Programming, I Don’t Like It

So i have been working on the talkie walkie and this time around, I have done something totally different. I am using a different SI5351A module and a different LCD screen. And here in lies the problem. To many changes at once and none of them wanted to work out the bag. Firstly I normally use a 16×2 lcd and I2C backpack. Cannot fit that in the case so its an 8×2 and regular parallel connection. Second thing is I used the Eat A Fruit style si5351, where i normally used the ones from QRP Labs and the drama there is the different crystals used and the fact that I was still using a very old version of the SI5351 library. So i upgraded it, and broke all my code, fixed all my code and now it actually works. Oh one last thing about the LCD, seems there are 2 different types of 8×2’s one with 16 pins and one with 14 pins. I think i smoked the 16 pin one, before my PSU went into current limiting. Not sure what the deal is, but anyway, after much messing around, this part of the project is actually working.

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A 21st Century Novice Station

So i got excited and wired up the tx rx switching and the extra relay and got on the contest tonight to see if I can make a qso or 2. Well i didnt get there, but hey I tried, that first qso will come soon enough. 5 to 10w in the contest was probably not going to cut it with the time of day and conditions not perfect.

But as a concept, it works. Its not all that practical, tuning both the transmitter and the receiver to the same frequency and then flipping the switch and keying the transmitter. I am really going to have to work on some auto switching method so i can just key and having a single vfo dealing with both the transmitter and receiver would be nice.

So next thing after the hand held will be a receiver to mate with this and some integration to make it work as a transceiver should. All in all though, this has been a good learning experience and totally worth it and its been fun also.

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Leaky VFO Fix

The fix for the Lo Leak was nothing more than a 2nd relay to disconnect the transmitter entirely when in receive. I am currently listening to 40m to whatever the contest is on tonight, and I am hearing everything other than my transmitter HAHA. I really should wire up the switching and make a few qso’s. But its late and its been a big weekend and I think i will go to bed instead. At least i know that this will work. I might take it portable up the road to one of the WWFF parks tomorrow and see if i can drum up the first qso or 2 with it.

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Pro Crastination

Pro as in very high skill level of doing nothing but looking busy. HAHA, Well i really should be building the hand held, I have the boards here and its just a matter of assembly and giving them a go. But, for some unknown reason I was digging in my box of arduino crap and came across these 2 modules. The first is an Analog Devices SSM2167 variable compressor and noise gate, the second is Maxim Integrated MAX9814 which is an AGC mic amplifier.

And this got me thinking because after the hand held build the very next thing will be a superhet receiver which will start life as a receiver to mate with the CW transmitter and later will become a ssb transceiver. Part of that build will be a single unified front end board, that has on it a micro controller, in this case its going to be an ESP32, a bunch of peripherals, like keyboard interface, SD Card slot, real time clock, SI5351 PLL Synth, a few push buttons for various radio functions, a couple of LCD screens, rotary encoder etc.

Also on the board will be the entire audio chain, which, when i designed it originally contained an pre amp with agc, select-able ssb or cw active filters and an LM386 AF amplifier because i was not thinking ahead of myself. I have a better AF amp ic that i can be using and it has a shutdown pin, so i can send logic level from the micro and mute the AF amp on transmit. And now, i have these 2 modules.

I can now see using 2 sets of these in an SSB transceiver build. First in the receiver, audio coming out of the product detector is routed into the compressor and noise gate, then into the agc before it is sent off to the audio filters and then to the AF amp. Seeing that I would have quite a bit of gain just in these 2 modules, I would not need to lots of gain from an LM386 and I could then use a much better sounding and higher wattage audio amp.

And then on the transmit side, have the exact same thing, compression, noise limiting and agc on the mic. Lets be honest here, how many home brew transceivers actually incorporate such luxuries as these. Each of these modules is around $5 aud off ebay, the Analog Devices IC i can buy for a couple of bucks and the passives I have, its a TSSOP package and something i can solder, so it is something i can integrate without mounting the module. The Mic AGC is a leadless package and not something i can really deal with at home, well i could, but i really do not want to. So that module will have to be mounted to the board.

People have been saying that 2020 is going to be a very interesting year for home brew, and from the way things look to me, it looks true. From the QSX probably making an appearance to novel items like AGC, compression and noise gates being incorporated, to other unique SDR based receivers like this one over on Circuit Salad https://circuitsalad.com/2020/01/06/compact-si5351-based-sdr/ things are already looking rather interesting indeed.

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I Sprung A Leak

So the story goes something like this. I am trying to mate the home brew transmitter with the SDRplay so I can get on air and see if i can make a qso or 3. The problem is, i have LO leakage on the transmitter. Now we are not talking a very big number here, presented to the receiver is -90dbm. A fly fart in the wind of rf, but the point of a receiver is it is meant to be sensitive and can hear a signal down to say -120 dbm. So my tiny leakage is orders of magnitude above the smallest signal the receiver can hear.

This poses an obvious problem, on receive I will never hear anything because I have an S9 signal screaming at me. The solution to this is rather simple. I need to add in an extra relay into the TX/RX switching to get greater isolation of the receiver from the transmitter. This will also help with the bleed over on transmit also as there is no way to mute parts of the receiver easily. Well i can mute the software but that is not helping the front end of the receiver seeing a large input signal from leakage.

Anyway, I have worked out the problem and determined a solution, its just a matter now of building it and seeing if that makes things usable.

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I Have Finally Built Something I Am Proud Of

So a few weeks ago I had sent and email to Pete Juliano asking some questions and showing off the progress of my projects, he commented that my work bench looked way to clean for me to have been productive, well I sent him this picture from this morning when I was hard at working building this project. There is currently CRAP EVERYWHERE.

Anyway, now onto the real story. The first thing I have built and designed that I am actually proud of. It all works as designed and I think i am finally starting to get up the learning curve.

So there is nothing revolutionary here. Its a simple CW transmitter biased Class A all the way. Why class A when there are more efficient biasing schemes for CW? Well, because I want reusable circuit blocks that I can use for DSB, SSB and other modes that require linearity.  The theory of operation is simple. We have an ATMEGA328 that controls an AD8950 DDS module, Q3 is a buffer so the gain stages do not load down the DDS. Q1 is a simple switching circuit. When the key is keyed, it pulls the base to ground, 12V can flow from collector to emitter of the PNP transistor, turning on Q4 and Q5.

I had to switch both of these and you will notice on the build board below a mod wire, as originally I had only one of these stages switching, but i was getting massive leak through. In a future build I will look at a better switching method to improve the overall usefulness of the transmitter and think about adding in some wave shaping and all the fancy stuff.

Q4,5 and 6 amplify the 300mv DDS signal to 10V p-p where it is then amplified by a single FET to around 5 watts. Its simple, but works, and getting those gain stages all working together was not an easy feat, I spend a lot of time in LTspice making them place nice with each other.

Here is a close up of the board. You can see the mod wire on the top side, on the bottom side of the board I cut a trace to isolate it.

This is most of the test setup, using only 1 side of the paddles because my straight key has the wrong size jack on it.

Current draw is around 0.7 amps key down.

Power out is 5w.

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Output on the oscilloscope.

And this is why you should never trust the output on the scope to give a honest representation of the harmonic content of the signal. First harmonic is -40db down and the second harmonics is only -30db down. Not good enough for the kinds of girls I go out with. So I need to spend sometime going over the build and see where added distortion is coming from that was not present in the prototype. When I put it on air later to test it out, I will add an extra 7th order filter to knock those harmonics on the head.

You know what. Its really nice when things start to click and what you build looks and works like the design. I get the feeling that 2020 is going to be a ground breaking year for my home brewing.

Cheers,

Rob.

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