Round and round we go, where we stop, everybody knows. 27 turns. HAHA
I dont remember ordering these transistors, but they are cheap, i have 100 of them and so i mounted one up on a test fixture to see what it could do. To my delight, I was seeing 1.5w out and I am thinking I hit the jackpot. I can push pull these as a predriver stage and its not going to cost much for a good amount of power.
A couple hours later, I noticed the scope probe was on 1x and the scope on 10x. My super awesome power just vanished out the window. Its still a good transistor, reasonable gain for small signal stuff, good enough bandwidth to be useful for most the HF bands of interest. Its just now a power house HAHA.
Oh well. It is what it is 🙂
So I pulled everything off the shelf to give this a test. Its working. What I have done in the pictures below was to use an DDS vfo as the incoming 7mhz signal. It run into as much attentuator as I have here, giving a final signal out of about -95dbm or there abouts according to the spec-an. This number is rather uncertain because that is right on the noise floor of the device and by my calculations, the actual signal is closer to -100dbm. Now I am still hearing a signal out of the speaker at that level. So its not an Icom, but its not totally deaf either. I do think i could use a little more gain in the audio stages. But, we will see how things pan out with some loudish signals on 40m this afternoon.
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.
So long story. I have had some trouble getting the Nano VNA to hold a calibration and display the correct information. I always had a -10db offset when using certain SMA leads i had there. It would display fine with semi rigid SMA leads, but, others would show -10db. Like the leads had loss in them.
Well anyway, I think i have resolved the issue and got the calibration right. 0.5db loss in my leads would be about right and that is what is showing now, they are after all cheapest crap leads from China, not high end leads you would use in a lab.
So anyway, popper calibration procedure.
Open: Open Load on S11 port.
Short: Short load on S11 port.
Load: 50 ohm load on S11 port.
Isolation: 50 ohm load on both S11 and S21 ports.
Through: Shortest high quality 50 ohm cable connecting S11 and S21 together.
Then save that. That is it, that should then give you fairly accurate, well as accurate as the NANO VNA is results. As you can see by the plot of a 40m bandpass filter above, it looks about what you would expect from a known design that has low insertion loss. A couple of dB, made up of the lead loss and the filter loss. This means my filter has about 1dB insertion loss. That is something I can live with.
Anyway, through no fault of my own, I broke the mini USB socket off the original NANO VNA i bought and tore the tracks off the board. Yeah not really happy with myself but it is what it is. So i bought another one, this time it seems to be one of the better clones, it even came with shielding and a battery. So anyway, i plan this time to ruggedise my NANO VNA and mount the whole think into an aluminium box, and have an panel mount USB port on the side that will take the abuse of me pulling and stretching and inserting a lead in and out on a regular basis. So i ordered one of these as well, yeah the new VNA came with USB type C not that stupid micro USB rubbish. Anyway, by the time i mount this in a box, i will never have to worry about breaking the damn socket off the board again.