Low Pass Filter Board: Part 3

So its been a busy day here in the workshop. Caps arrived today and we set about to making low pass filters for the transmitter. I started by sticking in the caps and then in turn winding the inductors for each of the 3 filters, 80, 40 and 20m bands and giving them a test by feeding in a square wave from my it makes nice distortion signal generator.

80m Low Pass Filter Under Test

The output was fed into the spectrum analyzer on the Red Potato. The spurs either side of the fundamental are crud from my signal gen, it does that when you start to turn up the output. This was a 10 volt peak 3.6mhz square wave being fed into filter and I through up a few cursors to get some levels, first harmonic is -60dBm down on the fundamental. That should be more than good enough.

I did the same for the 40m and 20m filter, again -60db down on the first harmonic, the above image being for the 40m band.

Something I was worried about was isolation on the relays. These are no uber doober premium rf relays, no no no, they are 10 for $1 Chinesium Grade floor sweepings and seconds off Ebay. So we stick the signal in the in port and connected the 100meg Hantek scope up to the other end and turned the volts per division down to the noise floor and looked for signal leakage. Nothing, maybe a couple of microvolts but that might have been an aberration. Either way, good enough for the kinds of girls i go out with.

Final board all together and soldered up.

 

A bit of an idea of how its all going to go together as a stack. I think things might actually work ok after all. Touch wood.  Tomorrows job is to program an ATMEGA IC to work as an iambic keyer, add in some switching and buffering to get the transmitter up and working. Not sure if i am going to have an external PA yet, or just use a couple of BD139’s and get 3 or 4 watts out. Will see how we go.

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Need A Tool-Make A Tool

I watch a bit of blacksmithing on youtube and something these guys often say when they are building something is Need A Tool, Make A Tool. Its a good way to think about life, why run to the store when you can make something now and do the job you are trying to do now.

So, with my breadboard setup, I am wanting to monitor the current demands of everything I am hanging off the one micro as it will eventually be running off solar and batteries and I need to make sure i have a few days of battery supply and enough solar to charge everything up within a days daylight. Don’t want my things turning off and losing precious data.

So i grabbed an Oled display i could not get working out of the box and got it chooching, a voltage and current sensor that i had doing nothing and made them all talk together with some software and she is skookum as frig. So all i gotta do now is wire it up with a nano stick it in a tiny box with some banana sockets and put it in between my power supply and my breadboard and I will always know how much current we are drawing.

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Low Pass Filter Boards: Part 2

So when i designed the filter boards one thing i took into consideration was that often you need to use multiple caps to get close to the value required. Now when i first started home brewing, i used to obsess about values, oh i dont have this value, oh i dont have that value, but now i really do not give a crap, as long as its in the ball park she’ll be right mate, mostly the values are not super critical as long as they are ball park.

I mean does it really matter if the corner frequency of a LPF is shifted up or down a few 10’s of Hz, or Khz for that matter because your cap values are 5% off ideal? I am using 10% caps as it is, so there can be a bloody lot of variation here. But I did manage to get pretty close to the numbers using the one range of MLCC caps that minikits.com.au most of which are also NP0 types also. So below are the schematics for the filters.

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Low Pass Filter Board

So I figured for my stacker transceiver project that I would start with the transmitter. First cab out of the rank will be the low pass filters to test out and ensure that my board design is working as it should be. Added in the switching bits to start with and jumpered out the coupling capacitor links used in band pass filters. I have even ordered NP0 monilithic caps for these and should have them in a couple of days and have this built and tested by friday.

The three filters will be for 80, 40 and 20 meter bands using 7th order Butterworth derived filters. Probably overkill for my transmitter design and power, but hey if you got em roll em.

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VK4FFAB Arduino Logger TFT Edition

So after procrastinating about this for such a long time I finally got around to porting the code over to make it work with the 3.2 inch TFT Display. And seeing as it was pretty much a huge code rewrite, while i was there I added in come new functionality to improve usability. All input functions will now accept the <BACKSPACE> key to erase mistakes. Still to be done, create and use a new logfile and add in some WWFF specific items like park to park and my park details. All in all I am rather please with how this has come along. Each time I spend some time on it, it comes out much more improved.

Arduino Logger TFT Edition Code: TFT_Arduino_logger

 

Here is a break down of the stack. 3.2inch TFT on the top, followed by the MEGA2560 and underneath it, the shield board with sd card, real time clock and ps2 keyboard interface. Not my cleanest work, but much cleaner than the last build LOL.

And this is the log file data from the video below. Nice clean ADIF that should by rights allow for importing into any other logging application.

And here is a video of the thing in action. Next on the agenda is boxing it up with a 5000mah single cell Liop, charge controller, BMS and Boost converter to make 5 volts, then add in the new code features and this choocher should be awesome as frig.

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Arduino Logger Update

I have made a little progress with the Arduino Logger updating the previous mess i had made in a box, to something that is going to be neat and tidy and use a TFT display where i can show more data than i could on the 20×4 lcd display.

Everything is now built into the shield that sits below the Mega. SD card will be accessible now without taking off the lid. The real time clock is in the shield board now and hidden from view. Next job will be to wire up a 1 Cell lipo battery pack, charger and boost converter to make 5v to run the thing, Oh and convert the software.

It is a bit of a power hungry thing, close to 200ma.

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Arduino Light Bulb Moment

You know when you are working on some grand Arduino implementation and you only have 2 available Gnd or Vcc pins and you need to then run a ground and power bus just to service the one extra module you are trying to add. It a pain in the arse for sure. But alas poor Yorick not everything is rotten in the state of Arduino, help is at hand. If you have a few vacant digital or analog pins, turn them into outputs and make them either VCC ot GND with a couple of lines of code and pull a pin either low or high. Code is better than hardware. Obviously current limits on pins apply and you cannot pull 100ma, but if the sensor or modules current limits are sparrow fart, then this is a nice way to solve a common problem.

A Ground Pin

A Vcc Pin

And here it is in action with a RTC.

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Taking Off The Training Wheels

Its interesting when you actually start designing for yourself and not just follow someone elses potential blunders, how many small things you have to overcome once you start prototyping your designs that you never considered would be an issue at all.

In my latest project i figured I would use a relay for the Tx Rx switching along with a micro controller and a push button that might one day become rf sensing. It works a treat does exactly what its meant to do and I am happy. And then i got to thinking, how much isolation is there between the 2 output poles of the relay. Its not an RF relay just the cheep 5v relay using in arduino projects, so i dont expect it to be great.

So i injected a 27dbm signal into the common of the relay, that about 0.5 watts and measured what was being seen on the isolated pole, 2dbm or about 1.5mw. So i have about 25dbm of isolation and enough power floating on the isolated port to burn your retinas out if you are a double balanced mixer, let alone the 25mw of Rf that’s going to be there when 5W is put thought it, assuming the isolation remains constant with power and im not sure it does, I do not have 5w to stick in there to see atm.

So i looked at the specs of a number of RF Relays, and they are only sporting 40 to 50dbm of isolation, much better but still soul destroying if you are the receiver DBM. So either way I have to find a way to protect the mixer and so im thinking either bidirectional amplifiers or just muting the rf preamp by cutting the power to the transistor. Hooray for me, I now have more thinking to do.

Thanks for reading an hombrewers lament 🙂

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