Putting the Doo back in Doohickey. I bought a new function gen, but this one is much fancier. Its like the licorice all sorts of function gens, does AM FM, PSK some things i have never heard of and well as Sin, Square, Triangles of various types and other kinds of squiggles. Seems to work well enough.
So i sat here this morning with a bag of 50 crystals playing a game of what is the resonant frequency. Thoroughly boring I can assure you and I am not sure how the ladies at uBitx Labs in India can sort through thousands to get sets of 8. But alas I have done enough for now to get 2 sets that are matched at a guess to within 50hz, being that my frequency counter is only good to 100hz.
One thing that I would not have predicted is the distribution in the resonant frequencies. Its like buck shot, all over the place and the other thing I would not have predicted is the number of dead crystals or ones with so low Q they might as well be dead being 8, that seems like a very high death rate to me. I still have 2 more bags to sort, but thats not happening today, i have 16 matched crystals I need for 2 filters and am moving on to the next thing.
Need a tool, make a tool as they say. Well i need to sort these 100’s of xtals into groups of similar frequency for use in xtal filters for an upcoming homebrew receiver project I have started. I have like 200 12mhz to sort through, so i should be good for a few filters at least. Its just a matter now i sitting here for a few hours and sort them all. Not looking forward to that. 🙂
No rocket surgery here, just your basic oscillator and buffer circuit that can be found all over the internet. Its simple, it works and it is reliable.
Built it up Manhattan style on a scrap of copper clad pcb. Yeah i love MeSquares to all you MeSquare fan boys out there. LOL
The Joy Of Oscillation, it works. 3 xtals checked, 100’s left to go.
Just A Bunch Of PSU Modules?
All good projects need a name don’t they? Sadly I think just a bunch of modules has been taken by Pete Juliano for one of his many radio projects. Oh well, such is life. I was thinking Buck This or Well Switched or Ebay Wonder might also do. Anyway, i have now completed the project. It has its lid on and it works. Here are the final images, a couple of them are even hi-res using the SLR and not my mobile.
Here is one of those Tribal Knowledge moments that we often hear about. This is 6 core shielded computer cable, I used it to wire up the pots. It helps keep things nice and neat.
It worked out quite ok, next time I will spend some more time measuring then cutting just to the right size.
I do not like playing with mains voltage or anything that can kill you. It is why i went for a modular option, so that when I am done there is no exposed 240v anywhere inside the case that dummy hear can touch while doing da-finger-poken inside the case. Not that the lid will come off hardly every, i just am very cautious of anything that can kill me.
Here i have started to wire up the DC power side of things. I am not all that much of a neat freak like my friend Simon 6MSC, who can make toriod winding look like an art form, but I am trying to keep things somewhat tidy.
Now we are getting into the meat of it, things are all wired up and zippy tied all over the place.
An overhead shot of the whole guts of it.
Now i do not pretend to be a metal worker. I was really happy with the bottom half of the case. But, alas i turned the top half into an abortion. First off i folded the metal on the wrong side of the line and made it to narrow so that it would not fit properly. So I had to try and fold it flat and start again. Sure i could have scrapped it and started over, but hey, why waste such a large sheet of metal. So i folded it a 2nd time, this time it fit, but one side is 5mm short. I can live with that. I know that the next time i make a case, which will not be all that far away, will be much neater now that I know what I am doing. Thanks for reading, hope you enjoyed the project.
So i got in and did a bit more work on the lab power supply. Finished off marking out and drilling every hole for every component so i do not have to go back and make a mess inside when i start to wire things up. Other than 2 tie bars across the top, the lower half of the case was done in the image below.
Mounted the switch mode PSU into the case, its held in with 4 M5 bolts and is not going anywhere, the fused, switched power connector and EMI filter are also bolted in with M5 bolts. The earth wire will be bolted to the case also with M5 bolts as well as serrated washers, so it cannot go anywhere. Go over the top on safety is my motto.
Mounted all the front panel items. From here its now just a matter of wiring up the spaghetti and making it all work together. But that will be another day, i do not think i have any wire suitable for 240v ATM and will need to go and get a meter of 3 core to pull apart for the internal wiring.
I have not had much luck with the inductance setting on LCR meters. Most of them are crap, inaccurate and just plane useless. I think this is the 5th cheapish meter I have tried and I have finally hit on one that works and works well. In the picture below is a 1.8uH slug tuned variable inductor. So yes, it works and is accurate and with 3 decimal places its going to be nice to use for getting those hand wound toriod inductors right. The test frequency is a bit low, it would be nice if it was in the MHZ range rather than KHZ, but for that you need to spend up big on high end gear, not $70 on ebay.
This is all kind of Schrodinger’s cat, but it turns out not all 6uH is made equally. 6uH can be 6uH, 6uH can be 12uH and 6uH can be a capacitor and the only thing different between all these 6uH’s is the frequency they have been measured at.
So tonight i got well and truly schooled on inductance and why my bandpass filter was looking rather bonkers. I built it with the right capacitors, i wound inductors with the right number of turns for the required inductance i assembled a really nice looking filter. But when it came to measuring the bandpass it was orders of magnitude wrong.
This is how my filter should have looked, well, at least something like that.
This was the bonkers mess I was measuring in the Bode Analyser.
So after much discussion with a much smarter man than me, and working out everything i was doing wrong, I ended up with this testy jig, 200R in series with the inductor, feeding one side with the signal generator while measuring both the input voltage and the voltage across the inductor. Changing frequency until my output voltage is 50% of the input and then using this following formula to calculate the inductance. L= R*sqrt(3)/(2*pi*f) that at least gets me in the ball park, and it turns out I was orders of magnitude off with the inductors i wound and were most likely acting like capacitors at 20mhz where my filter was peaking.
No No No, I have not abandoned the receiver project, I have just hit what is likely the first of many road blocks, and I have not yet worked out what is going on or why for that matter. So in the mean time, I am going to turn out some bits and pieces that I have been meaning to do for a while, while I contemplate what is going on with the bandpass filter and why its center frequency is bonkers off where it should be. More on that to come.
Anyway, I always need to connect A to B and B to C while i am building things, so i made up some double ended easy grips for just such an occasion. Hardly rocket surgery and defiantly not brain doctoring, but handy to have none the less. Next up is a test jig for measuring inductors on the oscilloscope.