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RADIO CONTROL BY BOB YOUNG AM versus FM: the real facts in the argument This month, we will take a look at some of the myths surrounding FM transmitters and receivers and see just how well they stack up against the old AM system. Some people really believe that AM is obsolete and will go so far as to claim that AM sets should be banned from flying. They are dead wrong. The Mk.22 series of articles brought forth a host of let­ters and telephone conversations, almost all of which were very positive. It certainly stirred up some interest around the coun­try. Yes, we do get the odd stinker but many are simple letters asking why 29MHz AM for the Mk.22, when all other manufacturers are producing 36MHz FM? However, the saddest letters are letter even indicated that flight training would cease unless he stopped using “inferior” AM sets and changed to FM. Very often the theme is that beginners keep crashing models; they are using AM sets, therefore the fault lies with AM. Well, I can state that there are plenty of reasons why models crash and the method of modulation is the last item that need be considered. This column What’s all the fuss about FM and AM? How did all of this start in the first place and why? We flew safely and successfully for 30 years on AM, so what has changed? those in which there is a genuine plea for help, usually from beginners who are under intense pressure from some club members to sell their “inferior” AM system and buy the latest FM-type transmitters, complete with LCD, bells, whistles and buzzers. They usually have a simple plea. “Should we sell and why? Your help please.” These letters often point to ridicule or lectures on why AM is dead. One 54  Silicon Chip is dedicated to those people who are under pressure from “experts” who should know better. What’s all the fuss about FM and AM? How did all of this start in the first place and why? We flew safely and successfully for 30 years on AM, so what has changed? Before we start I should point out that we are dealing with an extremely complex subject and it is easy to become entangled in a circular argument in which the main points keep getting lost. There are three branches in this discussion. The first concerns the relative merits of AM over FM under normal operating conditions. The second is the effect of interference on both systems and finally, there is the level of technology ap­plied to each system by the manufacturers. The question of oper­ating at 29MHz instead of 36MHz is a separate issue and we will deal with that another time. Cheap AM sets Let’s talk about the level of technology in both systems. As AM is much cheaper to produce than FM, it is the preferred system of modulation for those manufacturers going after the price conscious market. These manufacturers sometimes use dubious techniques to further reduce costs and the result is a system that provides minimal performance and reliability. This has more to do with the design and manufacturing approach than the method of modulation. To complicate matters there are also AM systems produced for model car operation, using short antennas. These were never intended by the manufacturers for aircraft use but were sold by the model trade as general purpose sets and thus found their way into model aircraft. These sets have played a large part in giving AM an undeserved poor reputation. Now “everybody” knows that FM is better than AM and, of course, so it is. But this applies to the FM used for radio and TV sound broadcasting. Fig.1: spectrum analysis of a 4-channel AM R/C trans­­mit­ter. This shows the occupied bandwidth as ±12kHz at -60dB. FM stereo radio is far superior to steam- age AM radio, and so it should be, with its frequency deviation of ±75kHz. That amounts to a channel bandwidth of 150kHz! That is true FM. By contrast, AM radio has a bandwidth of a mere ±9kHz; no wonder it is inferior. FM is not FM What “everybody” does not know is that model R/C equipment does not use true FM! To use the term FM to describe the method of modulation in an R/C transmitter is quite wrong. The system of modulation used in FM R/C sets is actually NBFSK. This stands for Narrow Band Frequency Shift Keying. This system is a form of direct frequency shift keying and is not to be confused with AFSK (audio frequency shift keying). This form of modulation uses narrow-band carrier deviation to transmit the data and let me tell you the emphasis is on NARROW! Typical frequency shifts are around ±1.5kHz to ±2.5kHz for a max­ imum channel bandwidth of 5kHz. That’s in theory. In practice, the deviation is more usual­ ly -400Hz and +2.5kHz for a system bandwidth of about 3kHz. In other words, the 36MHz carrier is shifted back and forth by a mere 3kHz. That is a world away from the 150kHz deviation applied in FM radio. Nor can anything better be expected with NBFSK. How are we ever going to get down to the coveted 10kHz channel spacing if we occupy more bandwidth? So why isn’t the correct term of Fig.2: spectrum analysis of a 5-channel FM R/C transmitter. This shows the occupied bandwidth as ±8kHz at -60dB (narrower than the AM transmitter shown in Fig.1). NBFSK used instead of FM? It really is misrepresentation. It never began as a deliberate policy but merely came into being as a matter of convenience to distinguish frequency-shift sets from AM sets. After all NBFSK is a form of FM and FM rolls off the tongue much more nicely than NBFSK, doesn’t it? The problem is, in the minds of many people, FM has come to mean something quite distinct from NBFSK. The term FM conjures up visions of wideband high fidelity stereo sound transmission systems, completely free of noise and interference. This is the underlying theme in the AM versus FM argument; AM is “inferior” because FM is so much better. But the argument is spurious and the question should be, “Is NBFSK better than AM?” or possibly, “Is NBFSK as good as AM?” Do you think I am being deliberately controversial here? Well, stick with me because you might be surprised. AM is really not AM Not only is FM not FM but just to confound the argument, there is one other thing that “everybody” does not know. The system of modulation commonly referred to as “AM” in the model trade bears no more relationship to AM radio than model “FM” bears to broadcast FM! Model AM is not AM! It is really a gated carrier system and many of the objections that apply to AM broad­ casting just simply do not apply to this system. It is a very robust system of modulation. Add to this receivers designed specifically for noise elimination and pulse shaping, with ceramic IF filters, audio slicers, audio filtering and decoding enable. What broadcast AM receiver is designed along these lines? The modern AM R/C receiv­er might look simple but it has had a long history of development and it works very well. Comparison tests With all of the above in mind we embarked on a series of tests to demonstrate FM and AM performance. We used a Silvertone Mk.22 receiver which is ideal for comparative testing as we could plug in the AM or FM modules ahead of the audio slicer. We used a loose form of antenna coupling to the signal generator which gave a practical dynamic range of 80dB. Both receiver modules were identical in sensitivity. The AM receiver circuit is that published earlier in SILICON CHIP and the FM receiver uses one of the Motorola receiver chips. The signal generator was set at 100% modulation for the AM testing while the FM modulation was set at -400Hz and +2.5kHz, mimicking a popular Japanese “FM” R/C transmitter. The external modulation was supplied from a Silvertone Mk.14 7-channel encoder. Measured under these conditions the signal-to-noise ratio of the AM receiver at the detector was -14dB at -70dB signal input, the point at which the audio slicer was about to shut off the pulse train to the decoder. The FM receiver measured -12.5dB (also at the detector), a figure 1.5dB worse than the November 1996  55 AM Receiver Fig.3: recovered modulation from the detector of an AM receiver, taken at a transmitter relative signal level of -60dB. Note that the waveform is clean and virtually noise free. AM Receiver Fig.4: same waveform as Fig.3 but with a transmitter relative signal level of -80dB. AM Receiver Fig.5: this shows the recovered data after the slicer, for a transmitter relative signal level of -60dB. AM receiver. Again the 70dB point is significant as it is the point at which the squelch is about to shut down the audio output of the receiver. Some idea of the relative signal-to noise-ratios of the two receiver modules may be gained by referring to the accompanying oscilloscope waveforms in Fig.4 & Fig.8. These were taken at a carrier level of -80dB, the lowest point at which a readable signal is present in both detectors. We took the above figures at these points because they are of interest when flying through weak signal areas. It is here that things will go pearshaped very quickly indeed if noise or interference are present. As you can see, these figures are completely at odds with the theoretical noise figures so widely available in text 56  Silicon Chip AM Receiver Fig.6: same signal conditions as for Fig.5 but with interference from the commutator of an electric motor. books and which form the basis of the “FM” versus “AM” argument. To my mind, the anomaly arises from the fact that the “FM” system uses such small deviations with simple receivers and the “AM” system uses a gated carrier with unusual receivers. They also take no account of the ambient noise levels in various receiver designs. In this case the FM receiver had a much higher ambient noise level than the AM receiver and it shows quite clearly in the scope waveforms. This point is important in the “level of tech­nology” discussed previously. So here we are well into the story and so far the AM system is ahead by a nose. It appears that we must dig deeper to find out why “everybody” believes that “FM is better than AM”. Never in its long history was AM ever considered perfect. The main weaknesses with the AM system from an R/C point of view are the AGC system and occupied bandwidth. The wider bandwidth of the AM transmitter is a result of the edge conditioning (ie, pulse shaping) of the carrier blocks which contain many harmonics. This is a most difficult factor in AM transmitter design. Blow the edge conditioning and you can end up with a spectrum a mile wide. If the time constants are not correct on the AGC rail, fast models can ex­ perience momentary glitches as the signal strength gyrates wildly on close passes to the transmitter. Here is a problem not experi­enced by communications receivers. The move to NBFSK, which began FM Receiver Fig.7: this waveform shows the recovered modulation from the detector of an FM receiver (before the squelch stage), taken at a transmitter relative signal level of -60dB. FM Receiver Fig.8: same condition as for Fig.7 but with a transmitter rela­tive signal level of -80dB. Note that noise is intruding serious­ly onto the signal and is much worse than the equivalent AM receiver condition shown in Fig.4. FM Receiver Fig.9: this shows the recovered data after the slicer of an FM receiver, for a signal level of -60dB. Note that this waveform is virtually identical to the AM slicer signal shown in Fig.5. largely in the very busy clubs in Europe I am told, was largely driven by the above two points plus the problem of electric motor noise. Electric flight is very big in Europe. They needed the narrowest channel spacing possible and stories circulated for years about 10kHz operation in Europe. Yet I was at the MAAA committee meeting last year that examined all of the latest sets, including the best from Europe and America, and that committee ruled that 10kHz operation was not safe with the current generation of NBFSK radios. So what are they doing in Europe? It is difficult to get a true story on how they are managing the frequencies in Europe but it appears that they allow FM Receiver Fig.10: same signal as for Fig.9 but with interference from the commutator an electric motor. Note that the data has been seriously disrupted, with an extra pulse appearing the fourth data block. the use of 10kHz spacing but only allow every second channel to fly simultaneously. Er, isn’t that 20kHz? Just recently, the MAAA has adopted a similar spacing for Australia. We were getting better results with AM sets in 1969 when I began testing systems for narrow band spacing. We actually flew Silvertone Mk.7 receivers on 5kHz and 10kHz spacing but we deemed this unsafe and settled on 15kHz as the closest safe spacing. We flew this spacing for many years without incident in several Sydney clubs. This is still true today as proved by the MAAA meeting in Melbourne last year. Yet there is a mystery here as a quick glance at Fig.1 and Fig.2 will confirm. The NBFSK transmitter has a slightly lower bandwidth and the receiver is more developed with very narrow filters, so how is it that the more simple AM system delivers comparable results from a band spacing point of view? Ironically, the big difference between the AM and NBFSK receivers in regard to band spacing is that the AM receiver has AGC (automatic gain control). This drastically reduces the signal levels arriving at the IF filters and thereby reduces the stress on these filters. In contrast, the NBFSK receiver with no AGC always runs at full sensitivity and the filters are subject at all times to heavy noise, carrier and November 1996  57 SATELLITE TV EQUIPMENT     Receivers  Feeds Positioners  LNBs  Actuators Dishes And much much more! C-Band Systems from $1495 Ask us for a catalog! B&M ELECTRONICS 469 Light Street, Daniella WA 6062 Phone/Fax: (09) 275 7750 Mobile: 041 99 0 55 00 Binders IF frequencies. Fig.8 shows the recovered data from the detector of a typical NBFSK receiver operating at a very low signal level (-80dB). Note the extremely high level of white noise. With the carrier off, this noise is very high. NBFSK receivers need to resort to trickery in order to get rid of this noise because if the transmitter is turned off or moved out of range and that noise got through the decod­er, the servo gears would be reduced to pulp within 30 seconds. The trickery consists of adding a squelch circuit which detects the loss of carrier and shuts down the audio preamp, thereby removing the noise to the decoder. All of this costs money of course and it all adds to the expense of an NBFSK re­ceiver. AM virtually noise free Fig.4 shows the detector of a Mk.22 AM receiver at the -80dB point and therefore running at maximum sensitivity. Here we are at the very edge of the range and yet note the almost complete absence of noise. This drops to a straight line once the carrier noise is removed. There is no need for squelch for there is not sufficient noise to get past the audio slicer in the decoder. The decoder shift register is fitted with a pulse omission detector to catch whatever stray spikes slip past the slicer. Servo gears are safe here. The reasons for this vast difference in two receivers of my own design is primarily the fact that I have no control over the choice of design and transistors in the FM receiver chip, whereas I had total control over the AM design and components. The tran­sistors in the AM receiver were the best I could find. Limiters These beautifully-made binders will protect your copies of SILICON CHIP. They are made from a dis­tinctive 2-tone green vinyl & will look great on your bookshelf. Price: $A11.95 plus $3 p&p each (NZ $8 p&p). Send your order to: Silicon Chip Publications PO Box 139 Collaroy Beach 2097 Or fax (02) 9979 6503; or ring (02) 9979 5644 & quote your credit card number. 58  Silicon Chip At this point it is probably opportune to raise the issue of limiters. One of the major advantages of the broadcast FM system over broadcast AM is the fact that the FM system makes use of very effective limiters which remove much of the unwanted amplitude modulation and noise from the system. The Mk.22 AM receiver is fitted with an audio slicer which also removes much of the noise from the system. This does essentially the same job as the limiter in an FM radio receiver. So here we are well into the story, having dealt with the two major complaints against the AM system. And what have we found? Technically, the balance is about equal with shortcomings in both systems, but the AM system is much cheaper to purchase and maintain. For those with more money than sense, I suppose this is not sufficient reason to give the AM system the good housekeeping stamp of approval. Therefore, let us dig a little more deeply. Here we move on to point two. A serious problem is that interference to the AM system tends to reduce the depth of modu­lation; if the interference is strong enough the modulation depth can be reduced to zero and all control lost. The equivalent effect in NBFSK sets is the capture effect. If the inter­fering signal is stronger than the carrier, then the receiver can lock on to the interfering transmitter, completely blocking out the wanted carrier. Again, all control is lost. The big difference is that the AM system is gradual whereas the FM system is abrupt. Capture is a strong point for FM radio broadcasts but a real drawback in NBFSK sets. When two signals are comparable in amplitude, the moment one signal becomes even a trifle stronger the response changes and the stronger signal assumes control. A similar effect occurs at low signal levels (almost out of range). The AM system will work right down to very low signal levels. Control deteriorates gradually at the lower levels, thus giving some warning that things are beginning to go pear-shaped. By contrast, the NBFSK system will often switch off abruptly with no warning when the squelch cuts in or capture takes over. These points are terribly important for it was demonstrated by Phil Kraft back in the very early days of proportional system development (1960s) that a system that shuts down abruptly was not as good as one that allowed the pilot to battle his way through noise and interference. There is a great deal more to this discussion but it all tends to reinforce my argument that FM is greatly oversold against AM. When you look at the true nature of each of the modulation systems, gated carrier (AM) versus NBFSK (FM), what the argument really boils down to is “weak FM versus SC super AM.”