RF Column 26 - December 1993 Copyright (c) 1993,1995 H. Douglas Lung ALL RIGHTS RESERVED TOPICS: SBE 1993 - Miami - Report on FCC & Regulatory sessions: Where is my tower? NAD 27 vs. NAD 83 Coordinates Spurious aural modulation in common-mode transmitters How to compare TV transmitter performance before you buy New dehydrators from Andrew Corporation --------------------------------------------------------------- Last month I ran out of space with my report on the SBE convention in Miami. I'll finish it up this month with some tidbits gleamed from Saturday's FCC/Regulatory sessions. I moderated a session on evaluating common-mode TV transmitter BTSC performance. There's more to evaluating a transmitter than looking at the data sheet. I'll pass along some tips on what to look for. I usually try to include items of interest to non- broadcasters using RF, primarily satellite operators. You won't find too much here this month, but drop down to the end of the column for some information on a nifty new product Andrew Corporation showed at SBE / RTNDA this year. During the morning sessions, Dane Ericksen, a senior Engineer at Hammett and Edison, pointed out that we are currently working with two sets of coordinates. The FAA is using the most current set NAD 83 while the FCC is still using NAD 27 coordinates. At some time, the FCC will convert all NAD 27 coordinates to NAD 83. The difference between the two coordinate systems increases the further you are from Kansas. What this means is that when you file an application that requires a notice to the FAA, the coordinates on the "Determination of Non-Hazard" will usually not match the coordinates for your site on the FCC application! At some point, the FCC will convert the NAD 27 coordinates to NAD 83. The best way to make sure errors don't occur on your applications is to specify which coordinate system was used in pinpointing the site. NAD 27 coordinates are the ones you get off of the Geological Survey topographic maps. NAD, by the way, is short for "North American Datum" and the 87 or 27 refer to the date on the datum - 1987 or 1927, Some GPS receivers can convert between the two coordinate systems, but the only approved method for conversion is contained in the NADCON program, available from the National Geodetic Survey Information Branch, N/CG174, Rockwall Building, Room 24, National Geodetic Survey, NOAA, Rockville Maryland, 20852, telephone number 301-443- 8631. I don't have information on the cost of the disks. My panel had a rather esoteric title - "Common-Mode TV Transmitters: What the FCC Expects, What you Should Demand" - but the discussion was lively. Panel members included Dane Ericksen (from Hammett and Edison), Nat Ostroff - President of Comark, Frank Svet - Director of Engineer for Harris Corporation's TV product line, Dr. Byron St. Clair, Chairman of T.T.C. and Bob Unetich, President and founder of I.T.S. The FCC was represented by Gordon Godfrey from the Engineering and Policy Branch of the Mass Media Bureau. Dane Ericksen pointed out that non-linearities and intermodulation in the common mode transmitter result in the video carrier modulating the aural carrier at sync rate and harmonics of the sync rate. Since the horizontal sync frequency is used as a pilot carrier for the BTSC (stereo) signal, it could cause false triggering of the stereo circuitry on TV receivers. Dane and the manufacturers had requested a clarification of the rules on BTSC pilot protection from the FCC. Gordon Godfrey, from the F.C.C. Mass Media Bureau, quoted from a letter the FCC released just before the SBE convention. The letter stated that the FCC did not perceive the lack of BTSC pilot protection to be a problem in transmitters not transmitting stereo. On the other hand, it left open the possibility of citation for over modulation if the total modulation, including the spurious products, exceeded the 25 KHz. peak deviation permitted for monophonic TV FM broadcast. I asked Dane Ericksen how you can measure this spurious modulation on a non-stereo transmitter. The conventional Belar or TFT mono modulation monitor will respond to the 15.735 KHz. modulation but it won't measure the harmonics of the sync frequency, which can extend out past 200 KHz. Dane uses a spectrum analyzer capable of measuring signals down to the audio range connected to the wideband demodulated output of a Hewlett Packard modulation test set. The spectrum analyzer reference level is calibrated using a tone modulating the carrier with 25 KHz. deviation, checked using a Bessel null function (see my previous column on FM deviation measurements for details on how to do this). The tone is then removed and the amplitude of the spurious signals can be read directly off the spectrum analyzer display. The amplitude of all the spurious signals are summed to give the total deviation. There has been some opposition to adding these spurious signals together. One alternative suggested is taking the square root of the sum of the squares of the spurious signals. This results in a lower total deviation. Dane said he did not see anything in the FCC rules that would permit using this method and that if it was permitted, what would stop a manufacturer from taking the cube root of the sum of the cubes to reduce the spurious deviation even further? While the panel didn't disagree with the assessment, there was some concern about including frequencies well outside the normal bandwidth of a TV receiver. Comark's Nat Ostroff commented that he had seen a case where there modulation of the aural carrier by the video sync and harmonics of sync was so extreme that in weaker signal areas the aural carrier couldn't be demodulated even though the video signal was still watchable. Nat explained this was because the spurious modulation of the aural carrier put much of the power for the aural carrier outside the detection bandwidth of the TV receivers, effectively reducing the aural power level significantly. Comark was hired to solve the problem and Nat Ostroff used the correction circuit he developed for Comark's high power common-mode transmitters. It worked. Frank Svet noted that Harris as well had developed a correction circuit that reduced spurious aural carrier modulation, but because it was new he was not able to release the technical details on it. Most of the common-mode transmitters manufactured by TV Technology Corporation (TTC) and I.T.S. operate at lower power levels - under 10 kilowatts. TTC particularly has a wide range of experience with common-mode Low power TV transmitters and translators. Dr. Byron St. Clair indicated that TTC has not heard of any problems with spurious aural carrier modulation in its translators or low power TV transmitters, but acknowledged that TTC would consider adding aural carrier correction circuitry to its higher power transmitters if the market required it. Bob Unetich said I.T.S.'s experiences matched TTC's. He had an interesting discussion with one of the engineers in the audience who noted that I.T.S.'s linearity correction circuitry works on the aural as well as the visual carriers. When this circuitry was properly adjusted, he noticed that the amount of spurious modulation indicated on a monophonic modulation monitor decreased significantly. After the session I was discussing the difficulty in measuring this spurious aural carrier modulation. Dane Ericksen told me earlier that he found that the newer Tektronix low end cable/broadcast spectrum analyzers in the 271X series go down to 10 KHz., low enough to look at the wide band aural output from a Tektronix demod. There is some question about how the TV set or demodulator affects this spurious aural modulation by video. In all TV's I'm aware of visual and aural carriers are amplified together and the aural carrier demodulated from the difference between the visual and aural carriers - 4.5 MHz. This raises several questions. One, which was noted by Dr. St. Clair, was that if the phase modulation by video sync of the aural carrier matches that of the visual carrier, the two should cancel out in the TV receiver. Second, while the spurious modulation of the aural carrier is corrected by observing the aural carrier only -- that is, it is detected separately on a precision demodulator -- when the visual and aural carriers are combined in the TV set's IF amplifier the circuitry itself will create spurious modulation itself in spite of the correction. Worse yet, different TV's will likely create different types of spurious aural modulation, making pre-correction for the TV difficult. I know of some tests being done on this receiver distortion and will be reporting on it here when I have solid data. For the present, I have to agree with Dane Ericksen and say the best approach is to transmit as clean a signal as possible. This should keep the FCC and your viewers (listeners?) happy. Whether or not you understand or agree with the arguments presented in this panel, I feel it shows how important it is to go beyond the spec sheet in evaluating a TV transmitter. The specification sheets on most TV transmitters match the FCC requirements, even though the transmitter may be capable of better performance. When buying a new high power transmitter, I work with the manufacturers bidding on the project to develop a tighter set of specifications. Notice I said I worked with the manufacturers on the specs. This cooperation is important for a couple reasons. First, the discussions should give you an insight into the skill of the manufacturer's engineers. Second, it will give you an idea how well the sales staff works with the engineering side of the company. It doesn't do you any good to have the salesman sign up for a great set of spec's if the engineers can't deliver it. Ideally, the company will be able to show you an existing transmitter that does meet the spec's you've developed. I recently rediscovered another way to compare transmitter performance. Before a company can sell a transmitter, it must be FCC Type Accepted. To obtain type acceptance, the company has to do a number of tests on the transmitter (refer to the FCC Rules parts 73 and 74 for details) and submit the results to the FCC's lab in Columbia Maryland. Request a copy of the FCC data from the manufacturer and take a critical look at it. Does it just meet FCC standards or does it exceed them? Pay particular attention to the spurious signals. Look at the lower sideband - it should roll off sharply 1.25 MHz. below the visual carrier. You can also ask for the proof of performance done at recent installations. Of course, none of these printed reports can duplicate supervised testing on a finished transmitter. Once you start tightening specifications beyond what the FCC requires, it is important to specify that all specifications be met simultaneously. If you've adjusted TV exciters, you know that it isn't too difficult to get excellent differential phase, gain and luminance non-linearity readings. The problem is getting all three to look good at the same time, at full power! Don McSherry, Andrew Corporation's manager of Media Communications, gave a huge packet of information at the SBE convention. If you've followed this column, you know I don't run many quotes or marketing material from manufacturers. I'd rather focus on the user. There were a few items in the package from Andrew that caught my eye. If you've been involved with RF systems, satellite, microwave or TV broadcast, you've probably dealt with those Model 40525 Andrew dehydrators with the screw on plastic canister of desiccant. I found that after a few refills (I usually cooked the desiccant to get the moisture out rather than replacing it), a few bits of desiccant would get around the top of the plastic canister and make it difficult to seal. Lately, I've found if I have a good, leak free Heliax transmission line, a tank of nitrogen and regulator is a lot easier way to keep the line dry. There are some applications where dehydrators work best - long runs of rigid line or large waveguide or areas where it would be too dangerous to have a nitrogen tank. Andrew has now replaced the old 40525A dehydrator with the new "MR-050" unit. They claim it draws low current (AC or DC) and can be adapted to a wide range of applications. The unit is quiet enough and has low enough vibration to permit rack mounting. Hopefully it will be easier to replace the desiccant in. It is more efficient and Andrew claims you won't have to change the desiccant as often. Sometimes you don't need to pressurize a line or feed - keeping it dry is sufficient. For these applications Andrew announced a small static desiccator, model SD-002 which can be connected right to the air fitting on the feed or line. You don't recharge the desiccant on these when it gets saturated, you throw out the whole unit. They claim a typical service life of approximately one year for a system with one cubic foot of volume experiencing a temperature change of 30 degrees F per day and a 40% relative humidity. This is a nifty idea for those roof top microwaves or satellite feeds where a dehydrator or pressurized line isn't needed. That's it for this month. I've been getting many calls about the Cheap Remote 2 in my October column. Next month, I'll pass on some tips about software for the Cheap Remote 2 and describe a new version of my remote control software available on CompuServe. If you have any questions or comments, please send electronic mail to my CompuServe ID, 70255,460. If you are on Internet, address the mail to "70255.460@compuserve.com". I haven't tried this yet, but it should work. ((8/95 > UPDATE! Use dlung@gate.net for Internet email)) Paper mail goes to Doug Lung at 2265 Westwood Blvd., Suite 553, Los Angeles, CA 90064. Fax stuff to me at 305-884-9661 or phone me at 305-884-9664. Please note I have a very hectic schedule. If you leave a voice mail, please indicate the best time to call and realize it may take a day or two if I'm on the road. Please do not call before noon as mornings tend to be very busy - the later the better. I'm usually in the office until 7 PM eastern time. Copyright (c) 1993,1995 H. Douglas Lung ALL RIGHTS RESERVED