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.
I have been slowly and by slowly I mean very slowly working on the audio amp stage for a receiver that I am building. Its a slow effort as i try things, fail things and then finally decide on an actual plan. So, I was looking at using a TDA2822 in bridge mode as i have a bunch of those in my parts box here, but after building it and blowing a few up I figured stuff it, its time for a different tack.
Then i saw these on ebay, I have used the PAM8403 boards before but in a different configuration with a dual gang pot on there, they are class E, uber efficient and sound nice, so i figured fuck it, lets stop trying to make the parts count high and lets just use something that works. And yes these things work.
So i busted out the digital calipers and took a few measurements and knocked up a component for Sprint Layout and can now get to the next stage in the audio stage plan, compression, filtering, pre-amp and AGC. That is, if i do not get distracted by something else shiny and play with it for a while. 🙂
Bottom of the cycle, bands will be dead they say. Well, perhaps not as dead as one expects. Little openings are there all throughout the day and in my case, VK2 was open most of the day. Bands are dead of course, not because there is no propagation, but because there is no one home to work. It is what it is and this WSPR thing is pretty cool. I did not learn anything i did not already know, USA in the mornings and Asia in the evenings. And a few other bits and bobs along the way.
I did have one interesting thing happen, a broken decode where I got no call sign but did get a grid square, I am not confident this is actually accurate, but if it is, then the station i was copying was in Egypt, someways south of Cairo. It would be cool if this was actually the case, but I won’t be holding my breath LOL.
So i got the dipole up now in its final position. The coax is now clamped to the mast so there is no flapping about in the breeze. The ends are up at their finished height. All up we have 7.6m apex and about 3m at the ends. Seems to be working ok, rx is certainly up.
SWR is as good as it is going to get and much broader banded than the old shortened dipole.
Photo bombed by Buster the Boxer. This is some fancy engenerding going on here. There is a 6ft star dropper 2ft in the ground right up the masts clacker providing the main support, the 2nd star dropper is screwed to the retaining wall and some big automotive clamps tying the 2 together. No idea what the safe working limit on all these things are LOL, but it did not move much during the big storm we had go thought last night. So i think it should be ok.
Today i pulled down the old antenna and mast and put up a new and improved version of each. The old shortened 40m dipole was only ever meant to be a temporary solution that stayed in place for 3 years. So it was time to bring her down and put it to rest.
I bought a flag pole cause it seemed like a quick and simple way of putting up a new dipole and a balun because i want to be one of those balun guys who has to tell you what kind of balun he is using even though its kind of not important to the conversation but the word balun gives the air of technical know how and ability especially when you say its a 9 to 1 balun which is really an unun or as I call them 9:1 onions, because endfails will make you cry, but I digress. <– this is called sarcasm to anyone who is going to get hurt feelings over their antenna of choice, don’t email me call your local politician and complain to them.
Erected the flagpole and hoisted up the balun and new dipole cut for 40m on the flag raising rope, saluting while humming the national anthem and got the ends up in a temporary fashion to check the long or short of it with the analyzer, when the wife gets home from work. She comes out the back, looks at the new flag pole and remarks, Oh great, its storm season and you go and erect a dirty great big lightning attracter. Chuckles to herself and leaves me to antennaring like i know what i am doing. And if my Pythagoras is correct, when i put the ends up about 4m in the air, i should have 50mm to spare. The joy of living on a postage stamp block in the city.