Today I built the PA and have tried to test it out. As it currently stands, I think my design might have some legitimate design flaws. I am not certain yet, but tomorrow I will know more. For now, I am off to bed. I am done for today.
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.
The fix for the Lo Leak was nothing more than a 2nd relay to disconnect the transmitter entirely when in receive. I am currently listening to 40m to whatever the contest is on tonight, and I am hearing everything other than my transmitter HAHA. I really should wire up the switching and make a few qso’s. But its late and its been a big weekend and I think i will go to bed instead. At least i know that this will work. I might take it portable up the road to one of the WWFF parks tomorrow and see if i can drum up the first qso or 2 with it.
So the story goes something like this. I am trying to mate the home brew transmitter with the SDRplay so I can get on air and see if i can make a qso or 3. The problem is, i have LO leakage on the transmitter. Now we are not talking a very big number here, presented to the receiver is -90dbm. A fly fart in the wind of rf, but the point of a receiver is it is meant to be sensitive and can hear a signal down to say -120 dbm. So my tiny leakage is orders of magnitude above the smallest signal the receiver can hear.
This poses an obvious problem, on receive I will never hear anything because I have an S9 signal screaming at me. The solution to this is rather simple. I need to add in an extra relay into the TX/RX switching to get greater isolation of the receiver from the transmitter. This will also help with the bleed over on transmit also as there is no way to mute parts of the receiver easily. Well i can mute the software but that is not helping the front end of the receiver seeing a large input signal from leakage.
Anyway, I have worked out the problem and determined a solution, its just a matter now of building it and seeing if that makes things usable.
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.
This is a bit of a redux. The fixed boards arrived today and I was able to build it using the correct value resistors. Got a nice and neat -40db of attenuation and being that the resistors in the top half of the divider are 1w, it should be able to deal with some power being put though it. Its pretty much overkill for the 100 to 200w that I will be playing with, but hey, the through trace will burn out before those resistors will. HAHA. Gain is rather flat to 30mhz. Beyond that, it increases, not sure why that is the case. But, i only do HF so its good enough for the kinds of girls I go out with.
One of the boards I have delayed is a bezel for 16×2 lcd’s. If you are like me, you cannot cut a straight line to save yourself and the holes look as crooked as a hillbillies tooth. So i am having these made and it should go along way to making my projects looks a bit better. The silk screen on the other side says VFO B, I am planning a dual screen, dual vfo transceiver in the near future.