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RADIO CONTROL BY BOB YOUNG Multi-channel radio control transmitter; Pt.3 Following the description of the encoder module last month, we present the long-awaited AM transmitter circuit. This has been carefully designed to keep harmonic content and third order intermodulation to an absolute minimum. Modern radio control transmitters place enormous demands on their designers due to the wide range of (often conflicting) features expected by the users and the standards required by the various watchdogs responsible for the safe and harmonious appli­cation of technology. This is particularly true of the transmitter module. Here the operator can cause a third (innocent) party to bore neat little holes in the ground. We treated this subject in some detail in the July 1995 issue of SILICON CHIP. Thus the designer of a modern transmitter module is charged with serious responsibilities. With this in mind the design of the RF module presented has proceeded slowly and cautiously. This has been far too slow for some, judging Interference takes on a very serious meaning for model fliers. If they allow their trans­mitter antennas to come in close proximity with their neighbours, they can cause a third (innocent) party to bore neat little holes in the ground. can intrude into the domain of his neighbour in a very big way. We are all familiar with broadcast and television interference but a new dimension has been added recently in R/C circles, at least in the form of 3rd order inter­ modulation inter­ference. This aspect of the interference spectrum takes on a very serious meaning for model fliers, for if they allow their trans­mitter antennas to come in close proximity with their neighbours, they by some of the letters and comments we have received in the period since the publication of the AM receiver. However, as they say, all good things come to those who wait, and so here at last is the long awaited transmitter module. Design philosophy Those who remember the discussion in the June 1995 issue may recall that at the time, I concluded that the best approach for an RF module with reduced third-order intermodulation would be a class-B push-pull unit. Initially, I proceeded to design a transmitter along those lines. I quickly discovered several important aspects of third order intermod­ ulation. First, direct injection can play an im­portant part in the process. Direct injection occurs where the interfering RF gets directly into the coils and PC board tracks as opposed to being picked up by the transmitter antenna. This form of injection has been minimised by the use of an aluminium transmitter case, a ground plane on the PC board, shielded coils and most important of all, by having the minimum number of stages in the transmitter. Secondly, I discovered that yes, the push-pull circuit did give good results but it had to be very carefully designed and was very tedious and expensive to build. What finally sunk this very promising development was the discovery that if the bias was set incorrectly the third order intermodulation was much worse than a class C output stage. This was a great disappointment since the class B stage proved to be extremely efficient and one module that we had out flying drew only 18mA and gave excellent results. I cried tears of blood over losing that 18mA output stage, especially when I had to wrestle with this new design to bring the current drain down to reasonable limits. More on that later but I still weep when I think of a transmitter with 12 hours of flying time on a 600mA.h battery pack, especially when you listen carefully to one of the modern computer radios and you can hear virtually the electrons roaring as April 1996  65 Fig.1: the circuit consists of a Hartley oscillator, Q1, driving Q3, a VMOS Mosfet critically biased by trimpot VR1. Modulation is applied to the output stage by transistor Q2 which varies the supply to Q3. they are sucked through the wires to keep up with the demand for current. At this point my attention was focused on the encoder design which took many months, leading to its presentation last month. In the intervening period I was able to formulate the approach presented in this article. The key aspect is the oscillator which delivers a very high drive level with good stability. In fact, you could almost hang an antenna off this oscillator and fly with it but it wouldn't really be practical. You would need to amplitude modulate the oscillator and the subsequent frequency modulation and pulling that with AM would cause all sorts of problems – not a good practice. So that meant an RF power amplifier (PA) with modulation. Here I ran into serious problems as the isolation between stages was poor – there was oscillator breakthrough and only 90% modulation. At this stage the project looked to be in serious jeopardy. The standard cure is to use a diode to set the bias threshold but this meant more non-lineari­ ty in the PA. This was completely contrary to the design philoso­phy which called for the output stage to be biased to the point of acting as a perfect transistor in order to reduce third order intermodulation. 66  Silicon Chip I could have used a buffer stage but again I ran foul of my own design requirements, set out above. At this point I realised that the emitter resistor was the main culprit in the third order intermodulation process and I set out on a search for transistors with diffused emitter resistors. The data books are full of them but you try buying one in this country. Up until then I had been concentrating on bipolar tran­sistors. I then had an inspiration and decided to use one of the VN series V-MOS FETs and lo and behold all problems vanished; well, almost. These FETs make ideal output transistors for transmitters, being almost indestructible and with good gain at 30MHz. Once the change was made to a FET PA, the problem of oscillator break­ through was minimised but it still remains in a very mild form, so care is need in this area during setup. The circuit presented also features some degree of latitude to make it useful in non-modelling applications. To this end I have indicated which components are not used for R/C work and those needed for matching into a 50Ω coax cable. As presented, the transmitter delivers close to 500mW into a 1.5 metre (60-inch) telescopic antenna with a total current of ap­proximately 120mA. This includes the oscillator, PA and encoder current. Useful operating time from a 600mAh battery pack should be in the order of four hours. Circuit description This photo shows the transmitter in early prototype form. The construction starts next month. Transistor Q1, coil L5, crystal X1 and associated compon­ents comprise a Hartley oscillator which is transformer coupled into the PA transistor, Q3. R6 and C5 are for decoupling and C4 is used to shunt any inductance in C5. This type of oscillator provides a high level of drive combined with good depending on the coupling bet­ween the oscillator and PA, too much bias can drive the FET into a very high current mode. Capacitor C7 provides a ground return for the RF flowing in the secondary of L5. In the early stages of development of this circuit, I had terrible problems with strong harmonics on 90MHz coupled with very high levels of current in the FET. This resulted in the FET almost steaming. Yet despite this maltreatment the five original FETs used in the prototypes are all still working very happily and I have yet to see one fail. As an added precaution, I have designed the PC board so that the ground plane and the transmit­ter case form a substantial heatsink. More power possible This spectrum sweep tells the story of how this new circuit is successful in suppressing third order intermodulation. The two large spikes represent the transmitter fundamentals of the Mk.22 at 29.745MHz and a standard imported Tx at 29.805MHz. The subsid­iary spike at right shows how the imported unit has substantial third order intermod­ulation at 29.865MHz but the intermodulation pro­duct of the Mk.22 Tx is well down, almost in the noise. Reproduced from the July 1995 issue, this spectrum sweep shows two conventional class C transmitters spaced 20kHz apart at 27.175MHz and 27.195MHz. The interfering signals, spaced 20kHz away at 27.155MHz and 27.215MHz, are only 30dB down on the wanted signals. stability. The 22pF capacitor C2 is used for fine tuning the crystal, if re­quired. Increasing C2 will pull the crystal lower in frequency although there is a limit to this. Bias for Q3 is provided by trimpot VR1, resistor R8 and diode D1 and is the core of the intermodulation solution. The setting of VR1 is fairly critical and the third order products can actually be tuned out when setting this trimpot. By watching the spectrum analyser and tuning VR1, the third order can be reduced to its absolute minimum. As this point is theoretically the point at which the FET is behaving as a perfect transistor, this point also corresponds closely to the point which gives the best harmonic suppression results. One word of warning here: This circuit is capable of further development and could eventually deliver up to 1W with care in regard to harmonic output. Coil L6 and capacitor C8 form a trap for 90MHz which can prove troublesome at high drive levels. These are not mounted in the R/C system but the PC board does provide for them. 1W is far too much power for R/C work but readers with non-R/C applications may find this of interest. The 10Ω resistor R5 is a “stopper” to prevent high frequen­cy parasitics while resistor R7 is there to discharge the gate. Q3 is loaded in the R/C circuit with L4. While provision is made for L3, it is not used in this circuit. Capacitor C10 swamps the Mosfet capacitance and provides some stability to the output stage. It also provides production repeatability and tunes L4 to 29MHz. The amplified RF is then matched to the antenna by an LC network consisting of capacitor C13 and coil L2. For those wishing to use a 50Ω coax output, C6 will provide adequate matching. This capacitor is quite critical and would probably be best made up of a fixed capacitor in parallel with a smaller variable type. Provision is also provided on the PC board for an additional base loading coil should the application re­quire it. These components are not used in the R/C system. This coil would be required, for example, if a short antenna was to be used. TB1 is the transmitter module connector and provides power, antenna and modulation connections. Transistor Q2 is the modulation transistor and is config­ ured as an emitter follower. Capacitors C11 and C15 provide RF bypassing and assist in the final shaping of the modulation waveform. This shaping is absolutely critical if the system bandwidth is to be held inside the ±20kHz allowed under current MAAA guidelines. At this stage of development, the Mk.22 Tx is rated at ±15kHz at 60dB. This is a little higher than I would have liked but well within the guidelines. Capacitors C16 and C14 are DC filters for the supply rail. This module will tune across the range of frequencies al­lowed for R/C work and should tune to 50MHz for non- modelling applications. The table presented in the circuit diagram gives some idea of the capacitor changes required for different operat­ing frequencies. The coils do not need to be changed. So there you have it. I promised you a module with reduced third intermodulation and if you look at the spectrum sweep in the accompanying photo you will see that this aim has been met. Next month, we will discuss construction SC of the transmitter module. See you then. April 1996  67