A 21st Century Novice Station

So i got excited and wired up the tx rx switching and the extra relay and got on the contest tonight to see if I can make a qso or 2. Well i didnt get there, but hey I tried, that first qso will come soon enough. 5 to 10w in the contest was probably not going to cut it with the time of day and conditions not perfect.

But as a concept, it works. Its not all that practical, tuning both the transmitter and the receiver to the same frequency and then flipping the switch and keying the transmitter. I am really going to have to work on some auto switching method so i can just key and having a single vfo dealing with both the transmitter and receiver would be nice.

So next thing after the hand held will be a receiver to mate with this and some integration to make it work as a transceiver should. All in all though, this has been a good learning experience and totally worth it and its been fun also.

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I Have Finally Built Something I Am Proud Of

So a few weeks ago I had sent and email to Pete Juliano asking some questions and showing off the progress of my projects, he commented that my work bench looked way to clean for me to have been productive, well I sent him this picture from this morning when I was hard at working building this project. There is currently CRAP EVERYWHERE.

Anyway, now onto the real story. The first thing I have built and designed that I am actually proud of. It all works as designed and I think i am finally starting to get up the learning curve.

So there is nothing revolutionary here. Its a simple CW transmitter biased Class A all the way. Why class A when there are more efficient biasing schemes for CW? Well, because I want reusable circuit blocks that I can use for DSB, SSB and other modes that require linearity.  The theory of operation is simple. We have an ATMEGA328 that controls an AD8950 DDS module, Q3 is a buffer so the gain stages do not load down the DDS. Q1 is a simple switching circuit. When the key is keyed, it pulls the base to ground, 12V can flow from collector to emitter of the PNP transistor, turning on Q4 and Q5.

I had to switch both of these and you will notice on the build board below a mod wire, as originally I had only one of these stages switching, but i was getting massive leak through. In a future build I will look at a better switching method to improve the overall usefulness of the transmitter and think about adding in some wave shaping and all the fancy stuff.

Q4,5 and 6 amplify the 300mv DDS signal to 10V p-p where it is then amplified by a single FET to around 5 watts. Its simple, but works, and getting those gain stages all working together was not an easy feat, I spend a lot of time in LTspice making them place nice with each other.

Here is a close up of the board. You can see the mod wire on the top side, on the bottom side of the board I cut a trace to isolate it.

This is most of the test setup, using only 1 side of the paddles because my straight key has the wrong size jack on it.

Current draw is around 0.7 amps key down.

Power out is 5w.

\

 

Output on the oscilloscope.

And this is why you should never trust the output on the scope to give a honest representation of the harmonic content of the signal. First harmonic is -40db down and the second harmonics is only -30db down. Not good enough for the kinds of girls I go out with. So I need to spend sometime going over the build and see where added distortion is coming from that was not present in the prototype. When I put it on air later to test it out, I will add an extra 7th order filter to knock those harmonics on the head.

You know what. Its really nice when things start to click and what you build looks and works like the design. I get the feeling that 2020 is going to be a ground breaking year for my home brewing.

Cheers,

Rob.

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FQP13N10

So i out one of these fets onto by board to test out. All the CBers tell me is the bomb diggety and makes more POWA than the IRF510. Well other than lifting a pad when i pulled it off the board, I did not find the device to be all that stimulating. I found it to be rather pedantic with a very narrow window of opportunity, with it being either off, on or in full oscillation. With its default being oscillation.

Ok, so it made nice power, I was getting 25 watts out of it, and those watts were nice and clean and linear. But for the life of me, it was persnickety. I had to turn off the gate bias, if it had much more than about 1/2 a volt it would oscillate, if the drive at the input to all those stages varied either side of 200mv by as little as 50mv it would either turn off, or it would oscillate. But while happy, it was very happy, but that window was so narrow that it would be impossible to use in a project.

Now i am aware that it is quite possible that it was just my board layout or design that was the cause of the oscillation, failure is always an option and what manifested with one device might not show with another that is more forgiving. Not something I can easily test for. But I do have a push pull board I can build up and drop a couple of fets into and see what they do. But that might have to wait for another day.

Oh I have respun this board into a CW transmitter and have sent off to china to have them made. That will give me something to play with while I think about the HT project and how I am going to proceed with it. I am having trouble with the receiver and then I need to start thinking about better integration between the boards and how I want to deal with tx/tx switching. I am thinking of turning off the whole PA board when in RX, which would require some redesign. But more to come on that when I work out how I am going to do it.

 

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Amplifier Voodoo

I have built a few amp stages, most of them either worked or worked and were total crap. This is the first time I have designed and built a multi stage amp and have things actually work how they are meant to. I wanted this amp to be very linear and to keep harmonics to an absolute minimum. And it is clean, each stage plays nice with the next stage, there is no loading up one stage, no distortion. Obviously this is not a huge power out monster, but there is 40db of gain in there so far with more to come from a fet final stage. So far I am pretty happy with how things are going.

 

UPDATE:

The complete mess 🙂

About 4 watts out.

Harmonics are -45db down or better.

Schematic for amplifier chain.

UPDATE 2:

 

So after a little bit of tinkering, its now making 5w out. After 10mins keydown it hardly gets warm, only 6 deg C above ambient measured on the case of the FET.

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10dB Gain

Its funny, I needed a 10db gain amplifier that used a 2n2222 and being lazy and not wanting to MATH i typed into google 2n2222 10db preamplifier and low and behold,  it came up with an answer that was what i actually wanted. Sorry to the OM i stole this from, i will edit this later with a link back to your site.

So anyway, I simulated in LT Spice and it was spot on, -6db input and +4db output, thats 10db and just what the doctor ordered. Who needs math when google can elmer like the best of them.

 

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IRF510 2×2 Push Pull

Well this one has been a long time coming. From the very first amp i built till now has been close on 5 years. Those first amps were rather scary contraptions dead bug on scrap pcb and had more than a few fets explode on me. And after the debacle that the 2×4 amp was, this time around i decided to cut it in half and make sure it is all working right and to have the mechanical aspects of the build right also, so if something does break and lets be honest something always breaks, that its easy to fix. I have not really started to push this yet, but atm with conservative biasing, drive and voltage i am getting a respectable 36 watts out. Its been key down for the last 15mins and other than the heatsink needing to be bigger to dissipate the heat its going well.

Here is the schematic.

Here is the PCB as laid out in my ecad of choice.

So rather than try and go the home made SMD route, i just went old school with the mechanical design. What i did was mount the fets loosely to the heat sink, then i snug them up, bent the legs with the pcb and then tightened them all down. It meant i could slide the PCB on and off the legs really easily until i was ready to solder them on permanently. If i fet dies, it should make replacing it a lot easier.

Setup for testing. I kind of just snuck up on it with voltage and drive. 13.8v and 200mw drive and I was getting about 7 watts out. Slowly and surely i brought the voltage up and the drive as well. In the end i was giving it 1 watt of drive at 24v and was getting 36w out. Its got way more to go, but its nice and stable.

A step in the chain, was about 25w here.

60v peak is 36 watts. Now i need to put some 200v caps in the low pass filter and make that look like a sine wave LOL

PDF of the schematic : irf510-2×2-amp

Gerber files for the pcb : REMOVED found an error when I ran 10A through it.

 

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DDS VFO CW Exciter

Here is the next part of my home brew CW transmitter. An Arduino and AD9850 are providing the DDS VFO signal. Q1 is a buffer to isolate the AD9850. I am not sure that it is needed, but its there to stop the gain stage from loading it up. Q3 is an amplifier that takes the 400mV P-P DDS signal and amplifies it to 8/9V P-P or there abouts into a 50ohm load. Thats about 200mW, enough drive to supply something like the QRP Labs 10w Linear. My plan is to design an amp to follow on from this that will provide 30 to 50w. More on that to come. Q2 is a PNP switch, pulling the base to ground turns lets power flow to Q3 thus keying the transmitter on and off. Simple but works.

Here is the schematic.

The PCB as laid out in Ecad.

Here is the board as built, if you are not colour blind and can read the colours on R5 you will notice i stuffed up here HAHAHA. Also Q1 silk screen is backwards.

Signal output into 50 ohm load at J2. This jumps up to about 10V P-P into the QRP Labs Linear amp. Not sure why yet.

Schematic PDF: CW-Buffer-Amp

Board Gerber Files: DDS-CW-Buffer-Amp

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7th Order LPF Boards

Ok, I have still been working at building stuff. Here is the latest piece of what will become a CW transmitter. These boards are 7th order Chebychev low pass filters.

This is the PCB as laid out in ecad. 4 boards per PCB and you can get 5 PCB from JLCpcb for $2 so that is 20 low pass filter boards. A life times supply.

My current project is a 40m CW transmitter, here is the LPF sitting on top of the TX/RX switch.

Filter response plotted with the NanoVNA.

If you want to have your own LPF boards made, here are the gerber files:  LPF_Singles

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100w Amp Repairs

So when i initially powered this up it was transmitting into a dead short. Obvious not a good thing, but on a good note, one bank of fets actually survived because I did not turn the bias way up on it by mistake LOL.

Anyway, this was way easier to pull apart than I was expecting it to be. Now i just need to run down the shops and get some solder wick to clean up the pads and drop in 4 new fets and then power it back up again. You will notice now there is a wire bridging the pads where the output relay is. Yeah this time it will be dumping rf into a dummy load. LOL

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