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REMOTE CONTROL BY BOB YOUNG Minimising transmitter interference This month, we will examine a startling new development which has radically altered the design parameters for the new Mark 22 transmitter. In past issues of SILICON CHIP I have often referred to projects taking on a life of their own, once some heavy duty effort is directed towards them. The Mark 22 project is one of the best examples of this process in action that I have encoun­tered during my long career in R/C manufacturing. It began with me being dragged kicking and screaming by the editor of this magazine once more to the dusty stool in front of my old drawing board. He said he (and his readers) badly needed this R/C project and that my feelings in the matter were of no account. As this argument had raged on for some time, I finally realised that further argument was futile. With that, I reluc­tantly set to work to modernise my old Mark 14 AM system. “That should get him off my back”, I mused. From there I have watched the Mark 22 develop into one of the most versatile R/C systems on the market today. So much so that the model aircraft fraternity have greeted it with a degree of enthusiasm that has taken me completely by surprise. As their enthusiasm has grown so has my own. Each step in the development process A classic scene at an R/C car track. This is much less than the recommended minimum spacing of three metres. How many of these transmitters are interfering badly with each other and possibly causing loss of control? 72  Silicon Chip has led smoothly (and not quite effortlessly) to the next logical step, to the point that I now find myself once more at the cutting edge of R/C development here in Australia. However, I have got ahead of myself a little and I must return to the February 1995 issue of SILICON CHIP which featured the new Silvertone frequency keyboard. This keyboard is the latest in a long line which began in 1969 and now incorporates the new frequencies on the 36MHz modelling band. The MAAA (Model Aeronautical Association of Australia) had released these frequencies as a result of the latest range of very elaborate R/C equipment arriving from overseas and I was asked to prepare a new keyboard in anticipation of the operation of 10kHz spacing on the 36MHz band. It was widely believed that this generation of equipment would allow safe operation on 10kHz spacing. Well, the upshot of recent field testing is that the MAAA has decided that 10kHz spacing is definitely NOT SAFE. The new 10kHz frequencies will be used but only at 20kHz spacing. That is startling enough but another problem has come to light because of this close spacing proposal which is potentially more serious and it has been there all along. I am speaking of direct interference between transmitters when they are physically close together and operating on adjacent frequencies. What happens is that if two standard transmitters are oper­ ated close together they both radiate extra signals and these extra signals will be on frequencies which might be being used by other radio control transmitters at the time. So here is the scenario. Two transmitters are being operated close together and they both radiate inter- Fig.1: this frequency spectrum shows two conventional class C R/C transmitters spaced 20kHz apart at 27.175MHz and 27.195MHz. Note the interfering signals spaced 20kHz away at 27.155MHz and 27.215MHz. These signals are only 30dB down on the wanted signals. fering signals at the same frequency as another R/C transmitter on the same field. The result can easily be that the third model loses control and has a crash! No-one has done anything illegal and the poor unwitting victim is left wondering why it happened. Has this happened to you? What we’re talking about is 3rd order intermodulation. This type of interference is generated when two non-linear (class C) transmitters are operated in close proximity of one another. The 3rd order products (P) are generated according to the formula: P = (2F1 - F2)+ (2F2 - F1) Let’s put some actual operating frequencies into this equa­tion. If we have two transmitters operating at 27.195MHz and 27.175MHz (ie, 20kHz apart), they will produce interfer­ ing fre­ quencies at 27.215MHz and 27.155MHz. Note that these interfering signals are “legitimate” frequencies on the same 20kHz spacing. The effect is shown in the frequency spectrum of Fig.1 which is part of a series of tests I did for this article. This photo shows the two operating frequencies as the taller spikes while the unwanted frequencies on either side are only 30dB down. (Note that this and the other spec­tro­­grams shown in this article were taken with unmod­ ulated transmitters to give a clearer re­sult). The power of these interfering frequencies is inversely proportional to the square of the distance between the antennas; so the closer they are, the Fig.2: this frequency spectrum shows a class B transmitter at 27.195MHz and a class C unit at 27.215MHz. Note the lower ampli­tude unwanted signal at 27.175MHz, the result of the improved linearity of the class B transmitter. worse is the interference. This interference can be very powerful and quite capable of shooting down an aircraft. And if the two R/C antennas touch, as they easily can in the excitement of a race, the power of the unwanted products can be almost equal to that of the proper signals. I knew of the problem but had no real concept as to its magni­tude. During these tests I generated enough 3rd order product to lock out PCM receivers and drive them into fail-safe. That is not the end of the problem as there will also be 5th order inter­ modulation products and these are demonstrated in one of the spectrum photos (Fig.4). Hence, as well as the interfering signals noted above, there will also be unwanted signals at 27.235MHz. and 27.135MHz, although their power level will be reduced. This problem is a well understood by RF engineers. When working with multiple transmitters on a single tower, they spend considerable amounts of time minimising 3rd and 5th order inter­modulation products. Why does it happen? When a transmitter is operated close to a second transmit­ter, some of the radiated RF is absorbed by the second transmit­ter’s antenna and its tank circuit. This unwanted RF energy finds its way to the base-emitter junction of the PA transistor which is operating in non-linear class C mode. Because of this, it acts as a mixer and so the unwanted difference frequencies are ampli­ fied and radiated along with the transmitter’s proper signal. The second transmitter affects the first transmitter in exactly the same way, so both transmitters radiate the unwanted frequencies. I must emphasise that this interference problem has always existed but it becomes much worse when the frequency spacing between transmitters is reduced. It is bad enough when a spacing of 20kHz is used and is quite capable of causing crashes. But with 10kHz, the problem would be a great deal worse. How do you stop it? So what measures can be undertaken to eliminate this problem, or at least minimise the risk? First and foremost, the transmitters should be far apart; ideally no closer than three metres between them. Second, any­thing that attenuates the incoming RF will help and so a metal transmitter case is to be preferred. The Mark 22 transmitter will (naturally) feature a metal case. Third, a good way to minimise the problem is use a more linear transmitter circuit. So instead of using the conventional class C transmitter, a move to class B transmitters is very worthwhile and this is demonstrated in the spectrum photos of Figs.2 & 3. Here, one of the transmitters is a class B model and you can see that one of the unwanted signals is greatly reduced. If two class B transmitters are operated close together, the overall radiation of July 1995  73 Fig.3: this test is the same as Fig.2 except that the transmit­ ters have been swapped; class C at 27.195MHz and class B at 27.215MHz. In this case, the lower amplitude unwanted signal is at 27.235MHz. interference signals is reduced even fur­ther. Demonstrate it for yourself Many people express surprise at the thought of a transmit­ter absorbing power and re-radiating it, but it is acting purely as an absorption wave­meter and this can be easily demonstrated, without any need for test gear. If you have two transmitters with RF meters, switch on one and move the other’s antenna in close proximity to it, you will see the meter of the OFF transmitter begin to read RF from the ON transmitter. Move them closer to­gether and you will see the meter on the OFF transmitter register a substantial signal. So you can imagine that when that second transmitter is turned on, all hell breaks loose and the interfer­ence is rife. Here then is an explanation for the completely transient and random nature of some interference. Over the years I have spent hundreds of hours going through sets which have come in with vague complaints of “interference” and all of the sets have checked out perfectly normal and few have ever returned. Was it 3rd order intermodulation? There is no way of knowing but it is highly probable. Having said all of this, I must state also that there is no need for panic. Safe field procedures will eliminate the problem completely and these include separation of each transmitter by a minimum three metres, field testing of suspect transmitters, placing adjacent transmitters at the opposite 74  Silicon Chip Fig.4: taken at a different screen refresh rate, this spectrogram reveals the presence of 5th and higher order interference pro­ducts, as well as the 3rd order signals. end of the flight line and finally if necessary, changing the frequency of suspect systems. Finally, if you see three keys in the keyboard on adja­cent channels and yours is one of them, then be doubly alert as to where the other two transmitters are located while you are flying. Why have 10kHz spacing? The proposed introduction of the 10kHz spacing system was primarily to enable large clubs to increase the amount of activi­ty per hour. Remember here, it is not that anyone particularly wants 60 aircraft in the air at once – nothing is more unpleasant than a crowded sky. The idea is to free up channels so that testing, motor tuning and field adjustments, all activities that tie up frequencies for long periods, may be carried out. Plus, the more frequencies that are available means less frequency clashes and fewer accidents. Proposed transmitter After these tests, I was faced with a dilemma. I have just designed a brand new (class C) RF module and now it is clear that a class B or better still a class AB (linear) PA would minimise the problem and thus make operation on busy fields that much safer. Hence, I have no hesitation in delaying the design to incorporate a vital feature for safe operation. I want the Mark 22 to be as good as I can make it. Thinking about it, I cannot understand why this problem has not been analysed and solved long ago. The Americans obviously understand it for they recommend a minimum of three metres sepa­ration and even go so far as to place boxes on the flight line three metres apart and each pilot must not leave his box. Some American clubs even go to extremes and recommend 10 metres sepa­ration. But even then they can run into problems, as indicated by the landing strip diagram of Fig.5. If we adopt the practise of separating two adjacent trans­mitters by putting them at opposite ends of the flight line, then the aircraft comes much closer to an interfering transmitter and the problem of signal strength ratios begins to become a factor. This is a separate issue to 3rd order interference and is purely related to system bandwidth. Fig.6 shows a simple go/no-go test for determining the safety of operating two R/C systems simultaneously. Here, the signal strength ratios are related to distance and a minimum of 12:1 is called for. To go closer than two metres to the receiver distorts the test due to overload of the receiver. In this regard I recommend that all transmitter antennas be telescoped when entering the pit area. Notice the similarity of this test to the conditions illustrated in Fig.5. Silvertone developed this test in 1969 and it has gained widespread acceptance all over Aust­ra­lia. So it is obvious there are advantages to the linear PA in R/C transmitters. If the 3rd order is eliminated or at least minimised, then operators can be MODEL 1 TRANSMITTER 1 Fig.5: a typical landing field with transmitters spaced three metres apart. This can place interfering transmitters much closer than the control transmitter, as the model comes into land. INTERFERING TRANSMITTER 11 11 TRANSMITTERS SPACED 3 METRES APART LANDING FIELD 30 METRES TRANSMITTER 1 grouped much closer in a much safer pattern, taking into account the signal ratio problem. Interference test procedure For those who wish to conduct a simple field test to deter­mine the safety of operating two R/C systems simultaneously, the following procedure is recommended. (1). Place model 1 on the ground with the antenna parallel to the ground 45ø STATIONARY MODEL CONTROL TRANSMITTER 33 METRES 90ø WALK IN UNTIL INTERFERENCE OBVIOUS RATIO TO BE BETTER THAN 12:1 INTERFERING TRANSMITTER Fig.6: standard interference test (developed by Silvertone) for two adjacent transmitters. and about 30cm above the ground. (If the antenna is closer to the ground, ground effect will distort the results.) If the model features a whip antenna, be sure that it is vertical. (2). Take the control transmitter for the model under test out 33 metres from the model, switch on the Tx, fully extend the antenna and hold it vertical. The angle between the receiver antenna and the transmitter should be 45 degrees. (3). Check the operation of the controls in the model to ascertain that all are working correctly. (4). Take the interfering transmitter out approximately 10 metres but on a line at right-angles to the first transmitter. Fully extend the antenna, switch on the Tx and hold the antenna verti­cal. This ensures that the receiver antenna is evenly polarised and receiving equal field strengths. (5). Take note of the operation of the control surfaces in the model. At these distances there should be no noticeable effect on the controls. (6). Walk towards the model with the interfering transmitter along the 45° line. Keep moving closer until the controls begin to exhibit some signs of interference. Note the distance from the model at which this occurs. The ratio of the two distances of the transmitters from the model should be greater than 12:1. Thus, with the control Tx at 33 metres, there should be no interference with the interfering Tx 2.5 metres away from the model. (7). Repeat the test using model 2 and with the original control transSC mitter as the interfering Tx. July 1995  75