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RADIO CONTROL BY BOB YOUNG Multi-channel radio control transmitter; Pt.4 In Pt.4 this month, we look at the features of the transmitter PC board and discuss its assembly. It is a double-sided board with plated-through holes. It has conventional components on the ground plane side and surface mount components on the other. When I am writing for SILICON CHIP, I am conscious of the fact that it is an electronics magazine and not a modelling magazine. I know full well that SILICON CHIP readers devour all kinds of articles and the knowledge gained is often applied in fields other than originally intended. This was driven home to me in no uncertain manner when I presented the Speed1B motor control in November & December 1992. More of those units went into non-modelling applica­tions than into models. They found their way into electric pow­ered fishing dingys, full size autogyros, wheelchairs and a myriad of other items. The demand was so widespread that I was forced to design an add-on pulse generator to allow these units to be used without a radio receiver. This was published several months later and required a new, small circuit board which is glued to the origi­nal Speed1B PC board. When I had finally settled on the circuit for the Mk.22 RF module, I sat back and contemplated what the readers might hit me with this time. Two days after the encoder circuit was published in the March 1996 issue, I had a request from a government de­ partment for transmitters and receivers. The Mk.22 was of great interest, Fig.1: this diagram shows the component layout for the surface mount component side of the board. Crystal X1 and trimmer capacitor are also mounted on this side. they said, because the RF modules came out so easily and this would allow them to use a fibre optic link without high drama. So it was obvious that it was going to be Speed1B all over again. This time I was determined to be one jump ahead. One obvious request would be for an NBFSK (erroneously referred to as FM in the modelling trade) transmitter. Another would be for a voice modulated unit. There is a very interesting band on 30MHz which butts up against the 29MHz band (it actually starts where the modelling bands stops) and allows the use of 100mW unlicensed transmitters for voice. We already make an FM simplex radio link for this band for several non-modelling customers. A data link is an almost certain application. Another was obviously the use on frequencies not approved for modelling, but for which the potential user was already licensed. Thus, the PC board presented this Fig.2: the component layout for the groundplane side of the board. Note that the crystal socket is attached from this side – see photo. May 1996  53 The groundplane side of the transmitter board carries only a few parts. Note the upside down crystal socket. Note also that the pins for TB3, adjacent to transistor Q2, have been clipped off flush with the board surface. capacitance for the modulation wave shaping and are best lumped in with the temperature stable mylar (polyester) capacitors on the encoder PC board. C11 and C15 have been reduced to NPO .001µF capacitors. Production spreads on the FET have since dictated that R7 should be 56kΩ or less. This plays a part in the modulation pulse shaping. During the final PC board layout I was forced to add a jumper in the form of a 1206 chip resistor. This is shown on the overlay as R11 and is 1Ω. There was also a typo on the circuit. Diode D1 is a BAS16 not BA516. These circuit additions have resulted in a transmitter which is a delight to tune and service. One of the big problems with checking frequency in a modulated AM transmitter is that the modulation blanks out some of the RF and the frequency count is always low unless you have a gated frequency counter. With TB3 in place, no problem. Just switch off the modulation by shifting the AM shunt to CW. The result, a carrier only transmitter on which it is a snap to check frequency. Want to check PA current whilst tuning the PA? No problem, simply remove the shunt from TB3 and insert the meter in series with the two CW pins. Dead easy! Construction This photo shows the surface mount side of the board. The only other conventional components visible are the crystal, trimmer VC2 and the multi-pin header, TB1. Note also that links across TB2 and TB4 have been installed on this side of the board rather than on the ground plane side, as depicted in Fig.1. month is a multipurpose unit and covers all of the above. Frequency range is from 25-50MHz with suitable coil and capacitor changes. Accordingly, it has provision for components which are not required for this R/C transmitter. Most of them are of no consequence for this project, but the programming pins TB2, TB3 and TB4 need to be dealt with for circuit continuity. TB2 is there to program the oscillator for various configurations and this is hard-wired to the AM position with a link. TB3 and TB4 are used to 54  Silicon Chip program the modulator but again TB4 is of no consequence. It is also replaced with a link. TB3 howev­er is a valuable asset when testing and servicing the board as it programs the transmitter for CW or AM modulation. Circuit changes There are some minor component changes introduced since last month. R8 has been increased to 22kΩ to restrict the FET gate bias range and improve the feel of VR1 when tuning. C11 and C15 form part of the lumped The component layout diagrams for both sides of the PC board are shown in Fig.1 & Fig.2. I must point out here that due to the stringent demands placed upon this module it is best tuned on a spectrum analyser. For this reason I strongly recommend that if you do not have access to a spectrum analyser, you should buy the module fully assembled and tuned. For those not familiar with surface mount construction techniques, I would suggest reading the article “Working With Surface Mount Components”, as featured in the January 1995 issue of SILICON CHIP. You will need a pair of magnifying spectacles, a fine-tipped soldering iron and a pair of tweezers with very fine tips. Begin by tinning one pad at each of the surface mount com­ ponents positions, as set out in the above article. This is a good time to clearly establish which components are not mounted by not tinning the pads for these components. When all of the surface mount components are in place, solder the jumper links as indicated on Fig.1. These may be made from the tinned leads of resistors. The longer jumper between J1 is made from the wire-wrap wire provided in the kit. Note that L1 and L3, which appeared on the circuit last month, are not used. Their positions on the board are actually bridged by the copper tracks out so you don’t have to worry about them. Coil winding The coil winding details for L2, L4 & L5 are shown in Fig.3. The direction of winding is not important but the number of turns are. However, it is important that the secondary on L5 is wound in the same direction as the primary. The enamelled copper wire provided is easy to solder and a hot iron will soon burn the enamel away. Tin one end of the lengths of enamelled wire provided. Only tin about 1mm of the wire to minimise the risk of a shorted turn on the coil. Due to the fact that 16 turns just fit on the coil formers, snip off half of the pin protruding on the winding side of the coil base on L2 & L5, leaving just enough pin to solder the wire – see Fig.3. Solder the end of the wire to the appropriate coil former terminal and wind on the correct number of turns using tight, close spacing. This done, apply a dab of super glue to the winding to hold it into place, then place the coil former on the desk to dry. When you return, remove the desk from the coil former. Having gone through the above ritual you now have three coils with one end free. Solder this end to the appropriate terminal and mount L2 and L4. Now wind on the second­ary of L5. Care must be taken here with the beginning and end terminals (see Fig.3) and also to ensure that the secondary is wrapped over the eighth and ninth turns of the primary. The physi­ cal location of the secondary plays an important role in the drive level and thus harmonic content of the output. Secure it after it is wound with another drop of super glue. Moving the secondary closer to the base of the coil (col­lector of Q1) will increase the drive level and harmonic content of the oscillator. Mount L5, taking care to ensure that the prim­ary and secondary terminals are correctly Fig.3: coil winding details for L2, L4 and L5. Fig.4: depending on how links are made across TB3, the transmit­ter can be set to CW (no modulation) or AM (normal operation). aligned with the PC board (primary terminals closest to the crystal socket). Final­ly, solder the shield into place, making sure that the coil former is centralised in the top hole. Crystal socket At this point, it is wise to deal with the next messy job which is mounting the crystal socket. The PC board is designed to allow the crystal to be removed from the back of the case and thus the crystal must be mounted vertically. However, this dictates that the socket must be glued into the PC board flush with the top (surface mount side) of the board. Thus, viewed from the ground plane side of the PC board, the crystal socket appears to be upside down. Do not get this wrong. If you glue the socket into the wrong side of the PC board you will have ruined both items. Fit the crystal socket into the hole in the PC board and ensure that it is the right way up and flush with the surface mount. Very carefully place a drop of super glue onto the junction of the PC board and the crystal socket from the groundplane side of the PC board. Once the glue is dry, solder the two connecting wires into the pads adjacent to the crystal socket and then solder them to the socket terminals. Care must be exercised here for the plastic used in the socket is easily melted. Tin both the socket terminals and the wire ends before soldering them together with just a quick dab of the iron. The rest of the assembly is a snap, with the only special care needed with terminal blocks TB1 & TB3 and trimmer capacitor VC2. VC2 is mounted on the surface mount side of the PC board for ease of adjustment when tuning. TB1 is likewise mounted on the SM side of the board and mates with the main power connec­tor for the module. The programming pins for TB3 are mounted from the groundplane side of the board with the long side of the pins pro­jecting through the board and out onto the SM side of the board. Solder them to the pads and then remove the black plastic from the back of the board. Snip off the pins on the ground plane side of the PC board as close to the board surface as possible. This also applies to the pins on TB1, as the antenna sits in the channel between the components and quite close to the PC board. These pins could short out the antenna if left too long. Finally, mount the output FET using the hardware kit pro­vided. The mounting of this transistor is designed to heatsink the transistor firstly into the groundplane of the PC board and then from there into the transmitter case via the mounting brack­ets. As a result the transistor runs quite cool, even with the antenna retracted. That completes the assembly. Put the unit aside for a period then come back and check once more that all components are correct. Ensure that the crystal socket is adequately anchored and that the contacts are free of glue. Plug in the crystal and place the micro shunt onto the CW position on TB3 (see Fig.4). Testing & tuning This section is a little ahead of itself as the module really cannot be completely tested and tuned until mounted into the transmitter case with the correct antenna. However, I will complete the tuning sequence for those using the module in other applications. This description will assume that the module is in the case and fitted with an antenna 1.5m long (wire or telescopic). May 1996  55 I reiterate that unless you have access to a spectrum ana­ lyser, you really can’t set up this transmitter module. However, I am presenting the following details for the sake of complete­ness. First, with a continuity meter test between the power and ground pins on TB1 to ensure that there is not a direct short to ground. Remove the crystal, hook up the main power connecter or apply 9.6V to the power and ground pins of TB1. Set VR1 to mid-range and screw the tuning slugs into the coil formers so they are flush with the SM side of the PC board. Remove the micro shunt from TB3 and connect a milliammeter (200mA range) in series with the CW pins then turn on the power. The PA current will be somewhere in the order of 15mA. Set VR1 for a quiescent current of 12.5mA. This should equate to a base bias voltage of 2.2V approximately. Remove the meter and replace the micro shunt on the CW position. Now quickly go over the board and check the voltages at the supply rail (+10.3V), decoupled oscillator supply rail (+9.54V), base of Q1 (+3.7V), emitter of Q1 (+3.0V), base of Q3 (+2.2V) and collector of Q3 (+10.37V). Plug in the crystal and hook up an oscilloscope to the collector of Q1. There should be a strong 29MHz signal present at the collector. Screw the slug out (anticlockwise) watching for an increase in amplitude of the 29MHz signal until it drops abrupt­ly. Screw the slug in (clockwise) until the oscillator starts and continue on for one half turn. At this point you should have about 5V RF signal at the collector of Q1. The oscillator is now tuned. Check the frequency with a counter to ensure that you are within ±1.7kHz Kit Availability Kits for the Mk.22 transmitter are available in several different forms, as follows: Fully assembled module (less crystal) .......................................................... $125.00 Basic kit (less crystal) ...................................................................................... $89.00 PC board ......................................................................................................... $29.50 Crystal (29MHz) ................................................................................................ $8.50 Post and packing of the above kits is $3.00. Payment may be made by Bank­­card, cheque or money order payable to Silvertone Electronics. Send orders to Silvertone Electronics, PO Box 580, Riverwood, NSW 2210. Phone (02) 533 3517. of the marked crystal frequency. The final frequency will depend on the brand of crystal you have purchased. The Showa crystal supplied will be within tolerance. The frequency may be fine tuned with C2; increasing C2 will decrease the frequency. Do not exceed 33pF for this capaci­tor. Set VC2 to mid-range and, using a wave meter, field strength meter or spectrum analyser, tune L2 and L4 for maximum amplitude of the output signal. At this point I should point out that the aim here is not to tune for maximum power but to achieve a balance between output power on the fundamental frequency against harmonic content. This is the problem that arises when tuning without a spectrum analyser. To further complicate the tuning process, VR1 is best set by tuning it for minimum third order levels. It is impossible to do this without a spectrum analyser. Once the transmitter is at maximum output, take note of the harmonic levels. VC2 is fitted for suppression of 60MHz and 90MHz harmonics. Adjust VC2 for the minimum harmonic levels and then retune L2 and L4 for the maximum difference between fundamental and harmonic outputs. It should be possible to exceed -60dB on all harmonic levels. At this point, the PA current should be about 65mA. Next, set up a second transmitter at a frequency 60kHz away, with the modulation removed (CW) and of approximately equal output to the Mk.22 transmitter. Place it on a bench with the antenna fully extended and switch­ed on. Switch on the Mk.22 and position it so that a good strong third order component is clear­ly visible on the spectrum analyser display (it will be the small spike closest to the Mk.22 fundamental spike). Tune VR1 for the minimum level of third order intermodula­ tion and move out until the two fundamentals are of equal ampli­tude. At this point, the third order intermodulation component of the Mk.22 should be approximately 15dB down on that of the adja­cent transmitter. Go back now and touch up L2, L4 and VC2 and tuning is com­plete. Check the PA current once more to ensure that it is under 100mA. Next month, we will discuss the SC assembly of the encoder. Fig.5: here are the full-size etching patterns for the double-sided PC board. 56  Silicon Chip