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The Mozzie CW Transceiver This nifty little transceiver is an unconventional design. It is suitable for Morse and RTTY and has a maximum power output of about 1 watt. It is battery powered and its output has a low harmonic content less than - 40dB. Design By CLIVE CHAMBERLAIN For a long time now there has been a crying need for a low-cost low-power transceiver which could be used by amateurs for Morse and RTTY communications. In the past, there has been a number of designs but there has been nothing which used readily available up-to-date components. Now that has changed and we can present the "Mazzie" which is 66 SILICON CHIP right up to the minute in its circuit design and performance. Designed and supplied by Australian Test and Measurement Pty Ltd, the Mazzie uses 5 integrated circuits and is built on a small double sided PC board. The top of this board is a ground plane which has been included for stability and freedom from noise. The Mazzie is housed in a low profile case. On the front panel there are two toggle switches, one for power and the other for transmit/receive [Tx/Rx) switching. There is also a volume control knob for the receiver function. On the rear panel, there are three insulated RCA sockets and a 6.35mm jack socket. The RCA sockets are for 6V DC input, a Morse key and the antenna connection, while the 6.35mm jack socket is for the headphones. The design could be adapted to operate anywhere in the amateur 6-metre band but has been optimised to suit the readily available American US colour TV intercarrier crystal at 3.58MHz - more precisely 3.579545MHz. We expect this frequency to become quite popular for Mazzie operation maybe it will become the "Mazzie Net"! Design features The Mozzie is a single channel CW (Continuous Wave - Morse Code) transmitter and receiver with a maximum power output of about 1 watt at 3.58MHz. The receiver is a direct conversion type (ie, not superheterodyne) which uses an oscillator frequency slightly offset from that of the transmitter. This is done by shifting the crystal frequency slightly when the transmitter function is selected. Now have a look at the circuit of Fig.1. This is split into two sections with the transmitter being along the top (IC1 and IC2) while the receiver is along the bottom of the diagram (IC3, IC4 & IC5). Both the receiver and transmitter use a common antenna with switching between the two performed by switch S1, at the top righthand corner of the diagram. Let's look at the receiver circuit first (Fig.1 ). Direct conversion receiver Incoming RF signal from the antenna is applied via switch S1 to the two bandpass filters comprising L6, L7 and two 33pF capacitors, and top coupled by a 6.BpF capacitor. These bandpass filters are centered around 3.58MHz and heavily attenuate frequencies more than about ± 50kHz either side of this frequency. This is necessary to block out broadcast radio, TV and any other transmissions not on the "Mozzie Net". Balanced mixer The heart of the receiver is IC3, a Signetics NE602 which is a double balanced mixer and oscillator. In this circuit, the internal oscillator is not used and an external oscillator, ICld, is used instead. Its output is fed into pin 6 of IC3 via a 5.6k0 resistor and l00pF capacitor. The "beat" output from IC3 is taken from pin 4. If both the transmitter and receiver were operating from exactly the same crystal frequency, there would be a "zero beat" from the mixer but as we have already mentioned, there is normally a difference between the two and this " beat" is an audio frequency. The beat note from pin 4 of IC3 The Mozzie uses 5 integrated circuits and is built on a small double-sided PCB. The top of the board is a ground plane which has been included to ensure stability and low noise. also contains a lot of the 3. 58MHz local oscillator signal which is filtered out by the 4.7mH inductor L5 and the a ssociated .022µ F and .033µF capacitors before being fed into the following audio stage. The NE602 does contribute quite a useful amount of conversion gain (about 100 times) and we need all the help we can get because the incoming signal is likely to be only a few microvolts of RF. So IC3 functions as a direct conversion receiver, with the incoming RF demodulated directly to audio without going through an intermediate frequency a s in a superheterodyne. This has the advantage of simplicity but some of the feature s of the superhet such a s RF derived automatic gain control to stabilise audio output level are sacrificed. The low level audio which may be at a few hundred microvolts is now passed to IC4a which is half of a low noise LM833 dual op amp. IC4a is connected as a non-inverting amplifier with a gain of 100. The audio output from pin 1 of IC4a now would be typically 30 to 50 millivolts. Limiter stage The next stage is IC4b which is the other half of the LM833. This functions as an audio limiter by virtue of the back-to-back silicon signal diodes D4 and D5 in the feedback loop. This provides a form of automatic gain control which prevents perforation of the ear- Where to buy the kit A complete kit fo r the Mozzie 3.58MHz transceiver is available from Australian Test and Measurement Pty Ltd, 28 Hotham Parade, Artarmon, NSW 2064, or fro m any of their dealers. See the advertisment in this issue. Phone (0 2) 906 2333 . The cos t of the kit is $84.50 plus $6 .50 postage and packing. MAY 1990 67 PARTS LIST 1 plastic instrument case, 140 x 110 x 46mm 1 PC board, 84 x 102mm 1 front panel artwork 1 rear panel artwork 1 6.35mm jack socket 3 insulated screw mount RCA sockets 2 1 0 way PCB connector strips 2 PCB mount SPOT miniature toggle switches 1 3.579545MHz crystal 1 knob 1 balun core 1 1OOkO linear potentiometer (VR1) Semiconductors 1 75451 dual peripheral driver (IC1) 1 7 4HCOO CMOS quad NANO gate (IC2) 1 NE602 double balanced mixer (IC3) 1 LM833 dual low noise op amp (IC4) 1 LM386 power amplifier (IC5) 7 1N4148 diodes (01 to 07) Capacitors 3 1OOOµF 16VW PC electrolytics 5 1µF 35V tantalum electrolytics 4 0.1 µF monolithics 2 O. 1µF metallised polyester 1 .033µF metallised polyester drums should a nice strong signal, say 50 microvolts, be received at the antenna. IC4b limits the signal at the function of the lOkO resistors to about 100 millivolts RMS. Note that this limiter is not suitable for speech signals since it clips heavily; the resulting distortion would be ghastly. With a continuous tone "beat" though, it is quite acceptable; it just sounds a bit "edgy". The signal from the limiter stage is applied to volume control VR1 and then to IC5, an LM386 amplifier which can drive an external loudspeaker or headphones. Thorough decoupling is provided for IC3, IC4 and IC5 with lOOOµF electrolytic capacitors. Without this decoupling, feedback via the supply line would lead to "howling" 68 SILICON CHIP 1 1 3 4 1 1 2 1 1 .022µF metallised polyester .001 µF metallised polyester 560pF polystyrene 220pF ceramic 1OOpF ceramic 82pF ceramic 33pF ceramic 6.8pF ceramic 6-45pF trimmer (VC1) Inductors (chokes) 3 2.2 microhenries (L 1, L2, L3) 2 33 microhenries (L6, L7) 1 180 microhenries (l4) 1 330 microhenries (L8) 1 4. 7 millihenries (LS) Resistors (0.25W, 5%) 1 1MO 3 100k0 1 15k0 4 10k0 2 5.6k0 2 4.7k0 1 1k0 1 4700 1 1000 2 1 000 ½ W (for dummy load) 1 4 70 ½ W (for supply current limiting) Miscellaneous 0.3mm enamelled copper wire, hookup wire, solder, 6V battery pack (4 alkaline AA cells plus holder) . due to the high gains and compact assembly. The ground plane construction of the printed circuit board is also a safeguard. Squelch feature D1 and D6 form an audio mute system to block audio from reaching the LM386 while the Mazzie is transmitting so as not to be distracting. When the Tx/Rx control switch is set to Transmit, about 0.8mA is fed through the diodes so that they become a low impedance. This shunts the audio signal away from the input of IC5 via a lµF capacitor and so the circuit is effectively muted. Crystal oscillator The crystal oscillator is based on IC1d, a standard HCMOS NAND gate biased for linear operation by the 1MO feedback resistor which keeps its output at about 3V. The crystal and the "tweaking" components, VC1 and L4, form a feedback loop which causes the NAND gate to oscillate at the crystal's fundamental frequency of 3.58MHz. The crystal oscillator runs all the time, whether in Transmit or Receive mode, but during Transmit, diode D7 is forward biased via its associated 10k0 resistor. This bias gives D7 a low resistance and it effectively shorts out VC1 and so slightly reduces the crystal oscillating frequency. IC1c forms an inverting buffer for the crystal oscillator's output from pin 11 and also drives a charge pump consisting of diodes D2 & D3. This charge pump produces a + 4V DC supply for the Morse key input. This is a safety feature for the power output stage of the transmitter. It proved neccessary because if the crystal oscillator does not oscillate no Morse key supply will be produced and so the transmitter cannot be keyed on. The 3.58MHz square waves at pin 11 and pin 8 of IC1 are of opposite phase and are fed via IC1a and IC1b which gate the signal through to the output stage when the + 4V Morse supply is applied to pins 2 and 4 via the Morse key. In this way, push pull drive is fed to the transmitter output stage. Transmitter output stage Readers who are familiar with transmitter output stages may be puzzled when they look at the photos and wiring diagram for the Mozzie - where are the RF output transistors? The answer is that there are no discrete transistors in the circuit. The RF output transistors are contained in IC2, a 75451 dual peripheral driver made by National Semiconductor. This is an odd device to find in a transmitter circuit. It is normally Fig.1: IC1, IC2 and crystal X1 form the transmitter while IC3, IC4 & IC5 make up a direct conversion receiver. Both the transmitter and the receiver share a common antenna which is switched between the two by S1. i I t ANTENNA ~ r II ---:--0 ~ 01 · ♦ tu m• 470{) f +6V S1 RX 14 220pF 10k 1M VC1 X1 3.579MHz 6-45pF D7 ol----'oo' • 180uH 100k 0.1+ 1N4148 L4 .,.. KEY • D2 1N4148 10k 82pF! T 0.1! .,. I f ! l 1 ~ 4.7k ... ~ ! 1000! l ✓.,_<r-Qsv S2 '::r:'DC .001 100k 220pF D1 1N4148 1000! o.,I D5 2x1N4148 I .,. 220pF ~ 1k ~ -< ..... 1+ co co 0 THE MOZZIE CW TRANSCEIVER ~ cc 1 +--- VD~~~E~ 1Dk 1000 SPEAKER OR HEADPHONES •·a:---:--v I " +· } .,.. ~ Fig.2: keep all leads as short as possible when installing the parts on the PCB and don't forget to solder on both sides of the board where required (note: ground plane not shown). Don't forget to solder the front flange of switch S1 to the ground plane pad provided. intended for use in high speed buffers, relay drivers and other peripherals for logic circuitry. It contains two TTL NAND gates and two NPN transistors which are rated for operation at up to 30 volts DC. The two NAND gates in ICZ are connected as inverters and they each drive an internal output transistor which then both drive pushpull transformer Tl and a tank circuit consisting of 11, 12 & 13 and the associated 560pF capacitors. The tank circuit filters out the harmonics of the transmitter waveform so that the signal fed to the antenna is a clean sine wave. Actually this gating and driving arrangement has more in common with a switch mode power supply than a radio transmitter, but modern high speed logic devices make this transmitter line up much simpler and more efficient than would probably be the case with tuned RF stages and an output tank, especially considering the low supply voltage of 6V! On receive, the current drain from the 6V battery pack is about BmA and on transmit up to about 250mA. If you use four "AA" alkaline cells as your battery pack, you should get a lot of air time, considering typical Tx/Rx duty cycles. Assembly As noted previously, the Mazzie is built on a double sided printed circuit board with the top function- ··w SECONDARY 6T, 0.4mm DIA ENCU PRIMARY 2x4T, 0.4mm ENCU BIFILAR WOUND ON CENTRE LIMB OF FERRITE BALUN CORE Fig.3: winding details for the RF output transformer. After winding, use your multimeter to identify the correct connections for making a centre-tapped primary. TABLE 1: INDUCTOR CODES 70 L 1, L2, L3 L6, L7 2.2µH 33µH red red brown gold orange orange black gold L4 180µ,H brown grey brown gold L5 L8 4.7mH yellow violet red gold orange orange brown gold SILICON CHIP 330µ,H ing as a ground plane. The board is housed in neat low profile plastic case which is supplied in the complete kit for the project which comes from Australian Test and Measurement Pty Ltd. Price for the complete kit is $84.50 while packing and postage to any part of Australia is $6.50. To keep costs reasonable, the double side printed board does not have plated through holes, so quite a few topside joints are necessary to complete the circuitry. These joints will be easy to spot however, as they will show through the top green solder mask. Start assembly by loading the lowest height components first , such as the diodes and resistors . Next, the ceramic capacitors can be loaded and soldered. Philips types have been used for the smaller values to 220pF. These have tiny flanges in their leads, just below the capacitor body, which fixes their height above the PCB and makes an ideal soldering point for the topside joints. The 0.lµF monolithic ceramic capacitors have a kink in both their leads which serves a similar function. The chokes can go in next. They look just like resistors and have the same colour code bands except they have thicker bodies. Colour codes for chokes (inductors) are based on multiples of microhenries. So, if a 4. 7 millihenry 5 % choke is called for, the colour is yellow, violet, red, gold. The colour codes for all the inductors are shown in Table 1. The ICs go in next, followed by the plastic, tantalum and aluminium electrolytic capacitors, while making sure that the polarities are correct. Once these are mounted the PCB-mount switches can go in'. Remember to solder the front flange of Sl to the ground plane pad provided. Before installing the volume pot, VR1, cut off the flattened section of the shaft. The RF output transformer, Tl, must be wound according to Fig.3, with the number of turns and direction of winding being exactly as shown in the diagram; 4 turns each for the primary windings and 6 turns for the secondary, all with The Mozzie transceiver is powered from an external 6V battery pack. Do not apply more than 6V DC to the circuit. 0.3mm enamelled copper wire. Strip and tin the four primary leads so you can use your multimeter, switched to a low "Ohms" range, to identify the correct connections for making a centre tapped primary. The twisted centre tap must be soldered to the top and bottom pads on the PC board. The other transformer leads can now be pushed through their PCB holes and soldered to the bottom side. Receiver testing The Mozzie should be tested before being mounted into the case, so first attach the four hexagonal standoff posts to the bottom of the PCB using the screws provided and use the 10-way pinstrips to connect to the matching socket strips on the PCB. For preliminary testing of the RF circuits, a dummy load consisting of two 1000 ½ W resistors in parallel should be connected between te AAA and GG terminals on the S2 connector and say 5 metres of hookup wire as a temporary antenna also to AAA. A pair of stereo headphones with both channels commoned or a small speaker can then be connected between the SS and the GGs on the S2 connector. For the 6 volt supply, a regulated power supply with current limiting is best for this procedure but a 6V battery pack with a series current limiting resistor of 470 can be used as an alternative. When connecting the supply, make sure that the polarity is correct, otherwise you'll damage the circuit. Set the volume control fully anticlockwise (minimum setting) and the transmit/receive switch to Rx. Turn the Mozzie on. Check that the positive rail (between + 4.5 and + 5.5V) is present at pin 14 of ICl, pin 8 of IC2, IC3 & IC4, and pin 6 of IC5. Now check for around + 3V at pin 1 of ICl, pin 1 of IC4 and pin 5 of IC5. Any major deviation from 3V could mean incorrect assembly or missing solder joints. Around 3V should be present at pin 11 of ICl. Also present at pin 11 of ICl should be the 3.58MHz signal of the oscillator. Now put on your headphones and gradually rotate the volume control clockwise. A hiss should become more evident as the control is advanced. If a loud and raucous noise is evident at even slight rotations of the volume control, look for bad joints around the pot and around Some component leads must be soldered on both sides of the PCB. Commercial boards will have a green solder mask so these joints will be easy to spot. M A Y 1990 71 The wiring connections between the rear panel sockets and the PCB are made via 10-way connector strips. Keep the wiring neat and tidy. open circuits around ICl or IC2. If you have an oscilloscope, it should show a sinewave at the oscillator frequency across the dummy load at about 20V peak to peak and after 10 seconds the dummy load resistors will become quite hot. To fully test and operate the Mozzie you will need at least one other Mozzie or another transceiver set to 3.58MHz in the vicinity. For best results too, an efficient antenna system is absolutely necessary for a QRP (low power) rig such as this. With another transmitter set to 3.58MHz, set trimmer VCl for the desired beat note which can be around 500Hz to lkHz or higher or lower, if you prefer. You should only need to do this once, before the cover is screwed on. Mechanical assembly All the parts on the front panel are soldered directly to the PCB. Route the leads to the connector strip as shown and make sure that all polarised parts are correctly oriented. IC5 or its associated electrolytic capacitors. If all is in order, disconnect the power and proceed to test the transmitter circuit. Transmitter testing With the 470 safety resistor, dummy load and temporary antenna still in place, throw the Tx/Rx control switch down to Tx and temporarily connect a normally open pushbutton switch between K2 and Kl on the Sl connector strip. About 80% of full supply - ie, about + 4.8V - should be present at pin 10 of ICl. About + 3.5V should be present at the K2 ter72 SILICON CHIP minal (anode of D2), indicating that the oscillator is running. If all is in order, it is time to short out the safety resistor and expose the Mozzie to the full awesome power of the four AA cells. An ammeter in series with the supply to the Mozzie and set to 1 amp FSD would be a good idea in the remote event that something is way out of whack in the remaining untested circuitry. Press the pushbutton gingerly several times and check that the ammeter shows a current drain of up to 250mA. Anything significantly more than that indicates shorts or Final assembly of the Mozzie is a matter of fitting the front and rear panel artwork, installing the PC board and sockets in the case, and then completing the wiring. The front decal should stripped off its backing sheet to expose the adhesive layer and carefully manoeuvered so that the circular legends surrounding the controls are centred. Shining a light from behind the panel will greatly assist this alignment. You will only get one crack at this, so take it easy. Now do the same with the rear decal. Run a scalpel or small craft knife through the decal and around the inside of the control shaft holes to remove the surplus paper and adhesive. The Mozzie PCB with standoff pillars fitted now slips into the case with the controls protruding through the front panel. Next, attach the nut to the pot and gently tighten, followed by the collett mounting knob and cap. The mounting pillars can now be secured to the base with the screws provided. The RCA antenna connector is screwed to the rear of the case on the right side, looking from the back. To the left is the 6VDC RCA connector with the centre contact being the positive connection. Install the other sockets and complete the wiring as shown in Fig.2. Do not apply more than 6VDC to the circuit or failure may result.~