RF Column Number 36 - October 1994 Copyright (c) 1994,1995 H. Douglas Lung ALL RIGHTS RESERVED TOPICS: Answers to questions about my Standard Frequency Receiver Comments on my column on power line harmonics Dielectric's wide-band "DigitLine" L-band interference and digital satellite receivers - cures? Acrodyne's new higher power solid state UHF amplifiers Shotmaster 100 "hand-held ENG" transmitter Transmitter tube life survey - initial request for info Parts sources for the standard frequency receiver/calibrator ---------------------------------------------------- Last month I promised to answer some mail I've gotten over the last several months. I'll answer some questions I received on the Standard Frequency Receiver I described several months ago, describe the test results Dielectric Communications sent me for their "DigitLine" coaxial transmission line system and briefly comment on info other companies have been kind enough to mail me. I've also uncovered yet another source of interference to our digital satellite transmissions. Fortunately there is a fix. Acrodyne showed me what they've been up to with LPTV transmitters during my visit there last month. Details follow. If none of this interests you but you work with UHF transmitters, skip to the end of the column to find out about a survey of transmitter tube reliability I need your help with. I received several letters and EMAIL messages regarding the Standard Frequency Receiver - Calibrator I wrote about six months ago. The most common request was for a more detailed parts list and list of suppliers. You may remember that I said that most of the parts weren't that critical and substitutions should work fine. That still applies - the guidelines for part selection is in the article. Table 1 is a list of sources for the parts. The latest Digi-Key catalog didn't include the NE602 I.C., but you should be able to obtain it from Ocean State or Ramsey. I don't have a PC board available for this project, but I see no reason why the HR-30 made by Ramsey Electronics, Inc. (see Table 1) couldn't be modified to match my circuit with a little trace cutting. This kit includes all I.C.'s necessary for the project. It also includes a tuned input circuit and components that replace C1, C2 and L1 in my diagram. You'll have to add the crystal and reference output buffer (MPF-102 FET) along with the related components. One letter raised concerns about my Standard Frequency Receiver's use of "direct conversion". The writer had used other direct conversion receivers and found them to have all sorts of problems. The secret here is to use batteries, an A.C. power supply or adapter to power the receiver. Do this and it will work fine! I did not design my receiver to be stable enough for use as a standard when it was not receiving an over the air standard frequency station for comparison. International Crystals (listed in table 1) sells a wide variety of crystals and oscillators if you want to build a more stable reference. Also, don't ignore oscillators you might have lying around in obsolete equipment. The old TFT Model 701 modulation and frequency monitor used a very stable 10 MHz. reference oscillator. The Standard Frequency Receiver - Calibrator isn't a difficult project to build. Drop me a line if you have problems locating any of the parts or have difficulty making it work. My column on power line harmonics in the June TV Technology also generated some comments. While most were horror stories about harmonic problems, one was from Richard Stephen, whose NAB paper was the basis of those comments. He wrote me clarifying two points, saying "Zero phase sequence filters (Zig zags) whether used in association with a UPS or not will increase the harmonic current between the filter and the load as well as exacerbating some fault conditions." and adding "The exploding transformer was at a TV transmitter site." Unlike many of the NAB presenters Richard was very low key in selling his company and services during the presentation and I missed his address and phone number. Here it is. For more information, contact him at IMMAD Broadcast Services, 3235 14th Avenue, Markham, Ontario, L3R 0H3, Canada, Telephone 905-470-2545, Fax 905-470-2559. Joseph Zuba at Dielectric Communications mailed me some information on the factory tests they did on their new "DigitLine" wideband coaxial line. If you're familiar with standard rigid coaxial line used at UHF TV frequencies, you know that this line is available in slightly different section lengths. This is necessary because reflections from the flanges will add if the flanges are spaced at multiples of the wavelength of the visual carrier frequency. As a result, no rigid line was considered suitable for use over the full UHF band. In those countries where sharing transmission line and antennas between stations is common, high power channels are usually grouped so that one antenna and line system does not have to cover the entire UHF band. At lower power levels, flexible line can be used which does not have this limitation. Dielectric's "DigitLine" rigid 6-1/8" 75 ohm transmission line was designed to handle the requirements of U.S. broadcasters for multiple channel transmission of Advanced TV as well as NTSC signals on the same antenna. It should also have application for non-digital multiple channel transmission in other countries as well. The results Joseph Zuba sent me show that the VSWR for a 1000' section of line was less than 1.1:1 from 470-806 MHz. -- the entire U.S. UHF band. A plot of a 6-1/8" elbow showed three VSWR peaks under 1.01:1 spaced approximately 120 MHz. apart. The VSWR at the two minimums was under 1.002:1. It is difficult to discern the pattern in the peaks in the straight line sweep. The peaks tended to be very sharp and Mr. Zuba said the average VSWR for the 1000' line was around 1.04. Its worth a look if you are considering replacing transmission line or building a new facility that needs full UHF band capability. Dielectric Communications is at P.O. Box 949, Raymond, Maine 04071 (U.S.A.), telephone 207-655-4555. After the last two columns, I promise I won't devote more than a paragraph to our digital video compression conversion this month. While investigating some interference problems to our digital downlinks I found a new source of interference, one that you may not be aware of. This interference will affect analog as well as digital signals, but the problem is more likely to be noticed in a digital system. Most of us are used to searching for C band interference in C band - 3700-4200 MHz. or thereabouts. I've found the new, low noise, inexpensive LNB's are also quite sensitive to signals in the L band range - 950-1450 MHz. You know about all the new paging and trunking gear going in around 900 MHz. A little bit higher in frequency and you'll find aircraft transponders. Up around 1200 MHz. the FAA has some long range (that is, high power) radar transmitters. Around 1250 MHz there are amateur radio operators. I found one interference cure that's always worked - installation of a .3700-4200 MHz. bandpass filter between the LNB and the feed horn. Such filters are available from Microwave Filter Company in New York (800-448-1666) and Dawn Satellite in Michigan (810-969-0377) for around US$600. It seems to me that a short section of C-band waveguide should also work - perhaps a couple 90 degree elbows back to back. The L-band signal shouldn't make it through the waveguide. This fix also moves the probe in the LNB away from the focal point of the satellite dish. I haven't received any comments back from stations as to whether this simple fix works or not. Joe Wozniak and Dr. Timothy Hulick from Acrodyne were kind enough to spend some time showing me their new LPTV products. Last month I spoke about how impressed I was with the design and construction of the I.T.S. Model ITS-830 1 KW transmitter. I hope to get to I.T.S. soon to take a close look at that model, which is currently shipping. The new transmitter I saw at Acrodyne undergoing testing for FCC Type Acceptance did not seem as revolutionary as I'd expected. Some of this may be due to my owning a range of Acrodyne solid state LPTV transmitters starting with one of the first and ending with two purchased late last year. The inexpensive exciter and upconverter system had some incremental improvements which significantly improved video performance with respect to group delay, frequency response and ICPM. More correction handles have also been provided, though it doesn't match the flexibility of the I.T.S. exciter or FCC Part 73 broadcast exciters. What I did find exciting about Acrodyne's new transmitter was the final amplifier stage. In conjunction with a semiconductor manufacturer they don't want me to reveal, Dr. Hulick developed a transistor which can safely handle over 100 watts per device. Furthermore, he has simplified the protection and bias circuitry, taking advantage of some of the characteristics of the new device. Looking at the circuits, I think the simplicity should make them more reliable. Dr. Hulick's digital transmitter (which I've written about before) required many UHF RF combiners, each of which had the potential to degrade efficiency. He's taken some of the technology he developed for this project and used it in the solid state UHF amplifier modules. What was the result? He was able to create a 600 Watt amplifier module in the same space previously used for a 300 watt amplifier module. The 600 watt amplifier module uses less parts, is simpler to manufacture and easier to adjust. What does this mean to the customer? The cost of a 1000 watt final amplifier stage using these modules is expected to be significantly less than that of the old style 1000 watt final. As a result, Acrodyne expects to be able to sell a bare-bones 1000 watt LPTV transmitter at a lower cost than before. On the other hand, if you're a customer like me who would like to see a little better quality, the savings in the final amplifier could be put into buying a higher quality exciter system. Watch the ads for info on this new product after its type accepted. This amplifier should also provide a good basis for building higher power solid state transmitters in the 5 KW and perhaps even 10 KW range. We'll see at NAB '95. This next item is almost a year old, but some of you involved with ENG and field production you may find it interesting. TransVideo Systems, Inc., 110 Pacific Avenue, San Francisco, CA 94111, telephone 415-564-2758 received FCC approval for a small microwave transmitter they call the "ShotMaster 100". The ShotMaster 100 operates in the 2400-2483.5 MHz. range under Part 15 of the F.C.C. regulations. It's designed to mount on the back of a cam-corder and transmit the video, audio, timecode and tally signals to a receiver up to 100 yards away. Unlike some of the 900 MHz. units available for home use, this is an FM modulated system. For longer range, more critical applications like transmitting a signal from a camera to a news van without cables, they sell the ShotMaster 200, which uses standard FCC Part 74 2 GHz. frequencies. Range is claimed to be over 1000 feet line of sight. I'd think interference from higher power 2 GHz. ENG units would be a problem in larger markets. I'd also expec t multipath to be a problem if the photographer moved around. If you've used these products, I'd be interested in hearing how well this technology works, particularly the unlicensed ShotMaster 100. While I was at Acrodyne, Joe Wozniak (as he usually does) took time to remind me how reliable their 25 KW UHF tetrode tube transmitters were. He mentioned stations were routinely getting life around or better than 20,000 hours from these tubes. I'd like to take a survey of readers using UHF transmitter tubes to see what kind of tube life stations are actually getting. Using one of the method of reaching me outlined below, please send me the following information: 1) Manufacturer of the tube (EEV, Varian, Eimac, Phillips, Thomson, etc.) 2) Model number of the tube (if available) 3) Description of tube (integral klystron, external cavity klystron, Klystrode, IOT, tetrode, MSDC klystron, number of cavities (if klystron). 4) Transmitter make and model 5) Channel and power output (per tube) 6a) Current hours on in service tube (or) 6b) Number of hours tube operated before it failed 7) Longest life on a tube of this type (either in service or hours before it failed) and power the tube was operated at. 8) Any problems associated with the tube or its associated systems? (Stability, arcs, cavity problems, cooling system problems, etc.) Don't worry if you don't have the hours down to the tenth -- "ball park" numbers are okay if you give me a range for accuracy (i.e. - 39,000 hours +/- 500 hours.) Any comments on the failures (how they failed, was other equipment involved?) would be helpful. All participants will be kept anonymous unless you have an interesting observation you indicate you wouldn't mind me sharing here. Varian had an interesting chart they distributed at NAB a few years ago which showed each MSDC Klystron they built and how many hours it had been in service. I've found at our stations we are getting 30,000 to 40,000 hours from both integral and external cavity klystrons. We have ten Varian integral five cavity klystrons in sockets, three Varian external four cavity MSDC klystrons and ten EEV external four cavity conventional klystrons in service now. All except the integral cavity tubes operate at over 60 KW output in visual service (to compensate for combiner losses) and from 12 to 18 KW in aural se rvice. Except for one MSDC that we had to return after six months because of collector arcing, all of our original MSDC tubes installed in early 1990 are still in service. A short description like this is adequate if you don't have time to do a full report. I rather get a short response from a large number of stations than a detailed response from only a few. Send your survey answers, along with any other questions or comments, to me at fax number 305-884-9661, via the Internet to "dlung@gate.net", via CompuServe to "70255,460" or via telephone 305-884-9664. I'm usually very busy during the day but feel free to leave a message on the voice mail. The best time to reach me is after things quiet down - usually between 6:00 PM and 7:00 PM Eastern Time or before 10:00 AM in the morning. I've been swamped since NAB, so I'll apologize here if I haven't responded to your letter or question yet. Jog my memory by computer, fax or phone and if possible I'll get back to you right away. I welcome your comments and questions. Mail is the slowest way to contact me and it usually takes the longest to respond to. If, due to location or telecom restrictions you must write, send mail to me at 2265 Westwood Blvd., Suite 553, Los Angeles, CA 90064. ---------------------------------------------------------------------- Table 1 - Parts source for Standard Frequency Receiver Source for resistors, all fixed capacitors (Panasonic), Sprague Goodman variable capacitors, CTS crystals, MPF102 FET, LM386 I.C., JW Miller RF Chokes (L3), connectors, battery holder and accessories (good general distributor): Digi-Key Corporation 701 Brooks Avenue South P.O. Box 677 Thief River Falls, MN 56701-0677 Phone 800-344-4549 FAX 218-681-3380 --------- Source for resistors, fixed capacitors, MPF102 FET, LM386 I.C., connector, batteries, battery holder and accessories: Radio Shack Division of Tandy Corporation Check local phone directory - U.S. and Great Britain -------- Source for toroid cores used for L1 and L2 (also good for any iron powder or ferrite core needs): Amidon, Inc. 2216 East Gladwick Street Dominguez Hills, CA 90220 Phone 310-763-5770 Fax 310-763-2250 -------- Source for semiconductors, including NE602 IC, toroid cores, crystals Ocean State Electronics 6 Industrial Drive Westerly, RI 02891 Phone 401-596-3080 Fax 401-596-3590 -------- Source for high stability crystals and crystal oscillators (TCXO and Oven): International Crystal Manufacturing Company, Inc. P.O. Box 26330 Oklahoma City, OK 73126 Phone 800-725-1426 405-236-3741 Fax 800-322-9426 -------- Source for kit of parts for tunable receiver that can be modified to match my design: Ramsey Electronics, Inc. 793 Caning Parkway Victor, NY 14564 Phone 716-924-4560 Fax 716-924-4555 -------- For information on construction techniques, see "The ARRL Handbook for Radio Amateurs", published by The American Radio Relay League, 225 Main Street, Newington, CT 06111-1494, phone 203-666-1541. Ask for a catalog -- they have books covering a wide range of RF topics. Copyright (c) 1994,1995 H. Douglas Lung ALL RIGHTS RESERVED