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8.2 Low Pass Filter
characteristics

These measurements show the filter characteristic of the
Low Pass Filter section only. An Advantest R3361C
Spectrum Analyzer with Tracking Generator was used. BNC
connectors were soldered to the LPF filter and output and
connected to the spectrum analyzer using 50-ohm coaxial
cables.

40m: right
30m: below left
20m: below right

8.3 Keying envelope

The oscilloscope trace below shows a single 24wpm CW “dit” (50ms duration) on 40m at 54W
power output.

50W QCX PA kit assembly

1.00q

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Summary of Contents for QCX 50W Amp

Page 1: ...F QCX Power Amplifier kit assembly manual A low cost 50W Power Amplifier for the QCX with Full break in QSK solid state Tx Rx switching PCB Rev 2 Designed and produced by QRP Labs 2019 50W QCX PA kit...

Page 2: ...2 3 13 Install capacitors C4 and C5 23 3 14 Install capacitors C8 and C10 23 3 15 Install diodes D2 4 6 8 9 and 10 24 3 16 Install diodes D1 3 5 and 7 24 3 17 Install inductors L1 2 3 5 8 9 and 10 25...

Page 3: ...ns 50 6 5 Low Pass Filter 52 6 6 Transmit Receive switching 53 7 Amplifier set up and operation 64 7 1 PA Precautions 64 7 2 Power supply 65 7 3 Connection to QCX 65 7 4 Adjustment 66 7 5 Supply volta...

Page 4: ...n it may be worthwhile experimenting with this but you will be in experimental territory This amplifier is intended for single band operation and may be built for 80 40 30 or 20m using the supplied co...

Page 5: ...ch have not yet been installed VERY IMPORTANT SOLDERING ADVICE when soldering a component pin that goes to ground you need to allow extra time with the soldering iron Ground pads are separated from th...

Page 6: ...16 1uF 50V code 105 7pcs C4 5 22nF 50V code 223 2pcs C8 10 47pF 250V code 470 2pcs C9 11 12 14 18 19 20 21 Band dependent capacitors see text C15 17 0 33uF 250V code 334 2pcs 180pF 250V code 181 2pcs...

Page 7: ...cs Heatsink 130mm x 28mm x 25mm deep 2pcs Enclosure kit OPTIONAL Enclosure 63w x 25h x 130d mm enclosure drilled cut and engraved Screw Enclosure front and rear panel mounting screws 8pcs Feet Self ad...

Page 8: ...50W QCX PA kit assembly 1 00q 8...

Page 9: ...50W QCX PA kit assembly 1 00q 9...

Page 10: ...at frequent intervals not more than 0 1 inches by vias 3 3 Wind and install output transformer T3 We start with the installation of the transformers and inductors which are generally the most difficul...

Page 11: ...y winding having 2 turns If you write on the middle part of the tape then it is less likely that the ink will be worn off during handling of the transformer during the installation on the PCB Wind the...

Page 12: ...ire ends are on the right side nearest to the transistors Q1 and Q2 whereas the 3 turn secondary winding wire ends must be on the left near inductor L4 Now use a DVM set to resistance mode or continui...

Page 13: ...time the wire passes through the hole of the toroid counts as 1 turn Wind the wire as tightly as possible on the core Wind the turns bunched up next to each other 2 When the number of turns is correct...

Page 14: ...is measurement If you do NOT see DC continuity then it means that one or more of the inductor wire ends is not properly stripped of enamel and trimmed this is a common error 3 6 Wind and install bifil...

Page 15: ...of determining which wire goes in which hole on the PCB It has to be done CORRECTLY or the kit will not work Step by step slowly and carefully will ensure success Scrape and tin the wire ends just fo...

Page 16: ...from the underside of the toroid goes in the hole near the Q1 text The corresponding black marked wire goes in the hole near C7 In this way the lower pair of wires Q1 C7 are from the twisted pair tha...

Page 17: ...op right hole nearest to C6 Check for good DC continuity with the DVM from the middle pin of transistor Q1 to the right hand side of capacitor C6 3 7 Wind and install trifilar transformer T1 The three...

Page 18: ...ientated as precisely as possible over the white circle drawn on the PCB silkscreen Put the correct wires in the correct holes Pull the wires through reasonably tightly still keeping the toroid in pos...

Page 19: ...seen here Finally repeat the same procedure for wires T 0 and B 0 Check for DC continuity between the left pin of Q2 s three pads and the same wiper pin of the trimmer potentiometer R5 If all is well...

Page 20: ...and C12 These two capacitors are 250V NP0 C0G types The capacitor value depends on the band the 50W PA kit is being built for as follows Band Value Code 80m 680pF 681 40m 680pF 681 30m 390pF 391 20m...

Page 21: ...0 C0G types The capacitor value depends on the band the 50W PA kit is being built for as follows Band Value Code 80m 390pF 391 40m Not installed 30m 180pF 181 20m Not installed The capacitors are ONLY...

Page 22: ...nd C7 are very close to the bifilar transformer T2 As long as you have positioned T2 nicely over the circle printed on the silkscreen there should be enough space for the capacitors Note that C16 hole...

Page 23: ...Install capacitors C4 and C5 These are 22nF 0 022uF 50V ceramic capacitors code 223 3 14 Install capacitors C8 and C10 These are 250V NP0 C0G types with value 47pF code 470 50W QCX PA kit assembly 1 0...

Page 24: ...lower edge of the board so that the black stripe is aligned with the corresponding white stripe on the silkscreen printing 3 16 Install diodes D1 3 5 and 7 These are 1N4007 diodes The orientation is...

Page 25: ...lded inductors Bend the leads as shown and install carefully 3 18 Install resistors R7 10 11 14 and 17 These are 10K W resistors colour code brown black black red brown Bend the leads as shown lying t...

Page 26: ...15 This is a 1 5K resistor colour code brown green red gold or brown green black brown brown 3 20 Install resistor R16 This is a 470 ohm resistor colour code yellow purple black black brown 50W QCX PA...

Page 27: ...is is the FQPF2N60C MOSFET in an all plastic TO220 package which is installed on the underside of the PCB It is important to install the transistor with the correct orientation as shown Bend the wires...

Page 28: ...are BS170 MOSFETs in TO92 packages Be very careful to check that the transistors you pick really are the BS170 since IC1 78L05 looks exactly the same except for the different writing on the label The...

Page 29: ...ular pattern on the PCB Make sure the flat of the IC body is aligned perfectly with the flat on the PCB silkscreen It s not necessary to push the IC all the way in a lead length of 5mm or so is fine 3...

Page 30: ...470 ohm 2W resistors with colour code yellow purple brown gold or yellow purple black black brown 3 28 Install resistor R3 This is a 22 ohm 2W resistor colour code red red black gold or red red black...

Page 31: ...ur code orange orange brown gold or orange orange black black brown 3 30 Install trimmer resistor R5 This is a 4 7K linear trimmer potentiometer Push the potentiometer pins gently but firmly into the...

Page 32: ...power input The PCB holes are quite large so the connector must be installed carefully to align it properly with the outline on the PCB silkscreen This is particularly important if you are planning t...

Page 33: ...the alignment is not accurate the hole and the barrel will not line up correctly Note two of the pads are very close together and you may easily create a solder bridge between them don t worry These...

Page 34: ...at the body of the LED Then bend the other wire down also but at a distance of 0 1 inches from the LED body Do the same thing for both LEDs Install the LEDs as pictured sitting with their flats right...

Page 35: ...pins upward of the IRF510 90 degrees right at the body of the IRF510 Make sure to bend them upward not downward you only get one chance at this if you bend them the wrong way then try to correct it th...

Page 36: ...s not ideal mechanically and you really should consider using a proper enclosure Use a DVM with one probe held in the screw head and the other touched to the central pin of each IRF510 You should see...

Page 37: ...embly BEFORE soldering to the PCB Otherwise there will be a small gap around 1mm between the enclosure heatsinks and the IRF510 tab Tightening the mounting bolt to eliminate that gap would put undue m...

Page 38: ...re lid Bend the pins upward of the IRF510 90 degrees right at the body of the IRF510 Make sure to bend them upward not downward you only get one chance at this if you bend them the wrong way then try...

Page 39: ...sure The PCB slots are shown here right By having the PCB upside down the IRF510 transistors which are NOT yet soldered will fall away from the enclosure lid allowing them to slide past the silicone p...

Page 40: ...of the top screw and the other probe on the middle pin of the top IRF510 Q2 which is the Drain Be sure to touch the probe to the pin not to the pad remember you have not soldered yet so the connection...

Page 41: ...of the front panel and two in the bottom corners of the rear panel see below left The top and bottom of the enclosure use a tongue and groove system on the long edges what this means is that the enclo...

Page 42: ...e of the details behind the circuit and choices made in the design and production of the kit In this section I will provide an explanation of the circuit sections including design choices Some of the...

Page 43: ...50W QCX PA kit assembly 1 00q 43...

Page 44: ...push pull circuit consisting of two IRF510 power MOSFETs in push pull configuration DC power is supplied to the transistor drains via transformer T2 which is bifilar wound on a ferrite toroid The RF o...

Page 45: ...o 10m operation and discuss the differences in gate bias arrangements The same DC power method is used here bifilar transformer T203 and the same output combiner impedance matching transformer T204 No...

Page 46: ...om the same batch will have approximately similar characteristics I am not going to adjust them for a precise bias current of 125mA each for linear operation I m going to adjust them just for Class C...

Page 47: ...t material and core topology to use The turns ratio determines the impedance transformation The impedance ratio is the square of the turns ratio We assume a 50 ohm output enforced by the 50 ohm dummy...

Page 48: ...her A beautiful binocular core typically used is the BN43 202 made by Amidon or an equivalent by Fair rite But these are expensive and they must be shipped from US and imported They are also rather sm...

Page 49: ...the following conclusions 1 The stacked Amidon BN42 202 binocular cores did indeed provide higher power output at a given supply voltage with higher impedance ratio However there was a high penalty 10...

Page 50: ...sign and the fact he is aiming for linear operation could inspire you to try this 50W amplifier as a linear Mike does not present any linearity measurement two tone IMD3 for example But I feel a lot o...

Page 51: ...45 C Generally it was around 40 C So considering an ambient temperature of 20 25 C we can say roughly a 20 C temperature rise to ambient So in this case at worst 45C the temperature rise would be 112...

Page 52: ...that they will not saturate and become lossy at 50W NP0 C0G capacitors which have a high enough voltage rating to withstand the peak voltage excursions seen at 50W 50W is a 141 V peak peak sinewave I...

Page 53: ...at the air core inductors containing more wire and hence higher resistance may be a lower Q than the toroidal ones and may also interact with each other resulting in signal leakage The result was slig...

Page 54: ...tive feedback situation will exist and power will travel around the loop from output to input in a self sustaining spurious oscillation that is at the very least undesirable and at the worst could cau...

Page 55: ...00kHz What about dynamic range and third order intercept point IP3 A properly handled 1N4007 should not impact the dynamic range or IP3 An excellent set of measurements by Claudio IN3OTD see https www...

Page 56: ...spikes at the instant of transmit receive switchover I myself noted these problems too but I worked hard on the design to find out the cause and fix it When the two rules are followed carefully the pe...

Page 57: ...path Note that it would be possible to just leave the PA connected all the time eliminating SW1 and SW3 and this is indeed a short cut that some designs take However I wanted to ensure that the anten...

Page 58: ...uring Receive transistor Q6 gate is 0V and this switches MOSFET Q6 Off P channel MOSFET Q4 is then also Off because its gate voltage becomes 12V 20V supply via R10 The supply voltage also feeds into t...

Page 59: ...960 ohms The current through R12 will therefore be 12 970 12mA at 12V supply and 20 970 20mA at 20V supply this current is shared between D3 and D5 equally The forward bias current in each diode is th...

Page 60: ...age never arrives instantaneously because there is envelope shaping provided by the QCX to prevent key clicks which by the way would be all the more important when transmitting 50W than at QRP power l...

Page 61: ...potential damage and the BS170 transistor survives However it s unpleasant to think of such unpleasant things going on You can even actually HEAR it in the QCX earphones during transmit it appears as...

Page 62: ...litude to zero gradually avoiding key clicks With a modern digital storage oscilloscope we can record a complete slow sweep such as this with a high sample depth I used 40 million at 12 bit resolution...

Page 63: ...om the loudspeakers through the losses of the air and into the microphone to be amplified and go around the same loop fast enough to create a spurious signal the squeal Here the same thing happens but...

Page 64: ...5K This discharge is faster than the charging of the SW2 capacitor C13 which is charged through 470 ohms but is 1uF The positive feedback situation is completely avoided The choice of component values...

Page 65: ...e later section 7 3 Connection to QCX The diagram shows an appropriate connection between the QCX and the 50W PA kit A simple coaxial cable is used from the QCX RF Out to the 50W PA RF In note Out In...

Page 66: ...consumption of the 50W PA kit For example suppose the current consumption is 85mA Then turn the R5 trimmer potentiometer very slightly clockwise a tiny little bit at a time until you see the current...

Page 67: ...voltage for the production models assembled for testing In each case each band 80 40 30 and 20m the input power was provided by a QCX powered at 12V supply The power was measured as 3 2W into a 50 ohm...

Page 68: ...yzer with Tracking Generator was used BNC connectors were soldered to the LPF filter and output and connected to the spectrum analyzer using 50 ohm coaxial cables 40m right 30m below left 20m below ri...

Page 69: ...of switch SW3 1 00d 13 Dec 2019 Corrected various typos thanks Fred VK2WS 1 00e 13 Dec 2019 Corrected one more typos thanks Fred VK2WS 1 00f 13 Dec 2019 Corrected more typos and fixed the problem wit...

Page 70: ...tion 1 00m 16 Jun 2020 Clarification of section 3 6 continuity tests 1 00n 16 Jun 2020 1 5K resistor can be 5 band colour code Removed 7 1 sentence about capacitor polarity there are none 1 00o 01 Oct...

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