RF Column 47 - October 1995 Copyright (c) 1995 H. Douglas Lung ALL RIGHTS RESERVED TOPICS: Tips on installing and maintaining UHF klystron low voltage pulsers FCC changes the rules on satellite uplink operations FCC requests comments on ATV proceeding, changes timetable --------------------------------------------------------- My columns the last few months have been about new technology -- Advanced TV, Diacrodes, high power solid state amplifiers, etc. This month I'm going to regress a bit and focus on old technology - klystron pulsing. Many stations can't afford a new klystrode, I.O.T., solid state or Diacrode transmitter. Older klystron transmitters can benefit from pulsing. I'll offer some tips on how to make it work. ((## Ignore the last two sentences if this column is used for the international edition ##)) I will touch on some currents news. The FCC's been busy this summer. I'll give you a summary of what's happening and show you how to get more information. Mention the word "pulsing" to an engineer responsible for maintaining a UHF klystron transmitter that's over ten years old and you'll likely hear either a litany of problems or a simple "no way!". The first UHF pulsers switched the klystron's mod anode voltage over an eight or nine kilovolt range in time with the sync pulses. This was done using high voltage switching tubes that had to fire at precisely the right time. If everything worked well, the klystron drew maximum current only when generating the sync pulses, resulting in a much lower average beam current and power bill. While I have seen mod-anode pulsed transmitters that had decent looking sync, frequent adjustment and maintenance was required to keep them this way. Some stations decided the substantial power bill savings weren't worth the loss of reliability and extra maintenance required. In the mid 80's klystron manufacturers came up with a much better way to modulate the beam current -- adding a grid like control element to the tube. It allowed the beam current to be adjusted with voltages less than 1,000 volts (with respect to the cathode). This allowed high voltage transistors similar to those used in home TV horizontal flyback circuits to be used for switching the modulating voltage. Use of transistors also eliminated the need for the complex timing required to compensate for the mod-anode pulser's high voltage switching tube characteristics. The low voltage pulsing electrode was called the "ACE" electrode by Varian and "BCD" electrode by EEV. Regardless of the name the function is the same. Our Los Angeles station, KVEA, has a venerable RCA TTU-110 110 KW UHF transmitter with three of the big Varian integral cavity tubes. Once the visual tubes were replaced with the Varian "ACE" electrode tubes, we had planned to convert it from mod-anode pulsing to low voltage "ACE" pulsing. This July David Gillenwater at KVEA and I made the switch. The KVEA project was the third "from scratch" low voltage pulsing installation / modification I've done. Lessons learned from the previous jobs allowed it to go smoothly. From experience I've learned it's most important to make sure the exciter and driver system are capable of supporting the pulser. Three factors need to be considered: timing, available RF drive power and modulator linearity correction capability.. The two previous low voltage pulser installations I've done used the Townsend pulser assembly. For KVEA, we chose pulsers based on the Townsend design from Advanced Broadcast Systems in Kentucky (606-282-7580). Even though the Advanced pulser unit looked like it was built in someone's garage, the design was solid and, except for a wiring error on a connector which caused one of the voltage adjustment motors to go backwards, it worked fine. I was impressed with how smoothly the pulser timing adjustments worked. Those on the original Townsend pulser control unit were incredibly touchy. Properly setting timing is perhaps the most difficult part of a pulser installation, especially if the transmitter has multiple visual klystrons. Here are my techniques for making it easier. First, it is important to understand how the various timing adjustments interact. When a klystron is heavily pulsed, it will have more gain at sync than it does at blanking. Therefore, it is important that not only must the sync level out of the exciter be adjustable, it has to be able to be adjusted (depressed) below blanking. The depression timing must match the video sync timing or sync distortion will result. Next, the pulser timing must match the sync depression timing. If it applies full beam current before the sync depression starts, there will be a large spike on the leading edge of sync. Finally, the ICPM (Incidental Carrier Phase Modulation) changes when pulsing starts. The timing of the ICPM correction must match the pulser timing. The first step in setting pulser timing to adjust the sync depression timing. Adjust it by looking at the output of the modulator at IF frequency, not at the output of the pulsed tube. Because ICPM can change the appearance of the sync pulses on modern TV demodulators using synchronous detection, I prefer using a wide band oscilloscope to view the modulated IF waveform. Sample the incoming video on one channel and the RF waveform on the other. Note that because the video will have to pass through the equalizing circuits and the IF will have to pass through a SAW filter, the two channels will not match each other in time. However, by carefully adjusting the triggering it should be possible to superimpose the video and RF waveforms. Use a distinctive transition in the video as a match point. Once this is done it is easy to align the sync depression on the RF envelope with the video sync pulse. If you can't get the two waveforms to overlay precisely on the `scope, measure the times in microseconds from the selected transition on the video to the start and end of sync. Adjust the sync depression timing so that it is in the same position relative to the video as the sync is on the incoming video. This sounds tougher than it is! Consult your modulator's schematic or block diagram for adjustment locations. Adjust the sync depression level so that the RF at sync is a bit higher than the blanking level and the edges of the sync pulse can be identified easily. The next step is to adjust the pulser timing. While it is possible to set this timing by firing up the transmitter and looking at the output of the klystrons, I find it much faster and much easier to first set the pulser timing by matching it with the RF waveform we were observing in the previous step. Move the oscilloscope's probe to the output of the modulator, after the SAW filter and all the IF correction circuits, just before it goes into the upconverter. Connect the other probe to the LED driver feeding the fiber cable to the pulser in the klystron cabinet. Because there won't be any video at this point, it is important both `scope channels be exactly in time. I find the "add" feature on the scope a handy way to insure this. Do not leave the scope triggering the same position used earlier. It won't work! First adjust the pulser "delay" or "leading edge" adjustment to match the rising edge of sync on the RF waveform, then adjust the "width" or "trailing edge" adjustment to match the falling edge of the waveform. If the transmitter has multiple klystrons, repeat this for each klystron's pulser fiber driver/ If you can't get the timing to match, stop! I found that the output of the Advanced Broadcast Systems pulser when connected to the sync pulse output on the Marconi B7500 modulator at KVEA would not match the sync on the RF waveform. The Marconi's sync pulse had too much delay for the A.B.S. pulser. I was able to bypass some of the delay stages in the Marconi and bring it in range. If you can't get timing to match and a modulator modification isn't an option, consider using a small, stable sync generator genlocked to the incoming video to generate the drive for the pulser controller. Once it is set, resist the temptation to make timing tweaks on it rather than in the pulser controller. Once these steps are completed, the pulser should require only minor adjustment when the transmitter is placed on the air. Shut off the transmitter RF drive. Without the pulser connected, set up the mod anode voltage for non-pulsed operation as suggested by the tube and transmitter manufacturers. Connect the pulser and set it for about 300 volts of pulsing. Without applying RF, bring up one of the visual klystrons. Verify beam current is less than without pulsing. Slowly increase RF drive. Verify there is sync on the output of the klystron and it is in the right place. As power is increased, if sync becomes too long, adjust the sync depression in the exciter to shorten it. Continue increasing RF drive until operating power is reached. Make a rough adjustment of exciter linearity correction. At this point, try increasing pulser voltage, which will increase sync level, then adjust the sync depression to restore it to its proper level. Readjust the linearity correction as this will affect the sync level. Depending on the capability of your exciter, you should be able to operate with 600 to 900 volts of pulsing. This procedure is somewhat different than what I've seen in manufacturer's manuals. Ideally, you should be able to set the pulser at 800 volts or 900 volts as required to establish a 75% blanking level then adjust the modulator sync level as needed to make 100% power at sync. In reality, several factors may prevent this from happening. First, increasing the pulser voltage reduces the current available to the tube for video. As a result, the tube becomes less linear and more linearity correction is required. Many older exciters don't have enough correction to allow the klystron to operate at maximum efficiency. Second, the tube has less gain at the lower current, requiring the solid state driver amplifiers to put out more power. If you take a look at the output of the modulator with the pulser operating as described above, you will likely see that more power is required from the driver amplifiers at black picture than at sync when the klystron current is much higher. It is possible that the transmitter's driver amplifiers can't deliver enough power to allow full pulsing. Setting up the pulser as I described above will allow you to stop increasing pulsing voltage before you run out of exciter correction or driver power. Once a reliable klystron operating point is established, I recommend checking the tuning of the tube with pulsing on. That is because the shift in current will change the tuning of the cavities. A few years ago I devoted a column to klystron tuning. Refer back to it if you need a refresher. Now you can make the final adjustment on the pulser timing. Using the "envelope" detector mode of your TV demodulator or using a diode demodulator, comparing the position of the sync pulse coming out of the transmitter with that going into the exciter. I found the easiest way to do this was to take a full set of measurements on the incoming video timing and then fine tune the pulser timing to match them on the output. Once the horizontal sync pulse timing is set, I like to use the transition from the vertical sync to equalizing pulses (line 6 with 525 line video) to verify everything is legal. It is possible to get combinations of ICPM, sync depression and pulser timing that will make the horizontal sync look okay but which will not properly handle the transitions in the vertical interval. While minor adjustments of sync depression timing may improve the sync rise times, make them with caution. Once these adjustments are completed, set the demodulator and waveform monitor for ICPM measurement and adjust the ICPM phase adjustment for least ICPM. Adjust ICPM timing for minimum over-shoot/under-shoot on the leading and trailing edges and recheck ICPM phase. The best looking sync pulses should occur with the lowest ICPM. If your test set up does not permit measurement of ICPM, adjust the ICPM phase and timing using synchronous detection until it matches the appearance of the sync pulse in the envelope detection mode. If your transmitter has only one visual klystron, that's it. If the transmitter has multiple tubes, adjusting the additional tubes is easy. Match the D.C. levels (un-pulsed beam current, beam voltage and pulser voltage) with those of the first tube. Remove RF drive, turn on the second klystron, bring up the drive and make minor adjustments to the timing (for that tube's pulser only!) as needed to match the input of the modulator and the first tube. Have some fixed attenuators on hand if you don't have a wide range adjustable attenuator for each tube. If, at full power, the klystron sync pulse is long or short, do not adjust the exciter. Adjust the pulser voltage to change the blanking level of the tube and hence the sync level. Some readjustment of RF drive to klystron may be required after this adjustment. If low frequency non-linearity looks bad on the second tube, try adjusting the pulser voltage. If after this adjustment the sync amplitude is wrong, a minor adjustment of the mod anode voltage will be needed. There are a lot of things that interact with pulsed klystrons. Here's a summary of the handles available when setting up a pulsed klystron transmitter. The modulator sync depression adjustment sets the RF power level at sync relative to blanking. The pulser voltage sets the current of the tube at blanking. Increasing pulser voltage reduces current, reducing gain and reducing power at blanking without changing power at sync. The end result is the sync will lengthen. Decreasing pulser voltage increases current at blanking, increasing the gain of the tube and improving linearity. The end result here is that sync will shorten, even though the tube is drawing more current. This is because the blanking level is now at a higher power level. Adjusting the mod anode voltage for more tube current will increase the DC power available to make sync, restoring the sync pulse length. I hope this explanation clears up some of the points I often find lacking in transmitter instruction manuals. A thorough understanding of what is going on in the transmitter is essential. I mentioned three factors at the start of this dissertation. Even though I touched on them during the timing and setup procedure, they deserve some more attention. Most of the problems I've seen when trying to make older transmitters work efficiently with low voltage pulsers stem from a lack of RF drive, often combined with insufficient exciter linearity correction. I recommend using a driver amplifier capable of delivering up 50 watts of drive per tube for high UHF channel 4 cavity klystrons or S-tuned 5 cavity klystrons. For lower UHF channels, 100 watts of drive power per tube may be needed. If limited correction is a problem, the best solution is a new exciter. The original transmitter manufacturer, if still in business, can often provide an upgraded exciter system with drivers. Harris completed a low voltage pulser upgrade at our NYC station, WNJU. The upgrade is working fine. The lower power consumption as a result of the pulsing has significantly reduced the heat load on the cooling system. I.T.S. (412- 873-1500) is another company experienced in building replacement exciters and driver amplifiers for a wide variety of older transmitters. One final note -- Often transmitter work is done late at night. During the installation and setup of a pulser it will be necessary to work near high voltage circuitry both inside the pulser and inside the transmitter. Do not take shortcuts. Do not take chances. Make sure AC power is off at the breaker before going inside the transmitter and use a shorting stick to make sure. the voltage is gone. High voltage capacitors can retain a nasty charge and sometimes breakers or contactors can fail leaving one leg of the power on. Have some one around in case something goes wrong. Next month's column will focus will on new technology when I summarize the World Media Expo / Society of Broadcast Engineers / Society of Motion Picture Television Engineers / NAB Radio / Radio TV News Directors' Association combo show in New Orleans. ((## US EDITION ONLY ##)) Before I close the column this month, I want to alert you to some new developments at the Federal Communications Commission. International Bureau Docket 95-117 proposes several changes to the licensing of satellite space and earth stations. On the earth station side, the F.C.C. proposes streamlining the licensing process by extending the license term for C-band transportable earth stations to 10 years instead of 1 year. IB Docket 95-117 would also direct reporting of transportable earth station operations to a central notification point in Washington D.C., rather than to the FCC's Engineer In Charge in the area of operation. Another provision eliminates the bandwidth limitation for digital carriers. Application forms will also change. While many of the provisions do not apply to the operations broadcasters typically use satellite communication for, it is worth a read if you own an uplink or transportable. You can find the entire text in the International Bureau's section of the F.C.C. Internet site (www.fcc.gov or ftp.fcc.gov) as file FCC95285.TXT. Perhaps the most interesting document to emerge from the FCC this summer is their "Fourth further notice of proposed rule making and third notice of inquiry" in the matter of "Advanced Television Systems and Their Impact Upon the Existing Television Broadcast Service", Mass Media Docket No. 87-268, released August 9, 1995. In the filing the F.C.C. recognizes that the nature of Advanced TV has changed from what they originally envisioned. Instead of one HDTV channel, broadcasters will be able to offer multiple SDTV (Standard Definition TV) channels and other digital services. Much of the filing deals with how to handle the programming and policy issues this change generated. What will interest my readers is the proposed changes in the implementation time table and the repeated statements that the Commission wants to recover contiguous spectrum from broadcasters, even if it requires stations to make additional channel changes. Here's a quote from the NPRM outlining one change in the time table the Commission proposes for the transition: Proposal. We propose to establish a procedure by which broadcasters have six months in which to make an election and confirm to the Commission that they want an ATV license. After that, they would have the remainder of the three-year period in which to supply supporting data as we may require. If they elect not to construct an ATV facility, or elect to construct but do not proceed to do so, their NTSC licenses will expire at the end of the ATV conversion period and they will have to cease broadcasting. This process would have the benefit of identifying early on locations where existing broadcasters do not want to transition to ATV and where applications from new entrants for ATV stations could therefore be considered. You can find more information on this and a full copy of the NPRM at the FCC's Internet site. The document is available in WordPerfect binary format only as file FCC95315.WP in the NPRM subdirectory of the Mass Media Bureau section. If you need help navigating the FCC's Internet service check out the links in the RF Links section of my World Wide Web RF Page site. The F.C.C. continues to improve offerings on their World Wide Web site. The complete engineering database for AM, FM and TV stations as well as the Directional Antenna databases are now available for downloading from the FCC's Internet site. You'll need a special decompression program to extract the files. Links to the TV databases as well as links to the files you'll need to extract them are available at my Web site. ((##END OF US ONLY SECTION ##)) That's it for this month. Be sure to check my RF Page on the Web for updates and more information. As promised I now have the complete MSTV proposed ATV channel allocations for all 50 states as well as Alaska and the Virgin Island posted. I've also updated the RF Links section to include links to tutorials and papers on MPEG compression and satellite technology as well as some interesting papers on the future of digital broadcasting. Check back every week or so for updates. I welcome your comments. My email address is dlung@gate.net or dlung@ix.netcom.com. Compuserve users can use my Compuserve PPN - 70255,460. You can also fax me at 305-884-9661 or phone me after 6 PM eastern time (when things quiet down a bit) at 305-884-9664. Both numbers are at the Miami Telemundo office, so expect a delay in a response if I'm traveling. My mail service address is 2265 Westwood Blvd., Suite 553, Los Angeles, CA 90064. Because I'm often traveling, if time is critical please contact me for a local address before sending items by mail. Copyright (c) 1995 H. Douglas Lung ALL RIGHTS RESERVED