MC3361 Mixer IC

What can I say, I like to trawl through parts catalogues and find somewhat obscure parts or ultra cheep parts to play with. So, when i found the MC3361, which is an FM Cordless phone IC, I thought why not. It costs 25 cents in one offs, and has some conversion gain and I assume its a gilbert cell in there, so its way cheaper than an SA612 and others.

LO in is on pin one, you can also add an xtal here for the LO. RF in is on pin 16 and the IF out is on pin 3. Now i did not try and match the impedance’s which are around 1800 ohms, i just fed in 50 ohms from my function generator into the LO and RF ports and then attached the spectrum analyzer to the IF port.

Other than some DC blocking caps and a 50 ohm termination resistor on the input there is not much to add to this. And as you can see from the plot below from the spectrum analyzer its working doubley balanced. RF in is suppressed and the sum and difference are the dominate signals. RF in was 7Mhz and the LO was 1Mhz for the sum and difference of 6 and 8mhz IF frequencies.

Is it as good as other mixers? I don’t know, but it works and its cheap, so i think its worth a go. Oh and it also has an FM de-modulator, AF amp and a op-amp for filtering in there also. I am yet to play with the other bits, i just setup the board for mixing only and well, mixing works.

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Just A Few Vias

I have been sitting here today working on the PCB for the WSPR amp and got to thinking about cooling for these transistors. Sure they run cool enough to touch, but im sure over time things are going to warm up some so I threw in a million vias around the transistor and though the source pad to wick the heat away. Then i can bolt this direct to some kind of heatsink and have more than good enough cooling to let her run for forever.

I have got a few boards here almost ready to send off for production. A new BPF board using shielded can inductors. an updated LPF board using SMD caps, an experimental audio filter board where I designed the CW and SSB filters, while I like the hypermite/SSBmite filters I probably cannot really in good conscience release my gerbers as they are a product sold by 4state qrp, so if these work, I can then release my gerbers. Finally I have this test board for the WSPR amp and another PA board for 2m/70cm to send for testing. So much happening at Rob’s Lab its not funny. 🙂

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ESP32 With Dual VFO’s

Hey look, its a home brew radio post. HAHAHA. So i have settled into this whole locked down for corona virus nightmare and have sorted out most things and have a game plan for how life will go on for the next 6 months of lock down and no work. And because i have everything in order, i have slowly been getting the enthusiasm to get into building some of the stuff i have in the pipeline and seeing if it works.

I have shelved the tuner for now, i really am not sure about it and it or how to correct its problems, which are both design and code and have started on the second iteration of the universal control board. First time round all the audio stuff worked just fine, more or less, but there were some design issues in the micro controller side of things. Namely, i screwed up things pretty bad and destroyed the 2 older ESP32 Dev Boards i had here which meant redoing footprints and all the other exciting things.

Obviously not everything has gone smoothly this time either, turns out that i laid out the board with to use an L7833 3.3v regulator as I am using a L7805 5v regulator as well, you know bog standard parts everyone uses. But guess what, the TO220 3.3v regulators I had were 1117A’s and they have a different pinout. Turns out, getting an L7833 regulator in Australia is not as simple as it seems, I had to order them from Element 14 for way to much plus way to much in postage. But, it got the parts and well got to getting this thing working. Lesson learned.

On the board, we have an ESP32 micro controller, the little red daughter board is a shift register module, ESP32 is 3.3v logic and most of the crap hanging off it is 5V logic, thus the need for shift registers on the I2C lines. The fuse is there from another lesson learned from the previous version of this board where I turned the solder on the micro into lava HAHAHA. We also have 2 headers for LCD screens, 2 rotary encoders, SI3531A module, yeah still using modules because the 2 times i have tried to roll my own using bare components, they never worked, and finally a real time clock. There is also footprints for 5 buttons, one of which is a reset button for the micro, 2 banks of 5 pins for band switching and pads for a thermocouple for monitoring heatsink temp of the final PA.

A bit blurry, but this is the initial fire up to make sure that I had things working right. Not much use spending hours writing code if there is an issue on the board.

Here I am starting to make some progress on the software, which is pretty much a ground up rewrite of one of the basic VFO codes out there, I think if i recall right was a very early version of a VFO by Jason Mildrum NT7S https://nt7s.com/

And finally, this is where I am at with this. All the buttons work, the rotary encoders work and both the displays work. The one thing i do not like about just about all the VFO codes out there is that they all follow the radix math method for setting the vfo increment value. I want to have a 500hz increment, and actually, when i start to build things for 2m FM i will want an 12.5Khz increment and the like. Now using radix math you only get to have things go up in orders of magnitude, 1, 10, 100 etc. So I changed how that all works and simplified things alot, by combining the increment and displaying into the one function that gets checks once per loop. I can now set my increments to whatever value I like to suit my purpose.

Finally before i close down this epic post, on Jason Mildrums etherkit github https://github.com/etherkit/Si5351Arduino/issues/66 some people have reported issues with ESP32 and the SI5351A library. I am not having any problems at all here. Everything works as expected and there are no I2C glitches. Well, that was a manuscript, thanks for reading and I will catch you next time.

Rob.

 

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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 🙂

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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 🙂 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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