So after much rooting around, I have this finally working and making ham radio noises. Its not brilliant, but it will do the job for now.
More things being tested for the hand held project. These are 1A power inductors. I need something much more low profile than a toroid to use in the low pass filter and also something not through hole. So i grabbed these power inductors to test to see how much loss they are going to induce. Turns out they are not all that bad and at least as good or better than the hand wound toriods i used on another board.
Other than the corner frequency being in the wrong place because i used the wrong cap values, the loss is less than 2db and that will be good enough for the kinds of girls I go out with. Oh and the hand wound type 2 iron cores? 3db loss. Go figure.
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.
I have been thinking for a while about using high speed op amps as IF amps in a receiver. As mental as that might sound to some, it actually makes practical sense in someways. Gain is easy to set, impedance is easy to set, being that the IF is at a fixed frequency you are not worried about being broad banded and can tailor the circuit to suit by using a suitable op amp with sufficient bandwidth to do the job.
And that is where the problem lies, op amp gain bandwidth is given at unity gain, IE a gain of 1 and as soon as you start adding gain, you start losing bandwidth. This means that you need a unity bandwidth of Gain x MHz to be somewhere close and then you also need a op amp with a fast enough slew rate to deliver the waveform amplitude you desire.
Now there are what are called current controlled op amps that give much better gain bandwidths above unity, but they are kind of expensive and so that leaves using voltage controlled op amps and working around all its limitations, but as you start to get up there in unity gains above 300 MHz even they start becoming non cheap items also.
So a few weeks ago I was on one of the Chinese parts sellers just looking at all the different crap they have and for some reason I ended up in the op amp section and found an op amp with a few hundred MHz unity gain bandwidth for pretty cheap. And by cheap, i am talking in the 40c each kind of space. So i bought a few to try out.
So i built up the non inverting circuit as shown above. Which is quite simple to set the gain and the impedance’s just by changing the value of a few resistors.
This is the circuit built on the test board. While the op amp is an SMD part, its SOIC 8 so its big enough that even a dummy with coke bottle glasses could hand solder, but I am kind of slack in that regard so I used paste and hot air, i mean why not. LOL
You can see from the S21 gain plot that there is usable gain from 40m to 10m. I am not sure what that notch is, but i suspect that its an artifact from the nanovna, because a manual sweep of that section of spectrum using a function generator and oscilloscope did not show that dip.
Oh I should say that I have the gain set to 6x for this test. And slew rate was not an issue for 7MHz to 30MHz, with the op amp able to deliver 1.3v peak to peak quite happily. Below that, particularly around 80m, the op amp could not deliver much more than 500mV peak to peak. So for a 9MHz or 12MHz IF amplifier, the op amp might be a credible option.