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QRP Labs QCX-mini CW, Assembly Instructions Manual, Page: 96 / 121

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QRP Labs QCX-mini CW Assembly Instructions Manual Download Page 96

4.13 Microcontroller

The ATmega328 microcontroller circuit controls many aspects of this transceiver. Below is 
this section of the transceiver circuit. Several points are worthy of discussion. 

ATmega328P processor

The ATmega328P was chosen because it has enough processing power and I/O to handle 
all the tasks required here. It is also common and inexpensive, and lots of QRP Labs 
products already used it, bringing economies of scale in both the kit preparation and the 
coding. The processor is operated at its maximum rated 20MHz system clock speed. 

QCX-mini assembly Rev 1.05

96

Summary of Contents for QCX-mini CW

Page 1: ...single band high performance 5W CW Transceiver with built in alignment and test equipment iambic keyer WSPR beacon mode and more Designed and produced by QRP Labs 2017 2020 pictured with Palm Radio pi...

Page 2: ...170 transistors 47 3 21 Install 2x3 pin in circuit programming header 48 3 22 Install 2x5 LCD header 49 3 23 Install 2x4 UI header 50 3 24 Wind and install toroid L4 50 3 25 Wind and install toroid L2...

Page 3: ...ogramming ISP interface 103 4 16 Test Equipment 103 4 17 5V voltage regulator 105 5 Fault finding 106 5 1 Blank LCD or blocked LCD 106 5 2 No back light at all 106 5 3 A row of blocks appears on the t...

Page 4: ...ilter with no ringing Si5351A Synthesized VFO with rotary encoder tuning Iambic keyer or straight key option included in the firmware Simple Digital Signal Processing assisted CW decoder displayed rea...

Page 5: ...eed further help join the QRP Labs discussion forum on groups io and post a message about your problem 2 Parts list Many components are SMD pre soldered to the PCB in the factory Only through hole com...

Page 6: ...3 6 32 34 36 39 41 48 50 52 5 0 47uF Label 474 C11 43 46 2 1uF Label 105 C21 22 2 2 2uF SMD C31 100 C100 on display board 3 10uF SMD C37 38 51 1 10uF Electrolytic tantalum In parallel with C38 may be...

Page 7: ...271 C27 28 2 680pF Label 681 C25 26 30m no C8 capacitor Qty Value Description Component numbers 1 22pF Label 220 C5 1 30pF Label 300 C30 2 270pF Label 271 C27 28 2 560pF Label 561 C25 26 20m no C5 or...

Page 8: ...bers 1 T37 2 toroid red windings depend on band L4 1 T50 2 toroid red windings depend on band T1 2 SMD 47uH inductor L5 6 Band specific inductors 80m Qty Value Description Component numbers 2 2 4uH 25...

Page 9: ...on T37 6 core yellow L1 L3 1 0 90uH 17 turns on T37 6 core yellow L2 1 0 40uH 10 turns on T37 2 core red L4 1 3 3 3 30 turns T50 2 core red T1 17m Qty Value Description Component numbers 2 0 55uH 13 t...

Page 10: ...IONS Don t try to be a hero and do it without instructions A jeweler s loupe is really useful for inspecting small components and soldered joints You ll need a fine tipped soldering iron too It is goo...

Page 11: ...As per standard QRP Labs practice the ATmega328P microcontroller has a 28 pin DIP socket in case you may wish to subsequently replace it for firmware upgrades etc Many of the components in this kit ar...

Page 12: ...be soldering a joint that is very close to nearby SMD components In this case angle the PCB and soldering iron so that you are approaching the joint and touching and heating the joint from the opposi...

Page 13: ...ls to the controls board So please read this and keep those off cuts safely 5 PRECISION I can t emphasize this enough this board is compact and the fit in the optional enclosure is precise with low to...

Page 14: ...ollowing diagrams show the PCB layout and track diagrams of the display and main PCBs of the QCX mini kit red track top side blue track bottom side there are no hidden layers QCX mini assembly Rev 1 0...

Page 15: ...QCX mini assembly Rev 1 05 15...

Page 16: ...QCX mini assembly Rev 1 05 16...

Page 17: ...QCX mini assembly Rev 1 05 17...

Page 18: ...de of the PCB On the display PCB the 2x5 pin header connector is installed on the reverse side so refer to the assembly manual steps very carefully Take care when installing integrated circuits All th...

Page 19: ...Main board Display board Controls board QCX mini assembly Rev 1 05 19...

Page 20: ...as well as the two push button switches The controls PCB needs to be slightly higher relative to the main board than the display PCB this is accomplished using two small spacer PCBs one which fits on...

Page 21: ...the manufacturing processes to panelize PCBs on a larger panel of blank FR4 If this is the case just gently snap off the excess strip of board It s also important to gently file away any rough edges o...

Page 22: ...issing from this photo So for a complete inventory refer to the parts list remembering to check the band specific components for your band Remember that the main kit bag contains several smaller clear...

Page 23: ...iagrams which enumerate each of the windings wire endings and holes on the PCB on both the layout diagram and on the circuit schematic diagram It should help to explain diagrammatically which wires mu...

Page 24: ...QCX mini assembly Rev 1 05 24...

Page 25: ...roid ring is not large enough to neatly hold all of those turns in a single flat tidy winding The large secondary winding WILL end up looking messy with overlaps in some places You should try to ensur...

Page 26: ...mb and finger and thread the first turn of the wire through from top to bottom Leave about 3cm of wire at the free end 2 Apply tension to the wire after each pass through the center hole to try to kee...

Page 27: ...keep the toroid in place while you deal with the remaining wires 10 Now cut the 3rd loop the one nearest the end of the winding work and un twist the twisted section you made near the toroid body 11...

Page 28: ...st which was between the 38th and 39th turns of the toroid winding Push the wire which came over the toroid body into hole 7 Now you have five turns of wire which make up secondary 2 between holes 7 a...

Page 29: ...ds or so to allow the enamel insulation to burn off Repeat for all the other wires one pair at a time until all eight are soldered If you have a DVM check for DC continuity zero ohms resistance across...

Page 30: ...the diameter of the PCB holes Try to position the socket as far towards the top edge of the PCB as possible oriented as the diagram This is necessary to create sufficient space for the Paddle and Ear...

Page 31: ...which way round the capacitors are installed However it is very good practice to install them all with the capacitor label facing in the same direction For example ensure all the 104 labels face to t...

Page 32: ...474 capacitors The five 470nF capacitors are labeled 474 and are capacitors C11 C43 C44 C45 and C46 3 6 Install 47nF 473 capacitor The 47nF 0 047uF capacitor is labeled 473 and is capacitor C9 QCX min...

Page 33: ...sen band The capacitors are located inside the separate Low Pass Filter bag in your main kit bag Refer to the following table to find the correct capacitor value for your band Band Value Label 80m 120...

Page 34: ...osen band The capacitors are located inside the separate Low Pass Filter bag in your main kit bag Refer to the following table to find the correct capacitor value for your band Band Value Label 80m 47...

Page 35: ...rtant note for the 60m band the capacitor requires both the 30pF and 56pF capacitors to be soldered in parallel but there is only one component position on the PCB For 60m you will need to install one...

Page 36: ...values Install the capacitor s appropriate to your band Refer to the following table to find the correct capacitor value s for your band Where none is indicated in the table do not install the corres...

Page 37: ...3 11 Install 1uF 105 capacitors C21 and C22 There are two 1uF capacitors labeled 105 which are C21 and C22 QCX mini assembly Rev 1 05 37...

Page 38: ...diode is used because the forward conducting voltage drop is less than an ordinary diode However on transmit the voltage drop across this diode can still be up to 400mV This reduction in voltage does...

Page 39: ...3 13 Install 20MHz crystal XTAL1 The engraving on this crystal is 20 000 or CE20 000 QCX mini assembly Rev 1 05 39...

Page 40: ...s crystal is 27 000 If you are going to install the 25MHz TCXO skip this diagram and refer to the following instructions If installing the TCXO module this is installed in the same position as the 27M...

Page 41: ...the board Thread the TCXO module carefully onto the wires Make sure module is pushed as far towards the 28 pin IC socket as possible Solder the wires to the pads holding the TCXO module firmly tight a...

Page 42: ...entiometer This 24 turn trimmer resistor is the small blue box component with label 501 It is R27 The trimmer resistor has to be prepared carefully before installation The resistor has little ledges o...

Page 43: ...he small blue box components with label 503 The trimmer potentiometers must be prepared before installation by cutting off the small plastic ledges with a wire cutter as described in the preceding sec...

Page 44: ...by the solid black bar the POSITIVE wire must be installed in the hole indicated on the PCB silkscreen and the layout diagram by the hollow bar and the symbol Electrolytic capacitors are also supplied...

Page 45: ...arefully and with the correct orientation which matches the PCB Cut the small pin stubs on the underside solder side of the PCB they only protrude a few mm but when installed in the enclosure could be...

Page 46: ...ther transistors Carefully bend and insert the wires so that the transistor s flat side is flat flush against the PCB surface and the body of the transistor is aligned with the square on the layout di...

Page 47: ...y aligned in the correct positions near the hole in the PCB After installation use the supplied 10mm M3 steel screw 12mm steel washer and M3 nut to bolt the transistors flat sides firmly flat on the P...

Page 48: ...er can be used to connect an AVR Programmer to apply firmware updates if desired Insert the SHORT end of the pins into the PCB Solder one pin in place first and check that the header is nicely seated...

Page 49: ...ring the other 9 Be sure to install the 2x5 pin FEMALE pin header connector see right not the male header connector which is installed on the LCD board Try to hold the socket as far towards the top si...

Page 50: ...efer to the following table Inductance values are approximate and will depend on variations in the core and how tightly you wind the turns Do not worry about these variations which are not critical in...

Page 51: ...toroid does not fall out while you are trying to solder the wires Cut off the excess wire using wire cutters leaving only about 2mm protruding through the PCB on the lower side Now apply solder quite...

Page 52: ...e toroids are installed laying flat on the PCB in the QCX mini it s much easier to spread out the turns if they were initially bunched together than it would be to bunch them up if they were initially...

Page 53: ...eck The number of turns is band dependent refer to the following table Inductance values are approximate and will depend on variations in the core and how tightly you wind the turns Do not worry about...

Page 54: ...connector orientated to match the PCB silkscreen It is important to install this accurately so that if you install the QCX mini in the optional aluminium enclosure the connector is correctly aligned w...

Page 55: ...r must sit squarely on the board with the metal body extending at 90 degrees to the PCB edge Cut the excess wire length of the center pin after soldering The metal body of the BNC connector is directl...

Page 56: ...osure if used The connectors have a tendency to sit at a crooked angle This must be corrected One way to ensure precise positioning is to angle some solder a few cm above the workbench then hold the b...

Page 57: ...by the uSDX group https groups io g ucx The small PCBs can be snapped out of the panel using needle nosed pliers or wire cutters or some such similar tool Breaking out the controls board is easiest j...

Page 58: ...st the PCB bridge to snap out using pliers or wire cutter Gently grip and bend to remove but be careful of the central spar of the panel PCB File rough edges of the cut outs in the panel PCB and the b...

Page 59: ...ut Bolt the LCD module ensuring equal gaps at top and bottom tighten screws firmly Drop component off cuts through the 16 holes their bottom ends sitting on the bench Solder the component off cut wire...

Page 60: ...ther nine Try to ensure that the pin header sits squarely and centrally in its allocated position 3 33 Install four 11mm nylon spacers Install four 11mm nylon hex spacers on the underside of the LCD P...

Page 61: ...B from sliding into the QCX mini enclosure later Therefore it is necessary to cut off the protruding feet using a wire cutter so that the potentiometer can sit flat on the PCB Follow the steps below t...

Page 62: ...pin side of the 2x4 pin header Insert the remaining length of the short pins into the PCB from the under side as shown The pins don t protrude from the top side of the holes However the holes are thro...

Page 63: ...switch pins Bend over the pin through 180 degrees so that it points to the front of the control Repeat the same procedure for the four remaining pins Install the rotary encoder first line up pins so...

Page 64: ...ar to the PCB Solder two diagonally opposite pins first then check the alignment and make any adjustments necessary when all is well solder the two remaining pins 3 38 Install gain control potentiomet...

Page 65: ...oint as possible so that there are no shorts when the boards are fitted into the enclosure 3 39 Install 11mm nylon hex spacer The final 11mm nylon hex spacer is bolted to the controls PCB using an M3...

Page 66: ...main PCB by plugging together the two 2x4 pin header connectors Fit an M3 6mm screw from the underside of the main PCB screwed into the 11mm nylon hex spacer pillar that is fixed to the Controls PCB a...

Page 67: ...d on and off or if the power plug is inserted while powered up then voltage spikes can be generated the voltage regulator can become unstable and fail It fails with the Input connected to the Output w...

Page 68: ...sier to solder in the next step The capacitor should be face label side down Pass the positive wire through next to the diode body solder quickly the negative wire to the left pin of the voltage regul...

Page 69: ...all blob of solder which will make it easier to the voltage regulator pin Tin the left hand voltage regulator pin too Put the capacitor lying horizontally in position with the positive wire passing be...

Page 70: ...lignment steps must be done BEFORE installation in the optional enclosure 3 44 Connections for basic operation The following connections are required for basic transceiver operation 1 Power supply A p...

Page 71: ...s also normal until you have adjusted the LCD contrast potentiometer R47 2 You can remove the Controls board to do certain fault finding Since R46 10K resistor was thoughtfully placed on the main boar...

Page 72: ...and alignment The first thing that you will notice when you apply power to the radio is that there is probably nothing at all shown on the display Perhaps the LCD back light will not even be on it dep...

Page 73: ...een as an intuitive bar across the bottom row of the display Using a screwdriver you adjust the trimmer component in order to maximize or minimize the displayed amplitude Unplug the antenna during ali...

Page 74: ...or so you can adjust the C1 trimmer potentiometer Now if you have earphones plugged in and lying hopefully on your bench you will hear a loud tone at 700Hz The display will look like this photograph Q...

Page 75: ...end of its range If it is this means the resonant circuit is NOT correctly peaked You need to adjust the number of turns on the long secondary winding of the T1 transformer Visual inspection will sho...

Page 76: ...e MINIMUM amplitude not the maximum we adjusted the BPF trimmer to We adjust for minimum because now the injected signal is measuring the unwanted sideband We want to MINIMIZE the unwanted sideband le...

Page 77: ...nfiguration menu and you should read the operation manual to understand and make use of all the features You may now bolt the two PCBs together by screwing the four M3 6mm nylon screws into the 11mm s...

Page 78: ...play board and the main board with the controls board bolted in position Peel off the plastic protective coating from the LCD module Here s the top half of the enclosure Note the PCB guide rails in th...

Page 79: ...his is how it looks from the DC connector end Determine correct orientation of the bottom half of the enclosure note the tongue and groove arrangement which means that the bottom half only fits one wa...

Page 80: ...knobs by tightening their grub screws Leave a small gap between each knob and the front panel to allow the knob to turn easily The grub screw of the volume control knob should be at the position of th...

Page 81: ...isplayed on screen if you have configured it The GPS produces two output signals PPS Pulse per second and RxD Serial data in addition to ground Optionally 5V may also be connected in order to power th...

Page 82: ...is PTT This has the disadvantage that plugging in a cable while the QCX is powered can short the 5V to ground Another disadvantage is that standard 3 5mm stereo audio cables can t be used For this re...

Page 83: ...I Q outputs are at audio baseband and go through a 90 degree phase shift network which cancels the unwanted sideband A 200Hz bandwidth CW filter is followed by more amplification and drives common ea...

Page 84: ...QCX mini assembly Rev 1 05 84...

Page 85: ...on the previous page The circuit diagram of the display PCB is shown below The circuit diagram of the uSDX daughterboard is shown below Refer to the uSDX groups io forum https groups io g ucx for deta...

Page 86: ...nth dividers and in this CW transceiver design this recommendation is followed The synthesizer section of the circuit diagram is shown here right The Si5351A datasheet dictates the use of a 25 or 27MH...

Page 87: ...documentation However QRP Labs has perfected the technique to put two of the Si5351A outputs into precise 90 degree quadrature which is maintained without tuning glitches as the frequency is altered I...

Page 88: ...he combination of the relatively large 470nF capacitors and the low source resistance results in a fast roll off of the audio response This is effectively a very narrow band pass filter since any inco...

Page 89: ...and pass filter is implemented by a fourth winding on the same transformer T1 with some fixed capacitors and a trimmer capacitor forming the resonant circuit It is only a single resonant circuit band...

Page 90: ...ter kit design by David Cripe NM0S available from the Four State QRP group http www 4sqrp com HiPerMite php thanks David for permission to use it here This is a high performance circuit specifically d...

Page 91: ...to be heard To prevent this diode D5 was added which bypasses the resistor R60 at the receive to transmit switchover It ensures that at the receive to transmit event the mute switch is enabled instant...

Page 92: ...sting the I Q balance and phase adjustment controls So some of the spare gates in the 74ACT00 are used to switch the signal generator on off and the RF Power Amplifier PA signal on off separately When...

Page 93: ...nce of the transistors normally an unpleasant lossy aspect is now a part of the tuned circuit Class E also has a reputation for being difficult to achieve All those intense mathematics Google will hel...

Page 94: ...with MOSFETs their resistance INCREASES as the temperature goes up so there is an inherent self balancing when multiple devices are used in parallel without any need for additional balancing resistors...

Page 95: ...rz com db W6JL though as he says this integrator type keying circuit is found in many published homebrew designs over the past 40 years or so so it is nothing new It uses a PNP transistor Q6 and R C i...

Page 96: ...on ATmega328P processor The ATmega328P was chosen because it has enough processing power and I O to handle all the tasks required here It is also common and inexpensive and lots of QRP Labs products a...

Page 97: ...can also be converted into noise detected in the receiver front end To combat the radiation issue the ATmega328P microcontroller is sited away from the RF and audio signal paths right at the front of...

Page 98: ...he time the pin can be left in a high or low state under processor control MOSFET Q100 is used as a switch in series with the backlight LCD It is switched ON by 5V on the gate pin However to avoid pot...

Page 99: ...an eye on all the buttons key paddle inputs and rotary encoder switches When button or switch closures occur as the operator activates a control the microprocessor responds immediately as required Th...

Page 100: ...crocontroller which is one of the Analogue to Digital Converter ADC channels No microcontroller internal pull up is used because I need to have precise control over the voltage levels The circuit frag...

Page 101: ...the microcontroller with precise timing in the Beacon and Iambic keyer modes It is routed to the RF envelope key shaping circuit discussed previously The Transmit Receive switch must remain in transm...

Page 102: ...lses would also work 9600 baud serial data sending standard NMEA 0183 GPS sentences TTL voltage levels i e 0V low and 5V high or at least below 1 5V and above 3 5V The GPS is therefore wired directly...

Page 103: ...iver kit DVM and RF Power meter The DVM and RF Power meter is implemented using the 10 bit ADC channel 2 input to the microcontroller When measuring voltage connect the voltage to be measured to pin 3...

Page 104: ...full ADC range is not really used The AUDIO2 channel connected at the CW filter input has even less amplitude so is not used for anything except the BPF Alignment procedure which has a very strong sig...

Page 105: ...of the 1 s digit OR by pressing the Exit button then the cursor will no longer be shown on the display Turning the rotary encoder 1 click anticlockwise shows menu 9 5 the frequency measurement Now if...

Page 106: ...r between the main PCB and the LCD board No back light can also be NORMAL If the backlight setting in the Other menu configuration in menu 7 is set to OFF then the backlight will be off naturally The...

Page 107: ...nd that in some of these measurements the act of measurement will itself change the measurement The input resistance of this DVM is only 13 3K R56 and R57 in series assuming high input resistance of t...

Page 108: ...e QSD integrating capacitors The voltage across each one differs slightly The following measurements show these small differences Left side of C44 2 44 Left side of C45 2 42 Left side of C46 2 30 IC5...

Page 109: ...back light cathode 5 5 RF Power output check If you go into menu 9 2 RF Power you can check your RF power output Your RF output should be connected to a dummy load since the RF power calculation assum...

Page 110: ...ing At this point the indicated power will be even less reliable than across the dummy load But at least see if it reads some watts If you see power output at the BS170 drains but not at the overall t...

Page 111: ...lyzer with 50 ohm tracking generator QRP Labs 50 ohm Dummy Load kit http qrp labs com dummy XONAR U5 24 bit external USB sound card Argo and Spectran audio analysis software from http weaksignals com...

Page 112: ...s show the power measurement using the oscilloscope to measure the peak peak amplitude across the 50 ohm dummy load Operation at output power levels above 5W is NOT recommended as it will put too much...

Page 113: ...gnoring the ringing artifacts due to poor scope probes etc the waveforms are correct for Class E operation 6 5 RF envelope key shaping The following oscilloscope images show the RF envelope when keyed...

Page 114: ...ows the close in response the other shows the response across 0 to 30MHz Note that in the circuit the Band Pass filter sits behind the Low Pass filter Therefore the low pass filter response is ADDED t...

Page 115: ...idth 367kHz Insertion loss 1 45dB at 5 357MHz center Tuning range 4 63MHz to 5 91MHz 40m band 3dB bandwidth 514kHz Insertion loss 1 97dB at 7 020MHz center Tuning range 6 04MHz to 8 36MHz QCX mini ass...

Page 116: ...h 645kHz Insertion loss 2 67dB at 10 120MHz center Tuning range 7 93MHz to 12 47MHz 20m band 3dB bandwidth 1 083kHz Insertion loss 1 30dB at 14 020MHz center Tuning range 9 69MHz to 22 54MHz QCX mini...

Page 117: ...stic of this circuit is an advantage because it effectively adds an additional narrow RF bandpass filter to the receiver preventing strong nearby signals from reaching the audio amplifier stages This...

Page 118: ...rove the selectivity of the CW filter but it does improve the intermodulation characteristics of the receiver 6 9 CW Filter response This chart shows the measured response of the CW filter combined wi...

Page 119: ...the I Q balance and audio phase shift adjustments and these curves are from one measured prototype Subtraction of these two curves results in the unwanted sideband level as shown in the following char...

Page 120: ...6 Beacon 6 1 Beacon On Off enable beacon operation 6 2 Mode CW or WSPR 6 3 Frequency beacon frequency 6 4 Frame frame duration in minutes 6 5 Start minutes past the hour to start 6 6 WSPR call Callsi...

Page 121: ...for details 9 Document Revision History 1 00 10 Dec 2020 First version 1 00 for QCX mini 1 01 11 Dec 2020 Edits in the circuit description and other sections corrections to many typos 1 02 12 Dec 202...

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