A Story Of Two Crystal Filters

While waiting for parts and boards to arrive for the main things I am working on, I thought I would turn my attention to the age old problem of crystal filters. Its not like you can easily buy filters off the shelf these days and even finding suitable crystals can be a problem. The low profile half cut crystals can be hit and miss and full size crystals can be expensive. Hayward wrote a few papers not long back on using 9Mhz crystals in simplified designs and pretty much if you buy 9Mhz 30pf crystals you should be able to duplicate his work with minimum effort. Problem is,  9Mhz crystals are non existent in Australia even in the low profile cases and they are $1 or more each in quantity on mouser. Screw that Jacko.

I have had mixed results making home brew filters, but this one looks the business. Other than being 4K wide, it looks like a filter should look. Fixing the width is just a matter of changing the capacitor values. The second filter is 600Hz wide for CW, thats just about where I like it, but its got -10dB loss, not sure the issue just yet but my gut feeling is that its one of the crystal is off from the others. I did measure them all, but you know, failure is always an option. I am going to suck all the crystals out and start with a fresh batch of 8 and see if that is the problem and then change the caps on the SSB filter to narrow it up some. I hope to have 2 very usable filters.

Typically i fix things until they are broken, but hey this actually looks alright. I need to narrow up the caps some, but now its not all loss and no stop band. -60dB rejection is more and good enough for the kinds of girls I go out with.

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New Broken Parts

Ever get that sinking feeling when you realize that the parts that you have been hoarding for 5 years are actually broken? Well, i have had these TFT displays in my parts draw for who knows how long. Long story short, I am working on a project that will probably use one of these displays. So i grabbed them out and a micro to test them with to ensure their is no library issues when I build one of these into the project. Good news is that there are no library issues, bad news is that both of my displays are broken, one more so than the other. This one worked just enough to show me that the code libraries worked with this display. So that is a good thing, now i just need a display that is not broken. To the ebay mobile batman.

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Prototyping Board V2.0

So after playing with the old version of my prototyping system for a while and building some modules for it and making some noises with it, i started to get a handle on its short comings and there were many. But i think that I have addressed them in this latest version.

The main differences here is that I have moved all the power stuff to the back, added in a audio amp and 2 speakers on the board. You can just see them poking out from under the LCD, they might be tiny but are rated at 2w each. The audio amp has a 2n3904 preamp followed by a TDA2003 amp and should be fine for 5w of output.

All the digital stuff has been moved to the main board now, rather than on a card and everything important has been broken out. The ESP32 is connected to WIFI and i can program it over WIFI so there is no more connecting cables and the like. Extra pads near each daughter boards means routing signals between modules is now much easier.

All around the improvements should mean I will be able to burn though a ton of though hole parts over the xmas break and get rid of them from my life HAHA. Most likely what will happen, I will build something I like and end up keeping it LOL. Once i have finished testing the board, I will add the schematic, PCB files and the code to get the VFO running, so everyone can have at it, if they choose to.

 

EDIT:
I did it again. Designed something and then used the wrong part. In this case I designed the audio amp using a TDA2003 and I have 30 of them coming from LCSC. I thought that I had a couple of them here already, but NO NO NO. I have TDA2050 which is a totally different part requiring different components. LOL, Anyway, i have 1 TDA2003 that i got gouged on from an Ebay seller in Sydney and it will be here probably early next week. So the final testing is going to have to wait until then.

PROJECT FILES DOWNLOAD: PrototypeBaseBoardV2

Everything is working and tested except for the TDA2003 audio amp. I have no reason to think it wont work, so downloader beware, use at own risk. Or wait a week until i have parts in my hand to test it.

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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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