requires from each licensee. I was surprised to find Low Power TV licensees have the most stringent monitoring requirements: 74.762 Frequency measurements. - (a) The license of a low power TV, a TV translator or a TV booster station must measure the carrier frequencies of its output channel as often as necessary to ensure operation within the specified tolerances, and at least once each calendar year, at intervals not exceeding 14 months. TV Broadcast and broadcast auxiliary licensees do not have any defined intervals for measurements. TV broadcast stations are covered by FCC Rules part 73.1540: 73.1540 Carrier frequency measurements. - (a) The carrier frequency of each AM and FM station and the visual carrier frequency and difference between the visual carrier and the aural carrier or center frequency of each TV station shall be measured or determined as often as necessary to ensure that they are maintained within the prescribed tolerances. The rule for Television auxiliary stations specifies no interval, but does require monitoring: 74.662 Frequency monitors and measurements. - The licensee of a television auxiliary broadcast station shall provide means for measuring the operating frequency in order to insure that the emissions are confined to the authorized channel. I can't find any reference requiring frequency measurements for satellite uplink stations. As with the other services, the requirement for maintaining the frequency tolerance is there: 25.202(d) Frequency tolerance, earth stations. - The carrier frequency of each earth station transmitter authorized in these services shall be maintained within .001% of the reference frequency. Most TV engineers are familiar with the frequency tolerances for TV broadcast transmitters. The visual carrier must be within one kilohertz of the licensed frequency (which may be offset by ten kiloHertz either direction from the frequency in the tables) and the aural carrier must be plus or minus one kiloHertz of the visual frequency plus exactly 4.5 megaHertz. The same limits apply to low power TV stations and translators operating with a specified offset. Non-offset low power TV stations and translators have a frequency tolerance of 0.02% for transmitters rated at not more than 100 watts and a tolerance of 0.002% for transmitters rated over 200 watts. This tolerance applies to the visual carrier frequency. The frequency of the aural carrier must be 4.5 megaHertz, plus or minus one kiloHertz, above the visual frequency regardless of the required visual carrier tolerance. TV auxiliary microwave frequency tolerances are specified in FCC Rules Part 74.661. The tolerance for stations operating below 13,250 MHz. is 0.005%, unless they are a TV translator relay, in which case the tolerance is 0.002%. Stations in the bands between 17,700 and 19,700 MHz. must maintain a tolerance of 0.003%. For the frequencies between 31,000 and 31,300 MHz., a 0.03% tolerance is specified. In the 38 GHz. band, the tolerance is 0.005% unless the output power is 50 mW or less, in which case it is relaxed to 0.05%. The same tolerances apply to fixed and mobile stations in this service. TV broadcasters need to be aware of one other frequency the FCC wants kept within tolerance. That frequency is the color subcarrier frequency. FCC Rules part 73.682(a)(5) states... "The chrominance subcarrier frequency is 63/88 times precisely 5 MHz (3.57954545 . . . MHz). The tolerance is 10 hertz and the rate of frequency drift must not exceed 0.1 hertz per second (cycles per second squared)." Many inexpensive sync generators do not meet this standard and cannot be used as a reference source for full power TV stations! This rule also prevents direct playback of hetrodyne color formats like 3/4", VHS and Hi-8 without time base correctors. How do we ensure our transmitter is within these tolerances? I'll cover that in detail next month. The FCC does not spell out a procedure for measuring frequencies, but does specify the standard in Part 73.1540: (b) In measuring the carrier frequency, the licensee may use any method or procedure that has sufficient precision to establish that the carrier frequency is within the prescribed departure limits. (c) The primary standard of frequency for radio frequency measurements is the standard frequency maintained by the National Bureau of Standards or the standard signals of Stations WWV, WWVB, WWVH and WWVL of the National Bureau of Standards. I'm designing a combination frequency standard / WWV(H) receiver for use in checking and calibrating counters and spectrum analyzers. Look for details on how to build it here in about two months. TV auxiliary application processing changes... While we're discussing the FCC, there have been some changes in how broadcast auxiliary applications on form 313 are processed. They are now handled by the Private Radio Bureau (PRB) in Gettysburg. Applications requiring fees should now go to "Federal Communications Commission, Broadcast Auxiliary Radio Services, P.O. Box 358700, Pittsburgh, PA 15250-5700. Applications exempt from fees go direct the Federal Communications Commission, Broadcast Auxiliary Radio Services, 1270 Fairfield Road, Gettysburg, PA 17325. The PRB has a reputation at the FCC for rapidly processing applications. I hear one way they achieve this quick turnaround is by being very tough on errors - one mistake and the application is returned, quickly. In April 1985, the FCC required applicants to use new emission designators in any FCC application with the exception of form 313, which the Mass Media Bureau insisted had to use the old designators (probably to maintain consistency with older records). The PRB will now accept either the old or new emission designators. Dane Ericksen at Hammett and Edison was kind enough to provide a chart comparing the old and new designators for common TV auxiliary services: Service Old Designator New Designator ------------- -------------- -------------- Remote pickups 12.5F3 12K5F3E 20F9 20K0F3W 100F3 100KF3E TV STL or ICR 25000F5 25M0F3F 17000F5 17M0F3F 25000F9 25M0F8F or 25M0F9F What do these symbols mean? FCC Rules part 2.201 gives a full description. I'll summarize some of the key points. The first four characters are the bandwidth. It must be expressed with three numbers and one letter. The letter takes the place of the decimal point and indicates the magnitude. The four letters the FCC chose are easy to remember - H = Hertz, K = KiloHertz, M = MegaHertz and G = GigaHertz. The next letter indicates the type of modulation: A = Amplitude modulation, double sideband (AM broadcast) C = Amplitude modulation, vestigal sideband (TV broadcast visual) F = Frequency modulation (FM, most microwave and satellite) The second symbol (a number) indicates the type of signal modulating the carrier: 1 or 2 = Quantized or digital information 3 = Single channel analog information 8 = Two or more channels containing analog information (subcarriers) 9 = Composite system with one or more channels containing quantized or digital information together with one or more channels containing analogue information. The third symbol, a letter, is the type of information to be transmitted: D = Data transmission E = Telephony (including sound broadcasting) F = Television (video) W = Combination of the above Klystron tips... Varian conducted a tour of their klystron manufacturing plant as part of the 1992 SBE conference in San Jose. I couldn't get to San Jose early enough to make the tour, but Eric Margeson at KSTS was kind enough to pick up a package of the information Varian was handing out with the tour. Some of the technical bulletins were almost 10 years old and I remember reading their predecessors when I first started working with UHF transmitters. If you work with UHF klystrons, I strongly recommend you give Varian a call and ask for the bulletins applicable to your type of tube. Varian's number is 415-424-5670 or you can write Varian Associates, P.O. Box 10800, M/S B-050, Palo Alto, CA 94304-1031. Varian has a 3/4" video tape available on basic klystron theory as well as a 5.25" computer disk (DOS) with catalog, data and troube shooting info. Publication #4511 is the theory book on integral cavity klystrons for UHF TV, Publication #2967 is the companion book for external cavity klystrons. Varian handed out a photocopied collection of articles called "Technical Tube Tips To Maximize Your Klystron's Performance". It doesn't have a publication number, but it is worth asking for. It gives tips on tracking down body current trips and replacing arc detector photocells, as well as general tips on what to look for when a tube won't make power. One item Varian discusses is filament voltage. Over the past couple years klystron manufacturers have been recommending operating their klystrons at reduced filament voltages. I've seen bulletins from Varian and from EEV on this. While you should refer to the bulletin for your tube type, generally they state that filament voltage should be reduced from the specified voltage until either beam current drops or power output drops, whichever comes first, then increased slightly above this point. This should be done after the tube has operated a few hundred hours at the specified voltage. Prepare to spend some time finding the proper filament voltage. Because the klystron filament has a large thermal mass, it will take half an hour or so for any change in filament voltage to affect the beam current or power output. Make the reduction in small steps, a tenth of a volt at a time and wait at least a half hour before making another adjustment. This procedure is very conservative and meets both EEV's and Varian's recommendations. Why reduce the filament voltage on a klystron? The obvious answer is to increase tube life. Reducing the filament voltage will increase the emission life of the filament. Perhaps more important, it will also reduce the amount of metal from the gun deposited on the ceramics near the base of the tube. It is important that the filament voltage be kept above the level where the beam current drops or power output drops. If it isn't, Varian warns it can cause "...emission instability which can cause beam defocusing, leading to increased mod-anode current and body current." You might wonder what effect that metal deposited on the ceramics might have on a tube. Varian's literature mainly talks about voltage breakdown. I experienced another problem with EEV four cavity klystrons at KTMD. These klystrons had over 35,000 hours on them and were not quite making full power. Checking the tuning on them, I found the input cavity tuned very broad - all it seemed to do was move the entire swept waveform on the spectrum analyzer up and down! Normally, reducing the input coupling would have increased the Q of the cavity and permitted seeing the peak. In this case, all that happened was the gain dropped. A phone call to Mike Kirk at EEV provided the answer. His response was one of "good news - bad news". The good news was we got 35,000 hours out of the tube. The bad news was we wouldn't get much more. What happened here? Mike explained that metal from the gun had deposited itself on the closest cool spot to the filament - the first cavity ceramics. This metal film lowered the Q and the gain of the cavity. These tubes had been operated at reduced filament voltage since EEV issued their bulletin, but that was after several thousand hours operation at only slightly less than the data sheet values. I suppose I shouldn't complain about 35,000 hours life on an external cavity tube. I was able to keep the tubes in service by increasing the input coupling almost to maximum. I tuned the second cavity a bit high, then tuned the input cavity low enough to give a 3 dB roll-off about 2 MHz. or so below carrier. I also increased the load coupling on the second cavity slightly to help flatten the response. Significantly more drive was needed to make 100 percent power - 7 dB more than that for a new tube tuned conventionally. Blue Earth Micro Correction... I found a small bug in my SETCLK.BAS program for the Blue Earth Micro remote control. It turned up when I tried to reset my remotes when we went off Daylight Savings Time. As it is now, the program will not allow hours between 2000 and 2300 to be entered. Change the "=3" part of line 375 to "=7" to fix the problem. Alternatively, if you have already run SETCLK.BAS to set the clock during the day, you can delete line 375. It is only needed when SETCLK.BAS is run for the first time. By the way - I received a call from someone at WCCO interested in the Blue Earth Micro. Unfortunately, as a result of my experimenting with new and unique CONFIG.SYS and QEMM 6.0 setups, I managed to lose the name and address. Some others may have disappeared too. If you requested information from me in September or October of last year and haven't received it yet, your name probably got crunched. Please give me another call. The full program for the Blue Earth Micro cheap remote as well as accessory programs are available on the Broadcast Professional's Forum (GO BPFORUM) on CompuServe. The latest version of my calorimetric power calculation program is there as well, along with my average to peak ratio TV power calculator. The files to look for are RMTCTL.BAS and RMTACC.ARC for the remote control, PWRCAL.EXE for the calormetric power calculator/log generator and A2PRAT.EXE for the TV average to peak ratio calculator. Next month... Next month I'll continue my series on frequency measurement. I hope to have some information on the current status of video compression for satellite transponders as well. Keep the comments coming. I'm still looking for tips on TV RF technology. If you respond to me via mail, please be patient as I spend a lot of time travelling and it can take a month for the mail to catch up to me. EMAIL on CompuServe is the best way to reach me - my ID is 70255,460. The mail address is 2265 Westwood Blvd., Suite 553, Los Angeles, CA 90064. Phone numbers you can try include 305-884-9664 or 818-502-5739. ((8/95 > UPDATE! - Use dlung@gate.net for e-mail!)) Copyright (c) 1992, 1993, 1995 H. Douglas Lung ALL RIGHTS RESERVED