I usually regret posting about something before I even know its working, I mean this blog is littered with the dead corpses of so many projects it probably should be called Robs Graveyard. Anyway, I thought i would post about where I am at with the 40m DSB Talkie Walkie. Other than having one small thing to test, everything has worked as designed.
Shock horror I know, but mayby after 5 years of killing parts, I am finally getting the knack. I have compacted the boards into 2. Yeah i could fit it all on the one board and in a future edition I might just do that, but for now I see a lot of advantages in keeping all the digital stuff on its own board with header interconnects.
All i am left to do is to send them off to China and have them made, build, test and make QSO’s with it. Wish me luck.
The Main PCB board
The Digital Board: Arduino and SI5351A
Ok, the long and the short of it. It actually works well. And by well, i mean it can hear me talking on the home station, it can make a tone from injected signals yadda yadda yadda.
So what was the problem, well ended up being a dry solder joint, the most basic of all problems. I had pulled it apart, cut traces, did eleventeen other things to it trying to work out why it was ringing its head off and making no sounds, and it turns out the LO input pin was dry and not making contact.
So what next? Well i start to integrate things onto one board now and start actually assembling something I can use to make a QSO with. Wish me luck.
So i was talking with a mate about crystals yesterday, he has an old 2m rig chock full of pairs of repeater crystals and the discussion was about overtone and fundamentals and all that. So for shits and giggles I knocked up a test fixture to measure some crystals on the VNA. And with the best crystals I have was getting the typical plot for a crystal.
And one thing lead to another and I built another crystal filter, this time for CW. I kind of did not make my target, of 500hz and was closer to 1200hz, but that is nothing changing the values of the caps cannot fix. Ripple is nice, the best i have made yet, the shape is acceptable and the stop band is down near the noise floor of the VNA, so all in all its not a bad looking filter and should be usable once i make it narrower.
Just about everything I am building these days is SMD. Parts a cheap, you can get parts that would other wise be hard to find in through hole versions and there are still some large size packages around of nice parts to use that you just cannot find in through hole. And because I am building a hand held, SMD is compact is the order of the day. In fact, going forward I do not think I will be building anything through hole at all. I have really gotten the hang of this SMD stuff.
That said, as you get older, the eyes are not what they used to be. I wear glasses and often when working with smaller stuff its not the size that is the issue, its not enough lighting. I have been using this LED ring lamp for sometime, mostly as an over head light. But of late, I have been using its magnifying lens as well, its 8 or 10 times magnification and for hand soldering SMD parts its really come into its own and I do not think i could live with out it.
The lens is 5 inches in diameter, so you get a really nice field of view, i mean, i can stick my face right up to it and look through with both eyes and see everything I am doing, plus, the LED light is bright and white, with very little shadow, so its just about perfect. So if you are like me and need to get your eyes checked and reading prescription adjusted every couple of years, add in a magnifying LED lamp and make your life that little bit easier. From here the next step if a microscope and I think that is overkill unless you are doing much finer pitched work than this. If i ever want to use some of those really cool leadless package IC’s i might need one, but for now, this will do.
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.
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.
The complete mess 🙂
About 4 watts out.
Harmonics are -45db down or better.
Schematic for amplifier chain.
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.