Broadcast Reject, High Pass Filter.
Project built, into plastic project box.
612 ABC, 50kW AM Station 10km away from me, Before:
612 ABC After.
10mhz band spread before.
10mhz band spread after.
What a day, what should have been a simple job, for a simple man turned out to be an all day endeavor to overcome a litany of problems. In other words it was a fucking disaster. LOL but hey, i got there in the end and now the weather station is about to be put back out in the weather to do its thing. So lets see, what went wrong, well the sensor i was using was different from the previous one, so i needed a different library, then the device address was different to what was in the code, then the micro controller I was using shit itself and died, then i had problems with my computer not recognizing USB devices, then the board library was being a pain in the arse and needed upgrading, and finally there was this problem with crashing because dumb arse here created an array and was causing a stack overflow because the array had no size.
Well, after all that, I now have a pile of crap on my bench destined for the bin and a nice space on my bench where the weather station used to be. This time i have spares and should be able to fix things if something breaks without having to wait months. Its been down for 5 weeks, and thankfully its back up and running. For now, touch wood. LOL. Now I need to create a page that will query the database and display the data.
So it turns out, that just by changing a DLL file in SDR# and changing from Quadrature to Q Branch, you can get HF on a dongle without an upconverter or modifications to the hardware. And most surprising of all is it actually works ok. I do have some AM broadcast breakthrough and I will need to add in a Highpass filter to remove it, but other than that, it looks like it works quite ok.
So i bought an RTL-SDR dongle to play with. And after fighting with windows 10 to install the drivers and then fighting with the software to recognize the dongle I finally got it working and able to receive signals. Connected to a 40m dipole, its not really the best antenna to be receiving VHF and above, but after a few tests listening to a local 2m repeater, a bit of simplex from my HT on 2 and 70cm, i figured it was time to actually look for some signals to listen to.
So with SDR# running, I tuned into Brisbane Air Port air traffic control for a bit of a stickybeak to see whats going on. Also at the same time, I have started to play with screen recording software and video editing software with the plan to start making the odd youtube on what I am doing. Anywho, with all that in mind, I do have some plans for different things to do with the dongle. I have some bits and pieces in the mail already to add to this and well watch this space, more to come. Oh and the reason why i got this was to use it to learn how to use GNU Radio. That is the ultimate plan and to learn some things as we go about doing that.
Here is a little video from earlier:
I just want to say that soldering TSSOP at home packages sucks big fat balls. What a pain in the arse this has been. I spent lots of tine chasing out solder bridges. But they are done now and I guess i can start to use them. Oh and if you want to know what it is, its a compander, to be used to make an audio compressor for the uBitx, Bitx and the receiver I am building, and maybe ALC as well.
So for the receiver I am building I figured that some sort of filtering is going to be required in the audio stages. Not entirely sure yet on the final make up of things, but to start with I thought that I would design and evaluate a high pass filter to cut out the low frequencies as this is likely to be fixed weather i am receiving CW or SSB. Though for the low pass filter, i do want either variable bandwidth or select able widths. More on that later.
As with all filters more orders, more betters LOL. And so i jumped online to a calculator tool and quickly designed up a 3rd order Sallen-Key highpass with a cutoff of 350hz. The simulation looked reasonable so i then simulated it in LTSpice just to confirm things and check the OpAmp i had chosen was going to be ok.
Schematic of 3rd Order Sallen-Key Highpass Filter.
Simulation Bode Plot
Next the circuit was built on a solder-less breadboard, the OpAmp is an NE5532 and negative supply rail is an LM2662 Charge Pump. This gives the OpAmp plenty of room to swing when powered with + – 5v.
For the initial testing the OpAmp was fed with 1vpp 600hz sinewave.
Dumb people do dumb things and I spend 10mins wondering why I had 10x gain in a circuit that should have unity gain, then i noticed i had the scope probe on 1x not 10x where it usually lives. There is my 10x gain.
For final shits and gigles i busted out the bode plotter and swept the filter from 10hz to 5000hz to see just how it really looks, and other than a little noise down close to DC, which i think is just the frequency generator not liking being that low, the filter itself is pretty much as designed. -40db at 100hz should be good enough for the kinds of girls i go out with.
Next job will be to either add in a couple of low pass filters for typical CW and SSB filter widths or have a crack with switched capacitor lowpass filters and make it variable. Thats a job for another day.