Short video looking at a different configuration for the RF power amplifier as mentioned in the last video, and an on-air test. To skip the QSO, go to 4:22. Thanks Wayne ZL2OZ for the QSO.
Hi Charlie. In my 40 years being in electronics I never had anything to do with short wave RF design. However, last November I passed the exam for F-license and now I am totally hooked up to RF design videos. (Specially when the wife is watching "The Voice"). I am actually mostly interested in how it all works. For my work I have designed wearables at 869 and 918 MHz but I really like the 20m band for the hobby, it's much more fun than all the 0201 stuff on small boards with everything on chip. Your videos are amazing, especially the way you take a piece of paper and explain each and every resistor and transformer. I have built an SSB receiver with a Tayloe detector and used a Teensy with your Hilbert transform software explanation to make a phase shifter. It works amazingly well. Now I try to built a 5 watt 20m transmitter with two times BD139 in push pull setting. I modelled it in LTspice and it seems to work reasonably well. However, if I drive the input transformer (3:1) with a 50 ohm generator LTspice shows distortion on the generator output going into the transformer. The emitter resistor of the BD139's is 2 ohm, if I understand your calculations right that should reveal some 25-30 ohm impedance at the input. Making the generator very low impedance tackles that problem but I wonder what's going on. I used that low-pass filter you explained lately and it gives a perfect, low harmonic output on the 50 ohm output dummy side. If I put a 220pF in parallel with the 50 ohm input the distortion on the input is gone. Somehow I get some interference out of the input stage.... am I right? I calculated with FT37-43, and have more than enough reactance. Can you give me some hints about what I am possibly doing wrong? Many thanks, Harold PA0AHL The Netherlands
Hi Harold. I'll have to put some thought into this as I don't have a answer right now. Sorry about that. Might pay to double check the inductive reactance of the smaller winding approx. 4-5x what it is connected to.
congrats on 5k subs! I've been going through all your videos and building my first rig. Got a receiver working and it sounds good! Working on a variation of this circuit now. For the output side I found a trifilar wound xfmr in a 1:2 configuration on the drain (right into a .1 cap) gives the most output. The mosfet sees 12.5 ohms at the drain with a 50 ohm load. I can get 5W out with 12v supply, admittedly with a 10v p-p signal from my function generator. I'm still playing with the drive circuit though. I'm finding a lot of the signal at the drain makes it back to the gate through the gate-drain internal capacitance. Since I want it for 40/20 band use I'm thinking to keep the 50 ohm resistor so it swamps (relatively) the very different reactance of the gate capacitor at 7 vs 14mhz. My next step is to try a common emitter amp to drive the mosfet. Anyway just wanted to share some ideas and say thanks for all this great info! 73 cheers!
That's fantastic. Sounds like you have had great success. I'm starting to play more with keyboard digital modes, which is interesting. Keep us posted on how you get on. Regards Charlie ZL2CTM
That's exactly what i picked up on. I had been monitoring the signal on several remote SDR receivers and it looked good. Always nice to hear it from another operator though. Very happy.
Very nice radio. if you do buy a case, get a bigger case than you need - this way you can open the case and not damage the radio. I damaged a MFJ Cub I built because i kept getting the radio out of the case to tweak it.
Thanks Charlie, I'm just starting to wrap my head around AGC for my rig. I'm thinking separate IF AGC and audio level control. I always look forward to your posts. WA8HTO
Thanks Jerry. I'm just debating about adding to this rig, i.e., add a flash TFT display, AGC etc. Not sure if that will be of interest to folks or not.
Hello Charlie - just to let you I'm copying (ish) your build and look forward to each video. I'm a rank beginner so the simpler the better for me. Keep doing what you're doing! One of the bigger changes I've made is using a RP2040 Pico (£3.90 plus postage) with Python rather than the Arduino - that makes the coding a lot easier for me (I got the Python hint from the YT-er NA5Y). Thanks Kevin M0XYM
No issues here Kevin. Just note these are not tutorials, just a video log of what I am getting up to. I know there will be errors here and there. I did some Python a while back for another project and it worked well. I guess I grew up with C so tend to stick with C/C++.
Of course you did say you wanted to keep it simple, so yes you are correct with no AGC. I have a real problem when designing things of going crazy with the features! You have to draw the line somewhere, unfortunately for me I keep moving that line which I think is the main reason a lot of my projects are never totally finished.
Yeah feature creep is a real problem for me too. Projects take longer and longer and the more time it spends on the bench the more ideas I have for it and thus it spends more time on the bench.
I hear you Brian. I'm thinking I might use this rig as a spring board and add other features. Not sure if that would be of interest to others or not. Perhaps add features with simple readily available components?
I would like to build only one rig and keep making it better and keep putting in features. I see it as a combat with commercial radios which are taking over our hobby more and more. I want to show, that home brew can match the commercial radios. And yes, my heart lies in the good old electronics, but I also have to go into digital filtering. Just to show that it can be done by amateurs. But my list is long and until now I have had a working day of 12 hours. I recently got started at my life as pensioned and now I got the time to live out the dream.
And for the output. The optimum impedance the IRF510 needs to see at 24V supply is 35R for 10W output. So your 5:6 transformer is about right to match a 50R output, but you have it the wrong way round !
Agree, it should be much more than 5W. Low power output may indicate either an impedance mismatch (the energy is reflected back to the driver) or a low drive level, or both. Personally, I would model the circuit in LTspice to get a better idea of what's going on. The model is usually quite accurate for such kinds of circuits. As a side note, there is a good 10W PA circuit based on IRF510s, that can be found in uBITX schematic. Requires 13.8V and I had no problems reproducing it. Unless the idea is to experiment with new designs, of course.
Thanks Alex. I did vary the driver output transformer ratio looking for maxi drive. Great QRP rig, but we can make it better! As for copying designs, I'm happy to do that only if I can reproduce the underlying maths/design. My preference is not to simply lift a circuit out of a book or off the internet. I just wish SSDRA and EMRFD and the like did that.
I'm looking at building my own version of the uBitX. Will have dual conversion with 45mhz and 11.2mhz IF's (found surplus cell phone 45mhz filters on Ebay, and the 11.2mhz SSB filter came from an old CB radio). Will use the si5351 for all oscillators (might use the two 11.2 mhz crystals from the CB rig for BFO/Carrier oscillator). Will re-arrange things a bit, on RX will put IF amps AFTER the filters, and will use separate IF amps / product detector / bal modulators instead of true bi-lateral. I have some MCL combiner/splitters I might use.
@@CharlieMorrisZL2CTM I plan on using Kits&Parts HYCAS IF amp kit for the receiver IF, and will base my two mixers on his diode ring kits. Will probably use MC1496 as balanced modulator and NE612 as product detector. I wired up the 30mhz LPF from the uBitX schematic and tested it with the NanoVNA. Works as described, drops off like a rock after 30mhz with deepest null at about 44mhz, then attenuation decreases slowly as frequency rises toward 90 mhz. I guess the inductors loose effect as the distributed capacitance takes hold.
Just had a thought of something new that may make the video more interesting when transmitting. I was playing with some online SDRs and noticed the other day some have a record function to them. Listening to both sides of the radio would be cool. :)
Every nice. I have some good power transistors i plan on using for my PA. Got two MRF453 from a CB amp i bought from a thrift store for $5 (mostly to make it work on a different band, plus its something illegal off the streets.)
A good conclusion to a great project. Thanks for making these videos, certainly has encouraged me to have a go. Which rig to box up? A difficult call, the first one would give you two bands in the field but the second would be smaller, lighter and more in keeping with the QRP and field ethos. At the end of the day it's only you who can make the call Charlie! Best 73s.
What is the voltage swing around the bias on the IRF510 gate? General wisdom is you need to drive an IRF510 pretty hard, and I doubt you're getting much out of the 2N3904 in a linear mode. If it's (generously) hitting 50% efficiency, it can't operate within specs and be linear with more than about a quarter watt of output (maybe a 0.5W with a mondo heatsink and driven out of spec). Getting about 5W means you are probably seeing about 13dB of power gain -- not too bad for a non-RF transistor. Asking for 10W or more is asking it to perform like something that it's really not. It'll only give you so much for so much swing at the input, and it has that massive 180pF input capacitance to deal with too. A stronger driver might work -- could even just make it one more IRF510 in between, so it's three stages... 2N3904 -> IRF510 -> IRF510?
Old Collins Guy Alc and agc would be nice, Diving into teensy as a dsp alternative, Their so Cheep. got a few for experimenting with, The demodulation and audio Compression etc. and running the whole thing. the 4.0 or 4.1 Teensy has a lot of possibilities got some others , had a DSP 10 2meter in past and lost it because of moving among other things. Seems to me the dsp would eliminate the If Filter and amps and add multi modes.
hello, I think you should add a decoupling capasitor on the ampplifier at the 24V input just befor the RFC. Otherwise your 24V wire participate to the inductance value of this RFC and may also be a big source of RF leakage. As a general rule place as much decoupling capacitance as you can on any volatge source at the point of connection. on4ldy
It's good to keep it simple! get rid of luxury that is not necessary indeed. what struck me is that 5 Watt indeed seems too little to me? when I make an AM Grenade transmitter I thought I was closer to 10Watt. otherwise search with Google for: grenade transmitter maybe it's the amplifier stage, (amplification from transistor?) I used a 2N3866 a little less maybe 2N2219A could be work too! And who knows, you just need an extra staircase behind that 2N2904 ?! And who knows, 2X 2N2904 may still work??? or also pay attention to the capacitors that they do not heat up at the output. Healthy Greetings from the Netherlands! Rob
Whoa. IRF510 has input cap of 180pF, bias RFC is 35uH and transformer secondary is 17uH. These are in parallel so inductance is 11uH. Which just happens to resonate with IRF510 input 180pF at 3.5MHz. So reason for higher output is you have a tuned circuit between driver and PA? For broadband use better to keep the 56R resistor and use the transformer to present a suitable impedance as a load on the driver. Remember the drive 2N3904 is a current source so what you get out is the load impedance x AC current. AC current can at best be that generated by Vcc - Vbe pk-pk... am I right?
At 24V supply a load of 35R should produce 10W output, You just need to match this to 50R nominal load. If you feed output directly to 50R (no transformer) you should get 6W output, One comment I would make is that any MOSFET has a significant capacitive input impedance so that is what your diver is looking at with the bias RFC. I'm following you closely on this as I am struggling also to understand/make a QRP PA. I have the driver done (input -10 to -15dBm, 33-46dB gain, up to +25dBm output, or 4Vrms/300m 50R). Now looking at two output stages IRF530 and 2SC1971 or other.... 12V supply used. For sure I would like to implement ALC, what range?
Well done again Charlie! Regarding adding things to your rigs, I was wondering have you ever tried adding some compression to the audio? I've got a little SSM2167 module which is a mic preamp with compression. I've not had much chance to play with it yet but I was thinking of trying it at the front of my transmit chain to get a bit more punch in the signal. Too much compression can sound terrible but a little bit might just help a bit. Just something else to think about! Thanks again. 73, Nick M0NTV.
Thanks for the update, it’s a great project. I’m surprised by the output transformer on the IRF510, I always thought that the transitor’s output impedance was well below 50 ohms, which doesn’t seem to be the case here. I played around with the cheap Chinese amplifier kit available on eBay/AliExpress (the miniPA70) which are delivered normally with IRF530. I tried out some IRF510 and IRF520, everything else being equal and the output power was definitely the highest with the 530’s Maybe worth a trial ?
Thanks Frédéric. Yes, I was a little surprised too. As for other MOSFETs, yes I know there are other ones that will perform better. I think I'll look into those at some stage, including the RD06HHF1.
I’ve tried the 70 watt eBay amp, too. The IRF530 puts out more power, but they blow easily compared to the 510. I installed a pair of 510’s in mine, changed the turns on the output to 3:1 (I think), and used the WA2EBY input transformer. On 13.8 volts I get 25+ watts out on 40 meters CW. Best 510 amp I’ve ever had.
@@richardchambless12 Oh that's very interesting. Because I noticed that the ferrites provided with my kit (at least for the output transformer) were not type 43 materiel as most of the people were using. And I noticed the materiel was heating up quite a bit. When I swaped the output transformer out for a type 43 ferrite, the ferrites were heating much less, the output power jumped from 45W to 55W (CW) and the efficiency from less then 50% up to 60%, all under 14V. I noticed that the input match wasn't that great ither, it's the next problem I wanted to adress. Thanks for the clue, I will definitely check out for WA2EBY input transformer. 🙂
For AGC is it possible to send some of the audio into an Arduino analog input for a to d sampling? Grab three digital outputs to a simple decoder chip for an 8 step output which can drive a resistor ladder for a log curve attenuator? Sounds complicated but may be fun to calculate the resistor ladder that can interface with a simple op amp. The a to d also yields signal levels to display on the lcd display. Sounds complex but you could easily model it, build it and display the test log curves on your scope. A to D also yields good data like a simple cw detector to drive a morse decoder.
I have thought about that several times in the past, but using a digital potentiometer to vary the gain of an IF amp. I'll add this to the list as I think it would be interesting to pursue. Thanks.
Revisiting old one is better :) And also because you are using the same blocks with minor changes may be it's time to draw PCB for some o them ;) It will save little time in future but also space for sure.
Hey Charlie, I really like your content. Do you have a patreon account? I would certainly throw a few bucks your way every month so you can keep these videos going.
Hi Randy. No I don't, and at this stage I'd prefer not to as I don't want to feel tied. Right or wrong. I'll keep documenting what I get up to for the foreseeable future.