What A Dummy :)

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 đŸ™‚

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No Annular Ring

I am building something where I needed some IO expanders. I have these modules sitting in my box of arduino crap and figured i could desolder the headers and reverse them and then plug these into my pcb board.

I got to starting on that when I realised, there is no annular ring on the top side of the board. There is no way to really solder the headers in the other way. Bloody stupid board designers doing stupid things like this.

Now to find a solution to my problem.

 

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Unstable Oscillations

 

For whatever reason, the MMIC amps in the receiver are super unstable. They have 2 states, oscillating wildly or not working at all. This is a real head scratcher, because the exact same circuit using though hole parts was super stable. The other thing is, if i stick my finger on the input cable, it becomes stable again. Go figure.

OK I got it sorted out and now its stable. The issue was the limiting resistor  R9 was to low a value. I am not sure why the through hole version was happy with 60ohms and the SMD version wanted 150ohms. But, either way, I learned a valuable lesson here with MMICS, if they are going bonkers and oscillating wildly, cut their current back by increasing their limiting resistor.

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AD831 Mixer Module

I have a couple of these AD831 mixer boards that I bought off ebay ages ago. Been thinking about building a receiver out of ebay modules just for laughs. So i got them out of the parts trays and fired one up to see if they are even working and functional. Looking at the output on the spec an, it would seem they work plenty fine and are within spec.

 

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Standards

So the other day I broke the USB connector off my board. Plenty of ways to fix this, but going forward I am going to start putting programming headers onto the boards I use and plug in an FTDI programmer to do the business. If you take a look at the picture above, you will see how I solved the problem in the short term, just cut a USB cable and wired it directly to the CH340 serial chip.

This is where I ran into trouble đŸ™‚ you would think that red and black would be Vcc and Gnd, but no, this cable is not wired up to the USB standard at all. Red is D-, Black is D+, yellow is Gnd and White is Vcc. I was scratching my head for a while after I first wired it up and it was not working, then proceeded to buzz out each wire with the continuity meter.

Put the correct wires in the correct place and BOOM, i was able to program the micro. More to come on this board soon,

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4.91520 Crystal Filter For CW

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.

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Op Amp Gain Bandwidth NanoVNA

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.

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Measuring Gain Bandwidth With NanoVNA

The NanoVNA is more than just a fancy SWR meter for checking your antenna. Its much more and a very useful tool for the home brewer building amps and filters and the like. Now i have been buying things like crazy for various projects that I would like to build in the future. Often these parts are spec’ed for bands not of interest to me. So what do you do? Well you measure them at the frequency of interest and see how they work yourself.

Below is a MMIC amplifier part that I found for really cheap. By cheap I am talking in the 40 cents per range, so i bought 100 of them. I mean why not, they are spec’ed for 100mhz to 3gig with +30db of gain. Worst case senario they are kind of useless at HF and I will have parts for when I actually want to build things at VHF and up.

So anyway I had a test board built with 4 different circuits on it for testing out various parts I have here and it includes Op Amp, Mosfet and BJT amp circuits. So i decided to start with the MMIC and see what it can do.

The good thing about MMIC gain blocks is the fact that they have such a low parts count. 2 blocking caps an inductor, bypass cap and gain setting resistor. Initially i set the bias resistor a little to high and was getting a lot of distortion, so I halved the value and boom it was providing 24dB of gain at 7MHz, which is my go to frequency for all these sorts of tests. Next though, I wanted to see how the gain bandwidth was. My bandwidth of interest is HF so 3 to 30MHz, so just for shits and giggles I measures it out to the 6m band.

The test setup was Port 1 of the NanoVNA to the input of the test board, the output of the test board to the RF Sampler i made the other day, which was also connected to a dummyload and then back to Port 2 of the VNA. Then an sweep of was performed and the S21 Gain was measured.

Things actually looked quite nice and rather flat. A few dB down at 80m and 1 dB down at 6m. That is pretty good for a part with a minimum frequency of 100MHz. So all in all, this part is a winner and something I can use in a project sometime soon.

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