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REMOTE CONTROL BY BOB YOUNG Building a complete remote control system for models; Pt.2 This month, we present the circuit description of the Silvertone Mk.22 24-channel AM receiver. Although designed pri­marily for the radio control of models, it also lends itself to a myriad of non-modelling applications. The receiver is a “three-PCB” arrangement, with PCB1 for the receiver, PCB2 for the first eight channels in the decoder and PCB3 for the last 16 channels. This month, we are describing the circuit operation, with the construction to follow next month. The design of any electronic device represents a series of compromises which eventually lead to a completed unit. In fact, many of the requirements imposed on the designer are conflicting in nature and we will discuss these conflicts as we go along. Basically, the design requirements for a receiver intended for use in the radio control of models are: small physical size, low cost, out-of-sight range on a low power transmitter (200-600mW), good noise rejection, ability to operate in close physi­ cal proximity to other transmitters (some of which may be only 10-20kHz away), temperature stability, and the ability to operate with one cell in the battery pack short circuited. Quite a number of prototypes were produced during the development of the Mk.22. For those who are curious about the Mk.22 designation, the last production Silvertone receiver was the Mk.14. Mk.15 - Mk.21 were proThis larger-than-life size photo shows the completed receiver assembly. Note the socket for the plugin crystal. The resistors, capacitors & transistors are surface-mounted on the copper side of the board. duced during the development of this unit. The main problems encountered were PCB layout problems causing front end instability, excessive noise, oscillator stability and local oscillator injection levels and coil phasing. You will note that all of these are essentially RF problems. The IF stages were no problem. The resulting receiver is a very useful little unit which gives surprisingly good results considering its simplicity. As my mate Klaus (who provided valuable assistance with this project, including the test flying) pointed out, there is not a lot that can be done with a couple of transistors and IF cans. Sensitivity Receiver sensitivity is approximately 2µV with about 1µV thrown away in the audio slicer. This results in a receiver of approximately 3µV sensitivity. Translated into practical terms, the result is about 600 metres ground range (depending on condi­ tions) and about 1.5km in the air or over water. In R/C modelling, it is important that the transmitter and receiver do not provide excessive performance. This is because many modelling sites are in close proximity to each other and excessive transmitter power or receiver sensitivity can result in inter­field interference. The trick is to provide just enough performance to do the job reliably. The band spacing on this receiver is 20kHz and this spacing can be used with complete safety. In addition, the receiver layout has a very small cross-section and this allows the board to be mounted at right angles to the February 1995  77 C13 2.2 R9 180k R10 2.2k C16 47 B E L5, L6 : TOKO M113CN 2K218 DC L4 : LMC 4100A L2 : LMC 4101A L1 : LMC4102A XTAL1 : 30MHz SERIES MODE 3RD OVERTONE R11 470  E C C VIEWED FROM ABOVE B R2 2.2k S Q3 BFT25 R5 1k E C10 4.7pF 4.7PF C B 30MHz XTAL1 C11 22pF V+ C7 15pF S R3 100k 78  Silicon Chip C9 .01 C12 .01 L3 D1 BFR92A S F F L5 ANTENNA 1 E B ANTENNA 2 C4 10pF 10pF C5 3.3pF C2 10pF 10pF C1 .0047 S F L6 F C3 .001 B R1 680  E E C L4 Q1 BFT25 R4 2.2k C8 .001 B R6 1.5k E R7 2.2k C6 2.2 R8 1M CF1 BFB455 B Q4 BFT25 Q2 BFT25 C Circuit details SILVERTONE MK22 RECEIVER B D2 BAS16 R13 10k L1 L2 direction of travel, even in the most slender of models. A plug-in crystal facility is also provided to allow the crystal to be quickly changed on the field. The machine-wound RF coils suggested are only suitable for 29MHz but with hand­wound coils, this receiver will tune over the range 27-40MHz. All in all, it’s a very useful little receiver which will satisfy all but the most demanding modellers. Q6 BC848 R12 1k C14 2.2 C C15 .047 V+ Q5 BC848 E C +4.8V TB1 Fig.1 (left): the receiver follows conventional superhet principles & features a crystal controlled local oscillator (Q3 & Xtal1), a double tuned front end feeding a conventional transistor mixer (Q1), two IF stages work­ing at 455kHz (Q2 & Q4), & the transistorised equivalent of an anode bend detector (Q6). The receiver follows conventional superhet principles and features a crystal controlled oscillator, a double tuned front end feeding a conventional transistor mixer, two IF stages work­ ing at 455kHz, and the transistorised equivalent of an anode bend detector. Fig.1 shows the details. The transmitted signal arrives at the antenna and is fed into either the primary or the secondary of coil L5, depending upon the application. Antenna 1 is intended for coax-feed remote antennas, while Antenna 2 is the normal model aircraft antenna (usually one metre of flexible hook-up wire). If signal-to-noise ratio is more important than range in your application, then use Antenna 1, even for the flexible wire antenna. This will result in a much cleaner signal at very low signal strengths but will cost about 6-8dB in gain. Diode D1 acts as a clamp to prevent mixer overload when the transmitter antenna is very close to the receiver antenna. This is a serious problem in model applications, as modellers often need to stand over their models in order to operate them unas­sisted. A common trick is to stand astride a model aircraft, for example, with the tailplane hooked behind the ankles whilst the motor is run up to clear the plug and check the mixture. This will result in a very high signal level at the receiver mixer if precautions are not built into the front end to compensate. earth/antenna and the transmitter antenna, these two signals (which are opposite in phase) can cancel each other out, the nett result being a com­ plete loss of signal and what is known as a glitch. This is a momentary loss of signal which clears itself almost immediately after it occurs. This problem can and does occur in most model receivers and accounts for some of the mysterious little hiccups which occur from time to time. Local oscillator This views shows the completed receiver (right) together with a companion 8-channel decoder unit (to be described next month). The two units can be fitted together inside a small metal case. Thus, D1 clamps the signal to 0.6V maximum. The downside to D1 is that it can introduce intermodulation effects at the mixer. For this reason, D1’s physical characteristics are ex­tremely important, if another transmitter is operated close by and on an adjacent channel. From experience, I know that a 1N4148 diode works well in this application. However, the Mk.22 receiver uses the base-emitter junction of a VHF transistor (BFR92A) for this diode and this also works extremely well. Coil L6 provides additional fre­ quency selectivity and also matches the 1-metre wire antenna into the base of the mixer. Before leaving the antenna coils, there is one very import­ant point to bring to light regarding the earth/antenna relation­ship. Ideally, the signal appears in its strongest form across the antenna and is balanced against a very strong ground connec­tion. In model work and particularly model aircraft work, howev­ er, there is no ground connection and the battery and interwiring have to work as a solid earth. The problem is, this wiring varies from model to model, depending on the size of the model, number of channels, servos and the neatness of the installation. In some cases, signal inversion can take place across coil L5, where the antenna is acting as a counterpoise (earth) and the earth wiring is acting as the antenna. In freak cases, de­pending on the polarisation of the receiver Fig.2: this scope photograph shows the output signal on the collector of detector stage Q6. Transistor Q3 functions as a local oscillator and runs at the carrier frequency plus 455kHz. In Australia, local oscilla­tors run on the high side of the carrier in the 29MHz band, due to possible image problems from the 30MHz band. The opposite is the case on the 36MHz band where the local oscillator runs on the low side of the carrier. Coil L3 forms the tank coil for the local oscillator, while its secondary provides low impedance matching for injecting the oscillator signal into the emitter of Q1 via C12. C7 provides the fine tuning for the crystal frequency. The crystal can be pulled about 1-1.5kHz by adjusting C7 and C10. The values presented on the circuit are for Showa brand crystals and may need some adjust­ment if different brands of crystals are used. Transistor Q1 functions as the mixer and the resulting 455kHz IF signal is derived from the composite signal by L4. C8 damps L4 to prevent ringing if it occurs. It is not fitted with the 4000 series coils provided in the kit but may be required if different brands of coils are used. Q2, L2, Q4 & L1 provide the IF amplification, with R6 acting as the main gain control. Increasing its value will reduce the gain (the value shown on Fig.1 provides near maximum gain). Ceramic resonator CF1 across Q4’s emitter resistor (R11) sharpens the bandpass characteristic of the IF stage by approximately 3dB and is a useful addition. Detection & AGC Q6 acts as the transistorised equivalent of an anode bend detector and provides the recovered audio signal as well as the AGC control voltage. Diode D2 and capacitor C15 rectify and filter out the 455kHz component. The recovered audio will be approx­imately February 1995  79 Frequency Control At Flying Fields The receiver presented in this article is intended for use on the 29MHz band and, in fact, the machine-wound coils recom­ mended will only tune from 27-29MHz. Hand-wound coils will allow the unit to be tuned through the full range of frequencies avail­able to modellers from 27-40MHz. However, its use on modelling frequencies outside the 29MHz band is not recommended for several reasons, as set out below. In addition, non-modelling applica­tions will need to take into account the relevant Department of Transport and Communications regulations. 27MHz Citizens Band (26.95727.282MHz): the original garbage band, cluttered with cosmic noise and thus given over to experimenters from the early days. It was heavily used by modellers for many years until CB traffic made it too dangerous. This band is very busy with CB traffic and now frowned upon by the authorities for modelling use. Two frequencies are given over to children’s toys and “toy” walkie talkies. 29MHz Band (29.72-30.00MHz): a specific modelling band allocated when the CB band became unusable (c. 1975) and the recommended band for this receiver. The frequencies recommended for use in this band are set out in Table 1 Crystals on these frequencies are available from most good hobby shops. This band is used extensively 3V p-p at high signal levels. The slicer in the decoder (to follow) rejects the bottom 1V of the audio output and passes only the clean, high level signal to the audio amplifier. As the signal strength increases, the 80  Silicon Chip Silvertone Keyboards are the recommended method of fre­quency control for all national events sanctioned by the Model Aeronautical Association of Australia (MAAA). Illustrated are the 29MHz board (standing) and the new expanded 36MHz two-board set. The expanded 36MHz band, soon to be released, now features 59 frequencies at 10kHz spacing. Table 1 Channel TX RX 10 29.725 30.18 12 29.745 30.20 14 29.765 30.22 16 29.785 30.24 18 29.805 30.26 20 29.825 30.28 22 29.845 30.30 24 29.865 30.32 26 29.885 30.34 28 29.905 30.36 30 29.925 30.38 32 29.945 30.40 34 29.965 30.42 36 29.985 30.44 by modellers favouring 2-channel equipment (cars and boats) but almost deserted now on flying fields due to the rush to 36MHz. This is a wise choice if you just want to go to the field and fly, free of channel clutter and waiting time. 36MHz band (36.00-36.60MHz): soon to be expanded and opened up for use with a 10kHz frequency voltage at the col­lector of Q6 falls towards ground and the bias supplied to Q1, Q2 & Q4 via R9, R2, R4 & R7 falls, thus reducing the gain of these stages. Capacitor C6 filters out any audio on the AGC line, while R9 & C6 together spacing. The Mk.22 is not recommended for 10kHz spacing and is thus not recommended for use on the 36MHz band. 40MHz band (40.66-40.70MHz): another of the original modelling allocations but now not recommended due to heavy traf­fic from hospital pagers and the like on 40.680MHz. Channel 50 (40.665) and Channel 53 (40.695) are still OK for 10kHz or wider bandwidth receivers in areas free of this traffic. The Silvertone Keyboard pictured above was designed in 1969 to allow the mixing of equipment with various bandwidth characteris­ tics at busy flying fields. It is basically a graphic representa­ tion of the frequency allocation laid out on a 1-inch = 10kHz grid. Each modeller is supplied with a key, the width of which is proportional to the bandwidth of his equipment. Thus, a 10kHz system uses a 1-inch key, while a 20kHz system uses a 2-inch key. To reserve a frequency block in order to fly safely, the correct width key is simply inserted into the appropriate slot in the board, thus reserving the frequencies required. provide the AGC time constant to filter out any flutter caused by rapid variations in signal strength. These can occur due to high speed aircraft flying by the transmitter or through weak signal areas. Finally, Q5 and C16 provide the power supply filtering. In operation, the capacitance of C16 is multiplied by the gain of Q5, thus resulting in a very simple and effective filter. Unusual Use For A Speed Control Unit Substitute at your peril Now a few general notes on the overall design of the re­ceiver. First, substitute values at your own peril. And to those who wish to do their own through-hole layout, the best of luck. Half of the prototypes were rejected because of layout problems. RF circuits are very sensitive to board layout and conse­quently the layout forms a major component in the design. Capacitor C14 is a layout compensation filter and must be mounted in the physical location shown on the component overlay. C13 is there to provide spike suppression on the power rail input. For those still determined to press on, use 2N3646 or BF494 transistors in the RF and IF stages. These will give the best noise and AGC characteristics. The surface mount BFT25 transis­tors used in the unit described here were chosen for the same reason and were selected after trying many types. Let me tell you, these are an expensive transistor but are well worth the money in this application. Also, use a BC847 in the DC and audio stages. Try not to substitute for the IF coils as they are the heart of the system and a change here can create all sorts of havoc. RF coils The only other components which are critical are the RF coils. These may be hand-wound and Neosid make a neat little 4mm coil former which will fit the PC board with only a slight joggle of the mounting pins. Use 12 turns of 28 B&S wire with a 33pF capacitor. The secondary consists of three turns of the same wire. Be sure to follow the start and finish instruction on the schematic. Reduce the capacitor to increase the frequency –there is no need to change the turns. They should tune to 40MHz with about 22pF of capacitance. You can use a 1N4148 diode for D1 but do not substitute anything else. In addition, make sure that you use NPO capacitors on all of the values up to .001µF. The rest of the components One of our readers, Peter Barsden of WA, has sent along some interesting photos of his gyrocopter (no details provided) which is fitted with a pre-rotator. This unit consists of an electric motor (located at the top of the mast) and this spins up the rotor before takeoff to reduce the takeoff distance. The electric motor is controlled by a Speed1B speed control unit fitted with a self- contained pulse generator, as published in Silicon Chip in November & December 1992, January 1993 and April 1993. Peter has purchased six of these units and appears to have convinced his friends that the Speed1B is the way to go. are not that critical. The resistors can all be 1/8W types. Surface mount components Finally, I failed to stress one important point last month on the hand assembly of surface mount components. The manufac­tures do not recommend surface mount components for hand assembly due to the risk of thermal shock cracking the substrate of some of the components. In practise, this can be minimised by heating the pad first and letting the solder flow from the tip of the iron to the component (ie, apply the solder to the tip of the iron and not to the component). Remember also that the iron and the solder (with flux) must be applied simultaneously to the joint. Do not try to transfer solder from the iron to the joint. Also, try to avoid touching the component with the tip of the iron. As you will recall, I suggested soldering one pad of each component first by sliding their ends into molten solder. This minimises the thermal shock. Looked at in this light, it is probably a good idea to immediately solder the second pad of a component after the first (ie, while it is still warm), rather than after all components have been mounted. In practise, I have hand-mounted thousands of these components with no signs of visible damage but do try to be as careful as possible. To recap my previous advice, use a low wattage iron (20W), keep the iron temperature as low as practical and avoid touching the component with the tip of the iron. Next month, we shall continue with details of the SC board assembly and alignment. Acknowledgement I would like to thank everyone at Borundi Electronics for the assistance and cooperation given to me throughout this project. Without the use of their proto­typing PCB facilities, I would have faced great difficulties in completing this design. February 1995  81