RF Column 19 - April 1993 Copyright (c) 1993,1995 H. Douglas Lung ALL RIGHTS RESERVED TOPICS: Disaster recovery - lessions from the World Trade Center bombing Digital video compression update - standards? Radiation hazards - new IEEE standard C.95.1-1991 Using a TV sideband adapter and mixer as a sweep generator -------------------------------------------------------------------- A few days ago, the last Friday in February, I was on the phone talking to Gunter Auerbach from KUBD when he mentioned a big explosion had occurred at the World Trade Center in New York City. Within minutes our news department told me our New York station was off the air, along with every other transmitter in the World Trade Center. Even though I was in Los Angeles, I got the distinct feeling they wanted me to do something to get it back on the air, fast! I fought my first impulse, to call the Chief Engineer and ask what was happening. I've been in "off the air" situations enough myself to know the last thing needed is management calling asking "what's happening and when will be back on the air". After things calmed down some, I found out smoke had reached to the top of the building and all electrical power had been shut off, but there was no damage to the transmission facilities. Until late Friday, no one knew how long it would be before broadcast operations could resume at the World Trade Center. Our contingency plan called for shipping backup antennas and a low power antenna to New York as soon as possible. Fortunately, I didn't have to dismantle one of our LPTV sites to obtain the transmitter. Both Acrodyne and Television Technology Corporation had transmitters they could ship us within 48 hours. As it turned out, none was required as power was restored to the W.T.C. late Friday night. This exercise got me thinking about backup capability if a site is lost. Many stations have backup transmitters. Some have backup antennas. Few have backup transmission sites. I'm considering putting together an "emergency restoration" package using a frequency agile transmitter operating at 1 kW. Higher power would be nice, but the size and weight of the power supply would make it difficult to ship via air. It will probably use a tube, since tubes offer high gain and are easily tuned over the band. Tube transmitters also tolerate wide temperature variations better than solid state designs. Most high power solid state LPTV designs I've seen are optimized for a particular frequency range. A series of notch filters will be necessary to reduce intermod products to FCC standards. We have to consider the antenna part of the package too. The antenna has to be able to work on all channels and be small enough to ship by air. I found that it was impossible to find anyone who could ship a 21 foot Scala SL-8 antenna we had in California to NYC by air over the weekend. Fortunately, I had some panel antennas available. Panel antennas are be easier to ship and can be made wide band. Sira (sold by Micro Communications Inc. in the U.S.) and Kathrein offer wide bandwidth panel antennas. These look like the best comprise. The individual antennas are small and can be combined for different patterns as required. Kathrein's turnstile is even self supporting. A small satellite dish and receiver would complete the package. Monitoring equipment would consist of a tunable TV set with video output, a waveform vectorscope and test generator with ID generator. (### the above section can probably run in the international version, use your judgment. The folks in Europe probably won't understand it, but in So. America/Caribbean they should. Do NOT include the following paragraph in the international version) Perhaps we can persuade some enterprising transmitter company to offer such a "backup station" package. A monthly "insurance" payment would guarantee availability along with a daily or weekly rental fee. I'd like to get your feedback on this idea of backup stations. What would you want in the package? How much would you be willing to pay up front and how much per month to "insure" delivery of a spare emergency LPTV transmitter? Fax me a note at 305-884-9661. I'll put them together, report the results here and forward the results to manufacturers capable of putting together a package. I'll then report the manufacturer's responses (if positive) in this column. Video Compression, again... It seems like every day brings news of a new development in video compression. I've even seen articles talking about running movie quality video over conventional phone wires using compression. One thing is clear, the use of video compression for satellite transmission is growing fast, even though a standard has yet to be decided on. The Telecommunications Inc. (TCI) order for a million cable decoders from General Instruments and AT&T seem to be heading towards a standard they call MPEG 2-EP. MPEG 2-EP doesn't use B frames and hence requires slightly less video RAM. B frames are used for bi-directional compression. Other users are heading for an MPEG 2 standard that includes B frames. None of the existing systems fully comply with MPEG-2 standards. Scientific Atlanta's system, based on an earlier MPEG variation, is currently being phased in on PanAmSat. In Mexico, TeleRay is installing General Instrument's compression equipment at their uplink site for use with the MultiVision cable network. It is likely the systems being installed today will be outdated with new compression technology by the end of 1993. At present, the cost of encoders is still very high. If the prices drop as expected, look for SNG operators to start using video compression to squeeze more signals on one transponder. If the cost of a video channel drops proportionally, SNG will become a viable option for TV stations wanting wide area news gather capability without the capital investment in microwave relay sites. Zapped!... A few months ago I discussed the new ANSI RF radiation exposure standards in IEEE C.95.1-1991. As I noted, the FCC has been taking a careful, measured approach to implementing this new standard. The recent scare over cellular telephone induced brain cancer has put pressure on the FCC to move faster to adopt the new exposure standards. It is unlikely the new standard will pose much problem for UHF stations with high gain antennas, however, FM and particularly low band VHF stations will have to demonstrate compliance with much stricter exposure standards. At the SBE National Convention, Bob Cleveland from the FCC's Office of Science and Technology indicated the FCC would have a booklet similar to the OST-65 document that broadcasters could use to mathematically show compliance when the new regulations were implemented. Look for more coverage of this in the next few months. Using a sideband Adapter as a sweep Generator... Back to more practical matters, I was impressed with the method Acrodyne's Steve Blisseti used to check the antenna system at our LPTV installation in Odessa Texas. I've found most TV stations have a Tektronix 1405 sideband adapter and a spectrum analyzer. This combination does such a good job few stations bother with sweep generators now. The engineers at Acrodyne borrowed a technique originally used by Harris for doing a broad band sweep of RF systems, including tube cavities and antennas. It can also be used to check the response of the aural transmitter, something difficult to do without a sweep generator, markers and patience. The setup is simple. Refer to figure 1. The video output of the Tektronix 1405 sideband adapter is applied to the IF port on a doubly balanced mixer. For my setup I used the Mini-Circuits ZFM-2H, however, other mixers should work as well. Be sure to check the power handling capability of the mixer. The ZFM-2H can handle up to +14 dBm on the IF (video input in this application) and RF ports and up to +17 dBm on the LO port. The LO port of the mixer is connected to an RF source centered on the frequency of interest. The video sweep from the sideband adapter mixes with the carrier frequency on the RF port to generate a RF signal swept around the carrier frequency. Here's a practical application for the circuit. If you are using any sort of tube type transmitter, you've had to sweep the tube cavities for flat video response. The most common way to do this is with a sideband adapter connected to the video input of the visual exciters. If the exciter permits the vestigial sideband (VSB) filter to be switched out, the transmitter response over a wide frequency range is easily measured. Problems develop when the VSB filter or other items such as linearity and group delay correctors in the exciter can't be switched out. The mixer comes in handy here. Remove the video from exciter and switch off any sync adding circuits so that nothing more than a carrier comes of the exciter. This method will work with sync on, but the display is cleaner with it off. Some exciters kill the RF when video is removed, so it may be necessary to move a switch or jumper to get it back. Since the spectrum analyzer will be tuned to the output frequency of the transmitter, we need to grab the RF at visual carrier frequency. This will be after the up converter. Connect this to the LO input on the mixer. Most doubly balanced mixers can't handle more than +13 dBm (about 20 milliwatts) and the common rating is only slightly more than one milliwatt so either insert the mixer before the up converter's output amplifier or use pads to reduce the drive to a safe level. Connect the input of the next stage to the RF port on the mixer. In other words, put the mixer in line with the RF going from the up converter to the next stage. The normal video out of a sideband adapter is about one volt. One volt into a 50 ohm load is 20 milliwatts of power, or +13 dBm. There are a couple steps you can take to protect the mixer. Turn off sync, it shouldn't be needed on a broad band sweep. Lower the APL below 50 IRE. Use a 75 ohm to 50 ohm minimum loss pad to match the video output of the spectrum analyzer to the somewhat reactive 50 ohm IF port on the mixer. A 6 dB pad will drop the maximum power down to +7 dBm. For best response, use a 3 dB or greater attenuator on the RF and LO ports as well. This will reduce the effect of mismatches. It will also reduce the power applied to the tube and cavity - which is advisable if you are tuning it for the first time. When using the mixer, spectrum analyzer and sideband adapter tuning is the same as when using the visual exciter. The difference is that the output from the mixer will have a frequency response comparable to that of the sideband adapter. This can be useful when trying to locate frequency response problems or off channel resonance. The mixer will modulate any carrier frequency with the sweep signal. I find it most useful in tuning the aural tube. Stereo and multichannel audio systems demand a flat response around the aural carrier. There are several ways to test this response. One quick and dirty method is to overmodulate the FM carrier with a loud tone, so a lot of sidebands are generated outside the normal bandwidth. Don't try this on the air! It quickly shows if the response is sloped high or low, but is isn't easy to use to see if the overall response is wide enough. Modulation Sciences includes a noise generator in their "SideKick" subcarrier and pro-channel generators which does a more elegant job of displaying response on a spectrum analyzer. These methods are fine for fine tuning a klystron or tetrode but do not have a wide enough sweep to see responses removed more than a couple hundred kiloHertz from the carrier. They are useless for tuning a klystron, where the penultimate cavity needs to be tuned six megaHertz above the carrier. The little doubly balanced mixer works well for these adjustments. A clean source of aural carrier is easy to obtain - just remove all the modulation from the aural exciter and use its on channel output. The hook up is the same as for the visual exciter sweep discussed earlier. You will find that it is difficult to see frequency response very close to the carrier using this technique. The sweep is generated at a TV line rate in the sideband adapter and does not really go all the way to zero. I suppose an audio sweep generator could be connected to the mixer for this close in sweep, but I haven't tried it. At the start of this discussion I mentioned Acrodyne used this mixer to test the antenna system. Apply the wide band sweep from the mixer to the antenna through a directional coupler (the one that is used on the transmitter for measuring reflected power may work or, for more accuracy, use a calibrated lab type directional coupler like those made by Narda). Connect the reflected port on the coupler to the spectrum analyzer and view the return loss of the antenna system over a 15 MHz. or wider frequency range. Depending on the coupler, you might have to run the output of the mixer through one of the transmitter's wide band, low level driver amps to get a clean indication on the analyzer. Several companies make wide band doubly balanced mixers. I used Mini-Circuits because you can order from them in small quantities and they take credit cards. The ZFM-2H comes standard with BNC connectors. Other connector options are available. Mini-Circuits main number is 718-934-4500. You can reach their distribution center at 800-654-7949 or 417-335-5935. (### for international: replace sentence with "You can reach their distribution center for North America at 417-335-5935 or in Europe at 44-25-283-5094. In South America, contact HiTech in Brazil at 51-11-531-9355. Mini-Circuits Yokohama's number for Japan is 81-4-5545- 1673.) That's all the space for this month. Next month I'll give you details on an inexpensive isolation amplifier that can replace expensive chopper/transformer isolation boxes in transmitter remote controls. I'm still working on the 10 MHz. frequency standard that can be calibrated directly to WWV (or other frequency standard stations outside the U.S.). My goal is to make the design easy to build, but sensitive enough to work with a piece of wire thrown outside the transmitter shack. If I succeed, look for the cheap, simple design next month. I'll also pass along some info on what to expect from and how to participate in the Broadcast Professional's Forum on CompuServe. Several manufacturers now have representatives there and engineers from around the world share ideas and information. As always, I invite your comments. Suggestions for column topics are welcome, as are your tips and techniques for RF work with TV transmitters, satellites or microwaves. Don't worry about putting the tips into perfect prose. Hand draw the schematic, make a few notes and mail or fax them to me. The best way to reach me is through CompuServe, ID 70255,460. I can also be reached by phone at 305-884- 9664 or 818-502-5739. The second number has voice mail. Mail comments to me at 2265 Westwood Blvd., Suite 553, Los Angeles, CA 90064. ((8/95 > UPDATE! - Use dlung@gate.net for e-mail!)) Copyright (c) 1993,1995 H. Douglas Lung ALL RIGHTS RESERVED