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REMOTE CONTROL BY BOB YOUNG Radio control receivers using amplitude modulation This month we will look at amplitude modulated receivers for use in R/C models. They have the advantage of being simple, easy to understand, and above all, reliable. Why use amplitude modulation (AM) when all the movers and shakers in the R/C movement talk endlessly about FM (frequency modulation)? There are several very sound reasons for my preference for AM. To begin, this column is written for beginners and sports flyers, and they do not face the demanding situations that the experts face in competition work. Secondly, beginners are never quite sure of whether they will be good at R/C work or even if they will enjoy it, to the level that they wish to stay in the hobby. So expensive equipment is a waste if it is just going to be resold or sit on a shelf. Thirdly, and most importantly, AM still gives the best value for the dollar. FM sets are expensive to purchase, much more expensive to repair and most important of all, much more expensive and sometimes more difficult to change crystals in. This is a very important practical consideration. Many modellers end up with lots of pairs of crystals, as the ability to change frequencies on a crowded field or on a race day is important. All of these disadvantages come for a very slender (if any) performance increase. Narrow band FSK The point to keep in mind here is that FM as used in R/C model equipment is not really true FM. It is more 110 SILICON CHIP correctly defined as Narrow Band Frequency Shift Keying (NBFSK) and believe me, the emphasis is on narrow band. The typical frequency shift range is from 1.5-2.SkHz. Now everybody "knows" that FM is better than AM and this may very well be true. But for it to be true, we are speaking of frequency shifts in excess of ±S0kHz. At this figure, there is a marked improvement in the signal to noise ratios over AM. With frequency shifts down around 1-3kHz, the signal to noise ratios are similiar to, if not worse, than AM and any talk of FM (NBFSK) R/C systems being better than AM systems in this regard is nonsense. NBSK advantages NBFSK systems do have two distinct advantages over AM. The AGC problem in AM receivers can be tricky and the time constants must be carefully set. A model moving very quickly past a Tx can create a lump in the AGC line which will show up as a glitch in the controls. This is particularly true of high speed model aircraft but can apply to boats and cars. Secondly, the frequency spacing can be moved a little closer with NBFSK. Typically, AM receivers operate on 20kHz spacing and are hard pressed to come down to lOkHz. It can be done but it is expensive. As many clubs only allow 20kHz spacing on their fields, this is really not a serious disadvantage. NBFSK disadvantage NBFSK receivers do have one very serious disadvantage. They suffer from "capture", in which an interfering Tx can override the master Tx. Control will thus pass to the interfering Tx if it is strong enough. On the other hand, AM receivers will often fight their way through interference. Even brief snatches of recovered control are sometimes sufficient to keep the model flying until the interference has passed. From a design point of view, the NBFSK system places much more stringent demands on the designer and the components he uses. To begin with, in an AM system, the crystal locks the transmitter oscillator onto frequency. Thus, the problems associated with the design of the supply rails, for example, are minimal. In other words, the crystal stabilises the electronics. The situation in a11 NBFSK transmitter is much more complex. Here the electronics stabilises the crystal; a "cart before the horse" situation if ever I saw one. To achieve the frequency shift required for modulation, the electronics must be able to pull the crystal "off frequency" and herein lies the danger in the NBFSK system. Supply rails to the transmitter oscillator must be well stabilised with regards to voltage and must be heavily decoupled. Also, the temperature stability of the components must be excellent and the oscillator design very precise. The crystals used in AM systems 07 BC548 A 2.2k 0.1+ + ANTENNA 50.I-.,. C B .047I . T1 YELLOW I 01 .0022I 470n • . . .,. 2.2k 2.2k 220k AGC + C 0.47,2.2.:r pF A --------+--......--------+--+------+---♦+4.BV + .047+ 50.r 100k 14 B 100k .,. .,. .,. .,. 13 Fig.1: this is the circuit of a simple AM R/C receiver which has been thoroughly proved over many years. The CMOS ICs provide audio gain and squaring (ICt) and serial to parallel conversion for the 8 channels (IC2). are simple third overtone series mode types which are very cheap to produce at 27-40MHz. However, they cannot be pulled off frequency very easily. This makes it sound like a bad thing when in fact it is a good thing; such is the nature of the AM/FM conflict. Typically, series mode crystals can only be pulled about 1-1.5kHz in our bands. As a result, fundamental crystals are required for NBFSK systems and these can be pulled up to 5kHz off frequency easily. They are, however, much more expensive to produce. In addition, they are even more expensive if cut to 30MHz, which is about the limit of fundamental crystal technology at the moment. Thus, we have an additional problem in that the crystals used in R/C sets are cut to one half of the Tx output frequency and doubled in one of the Tx stages. All of this amounts to much greater complexity and a much higher price. Now the important point here is that 90% of all modellers only require one thing of their R/C systems and that is that the integrity of the radio link must be perfect. In other words, the commands sent must equal the commands received. AM will do this at a much lower cost/complexity factor than FM. The reliability of AM sets is also better than FM sets, mainly due to the simplicity involved. This is not to say that FM is unreliable; far from it. In fact, it is a tribute to the modern component industry that this system works as well as it does. But AM will do the job with less fuss and a much greater cost effectiveness. I always feel sad when I see a beginner stagger out of a hobby shop loaded down with expensive equipment he does not really need, or for that matter, know how to operate to its full potential. Nor is he ever likely to reach this level within the lifetime ... of his first radio. Such is modern merchandising. AM receiver circuit Let us turn now to Fig.1 , which is a typical AM single conversion Rx of the type used throughout the . R/C industry for many, many years. It gave excellent results and a lot of fun to untold thousands of modellers. This type of Rx, incidentally, is still used in the current generation of two channel systems. Even here though, the relentless demand for increased complexity in all things (the "Gingerbread Syndrome", I call it) is forcing the pace on the development of much more complex voltage regulation circuits, for example. In Fig.1 we see a simple superhet Rx using a local oscillator (Q6), mixer (Ql), two IF (intermediate frequency) stages (QZ, Q3) and an active detector (Q4). Audio amplification after the detector is provided by a transistor (Q5) and this is followed by several stages of squaring using a CMOS 74C04 inverting buffer (ICl). The seDECEMBER 1990 111 The circuit of the 8-channel receiver shown in Fig.1 can be built into a very small box as this original Silvertone unit shows. The circuitry was on two small PC boards and there was provision for crystal changing. The slightly larger Futaba receiver at left is a 3-channel unit. rial to parallel conversion and decoding is performed by the 74C164 shift register (ICZ), which gives eight channels of decoded information out. (In R/C work, a control output is referred to as a channel, hence eight channels can control eight separate controls, each giving left and right, up and down, etc). The designer of a receiver for R/C use faces several problems which are relatively unique in Rx design. The overriding factor is that the final unit must be small, light in weight and cheap to produce. In addition to this, it must have good sensitivity and be capable of sustaining crash after crash. Some of these crashes can provide "G" forces that can wrench components completely out of the PC hoard. At all times, the unit is subjected to high levels of engine vibration and high levels of in-flight "G" forces. Thus, all tall components or components with thin leads must be bonded to their neighbours at the top end with contact cement or similar. I have had components fall completely out of the PC board under extreme engine vibration. The salt water hazard The receivers in model cars and boats are regularly immersed in water (salt, brackish or fresh). Salt water can electroplate the copper from the PC board to the plastic Rx case in 10 minutes if the board is not correctly coated. The moral here is get that model out of the water quickly, get the power off pronto, wash it out with fresh water from the bottle you carry espe112 SILICON CHIP cially for this purpose and then flush with methylated spirits to absorb all residual water. The metho comes from the other bottle you carry just for this purpose. The reason you carry these bottles, of course, is that you are operating near water regularly. You do carry these bottles don't you? Another serious problem is that of the Rx coming into extremely close proximity of anything up to 15 or 20 transmitters, all on very close frequencies. The typical frequency spacing on most club fields is 20kHz. The Rx design shown in Fig.1 was one of the first in this country designed specifically for close band spacing. Prior to Silvertone introducing this system in 1969, the band spacing on club fields was 50kHz. While the design is very basic and cheap to build, it gave good performance on 15kHz spacing and revolutionised frequency utilisation in this country. We had to devise a new method of frequency control to handle the number of transmitters on the air at any one time, and this system is now the Australian standard. We also had to pull a few devious little tricks to achieve this spacing from such a simple design and here are a few of them. One of the big problems faced in model transmitters is the "hole" or weak signal area off the tip of the Tx antenna. This can be overcome with more Tx power, greater Rx sensitivity or more effective antennas. This Rx used a bifilar wound antenna coil (11) which was intended for use with a centre-fed antenna. We envisioned self-adhesive burglar alarm tape on the wing leading edges. The.se would have doubled as turbulators to enhance lift. As it turned out, this Rx was sensitive enough not to require such an antenna system but it did work well in practice. Diode D1 across 11 clamps the input signal to 0.6V and helps prevent front end overload. AGC (automatic gain control) was applied to the mixer (Q1) and the two IF stages (Q2, Q3). This was unorthodox but it definitely helped when others stuck their Tx antennas through the covering on your wings. The active detector Q4 gives good AGC and a high level of audio output. This audio is then "sliced" at approximately 1V above ground, thus eliminating the low level noise and adjacent channel interference present in the audio output. There are, of course, more modern and elegant ways to achieve this slicing and op amp slicers are great. The output of Q5 is a straight line with no noise with no carrier present. Filtered audio is applied to pin 9 of IC2 (enable). Thus, with no carrier, IC2 was switched off and this further ensured that there were no spurious servo output signals. The rest of the circuit was fairly straightforward and followed conventional superhet practise. This was a nice little Rx and it stayed in production for 15 years. I can still recommend it to anyone who just wants to fly and fly and fly and not get bogged down in endless discussions about the latest and greatest in the gingerbread line. For those who prefer to roll their own, I have included circuit values. Go and have some fun!