Here are some measurements of 3 filters off the low pass filter board I designed and built. First harmonic of each band is at least -50db down. The 4th band which in my case was for 15m, was well out and I need to look at it to work out why, cap values is going to be my initial guess LOL. I built this thing and have never used it LOL. Though I got some new boards coming from JLCpcb this week and i hope to have a transmitter happening that I will use in combination with the HackRF One as a receiver.
There was a post over on EEVblog that got me thinking. What do you do when you are unable to, for various reasons not be able to procure iron core toriods. Well, obviously air core inductors are a thing and highly practical for the low values of inductance used at VHF and UHF. But, what about at HF?
Well it turns out that as long as you do not mind a little bit of size to the inductors, they are still a vary practical option. So what i did was grab out some 1mm enamelled wire and wind 9 turns on a bit if 22mm OD pipe i had laying about to see what sort if inductance such a coil would create. Turns out about 2uH. That is highly usable value in low pass and band pass filters in the lower HF range. And with some thought on the actual construction and mechanics of the coil, very usable inductors could be made this way if you are unable to buy iron core toriods for whatever reason.
Anyway, that is some food for thought. There is nothing magical about iron core toriods, air cores will work just as well and should have a reasonable Q equal to or better than a toriod. So if you are stuck, there is a solution.
As i explained in a previous post, I am building an amp that is going to be capable of doing more than 100w. No biggy for most people but when everything you have built previously has been qrp there becomes a whole new set of challenges. With qrp you can just she will be right mate and do what ever you like “Almost” and it will work to some degree. So with this in mind, i set forth on working out what voltage rating i would need for the capacitors in the low pass filter.
What i really wanted to know, was the WHY behind the decisions others had made in their design choices and I started to ask others WHY as well. Often the reply was just use Mica Caps. Did not matter what power level, the answer some gave was just use 500v Mica and you will be right. No explanation, just this is what I do.
So that just never sat well with me, the answer cannot always be use $60 worth of 500v caps in your low pass filter. It took a while but i finally came across some information that seems credible and more importantly reliable and my gut feeling was right, Mica while good, might often be over kill, but still there are some caveats that are important to understand.
Firstly the voltage that a capacitor in a LPF sees is the peak to peak voltage of the AC wave form. Might be obvious to some, but its not information that is easy to find on the internet. So knowing that we can then use the power formula to find out what voltage might be present in the filter. Power(watts) = Voltage^2 / Load(Ohms) where in this case, the voltage of the signal is RMS Volts and once you have solved for it, you need to convert RMS to Peak to Peak by multiplying by 1.4.
So in my case 100w = 70.7vrms/50ohms :: 70.7*1.4 = 100v peak to peak. Now obviously there can always be an impedance mismatch between the low pass filter and the antenna and with that a change in the signal voltage. In my case i also looked at 100w at 25ohms and 100ohms load to get a better picture of what might be expected of the capacitors in the low pass filter. Being that I am actually designing the amp deck to be capable of 400w even though i am only running it at 100w, it means the capacitors need to be able to withstand a peak voltage of about 350V.
So onto the next point, I wont be using Mica Caps, for the very simple reason that $50 worth of caps is a lot of money to throw at something that might be a dismal failure. I will be using SMD ceramics and here is something i learned just recently, ceramic caps derate in capacitance the closer you get to the maximum voltage rating. According to a TI white paper i was reading, you should probbaly derate ceramics in filters by at least 1/3. So I need at least 360V rating, that means using 1Kv or 2Kv ceramic caps should be perfectly fine. The good thing is, Kv rated SMD caps are cheap, in the sub 50 cent range in 1 of qualities on mouser and they are NP0 and 5% tollerance. So i have solved this problem now and can move onto other things like output transformer and core sizes i need to use for the power level.
So here’s the thing. LOL how many times have i started a new blog post with So. You know what it means though, that something did not go to plan, did not work and just became an exercise in learning. Yes, this is one of those times when I learned a lot and still have nothing functional and working at the end of it. 🙂
So JLCPcb were running a special of 30% off on boards. I had been tinkering with an audio amp and filter board for a while but it was no where near ready. I really had been lazy and just not put any time into finishing it. So while searching around for something else I came across this Indonesian site that contained downloads of Gerbers for some of their projects. So i downloaded one and looked over the schematic, it looked interesting so i then rushed into getting my audio board ready to make and sent off for the boards.
Now I did do somethings very wrong here, I played the main components like the relays, IC’s etc and locked them in place and used the autoplacer and autorouter to place all the passives and route it up. Obviously not the best idea, I also should have put a via fence around the negative voltage inverter and the same to isolate the 2 audio filters. So anyway, i got the board populated and started checking voltages. No 12v rail to the 2nd opamp, audio preamp or the audio amp. Seems that the 12v net did not get autorouted in 2 places and I did no checking to ensure things were right. Noob mistake, but i was rushing. Bust out the MOD WIRE.
So does it work? As pictured NO. The relay below the voltage inverter on the right hand side of the board is acting as an antenna for the voltage inverter oscillator frequency, which in turn is causing the audio amp to oscillate like crazy. Removing that relay and bridging the pads fixed that problem. The second major issue was the values I used for the filters.
Normally i would simulate in LTSpice just to confirm things are ball park. Turns out that the calculator i used, uses the frequency supplied as the -6db point. Kind of useless for a CW filter where you want 600hz peaked not attenuated by 3db LOL. So i will suck out the parts and replace them with something more appropriate now I have redesigned the filter response and simulated it.
These are not going to be the sharpest filters in the shed. They just have Salen-Key responses. I did not want brick wall filters, I just wanted something to make listening more pleasant. Eventually I will do the aggressive filtering in the IF using DSP. I have a partial design for this happening already, but it is a long way off being ready. Crystal filters it is for now, but DSP IF is coming, it just takes time.
My old breadboard was starting to get a little tired so I thought it was time to update it and to something more useable. The old one was homebrew more or less, but how it was being powered was becoming a nightmare to use.So i grabbed one of these of ebay, and with a breadboard power module i have 12v 5v and 3.3v as well as USB power out. All in all this will be much more usable than the old board.
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.
So I have had this pile of Ebay sitting on my desk for sometime and today I have gotten excited enough to actually start taking a look at it all and seeing how it works. I got these frequency generator chips for like a buck and after setting them up with the test circuit, i could not get them to work, it happens, now and again you get Ebay’d in the butt.
So I figured next i should test out the TDA2822 audio amps. I for 50 for 2 bucks which is a lifetimes supply. Do they work, well, yes they do and here are the results.
Here is the test circuit straight out of the PDF. As you can see parts count is low. So I put the IC on the breadboard and used just 1 1/2 for a mono amp. I also used just 2 caps, pin8 to ground, 470uf as it was already on the breadboard and the input cap on pin 1. Powered with 8v as its a handy voltage i have on my breadboard. 12v would probably be a better option to allow for a larger voltage swing.
As you can see, nothing fancy here, just the IC and 2 caps and my signal gen and oscilloscope probes doing there thing allowing the pixies to in and out and display them in the screen.
So we stick in 0.1v 600hz sinewave and see what happens.
Well, we actually hit the voltage rails and clip somewhat. 0.1v in almost 8v out, that is the voltage gain there. And when i do some da finger poken, the IC itself is cool to the touch, not warm, not hot, but about the same as ambient temperature of the room. So i am thinking Bye Bye LM386, and hello life time supply of TDA2822. And being a stereo IC, I can also bridge the left and right for even greater output. Not that I think i would need it.
50 audio amps for $2. Seriously, this is like a life times supply of audio amps. In bridge mode they will put out a good watt or more into a 4ohm speaker and have 40x gain a lower parts count than an LM386 but sound significantly better also, coupled with the 2n3904 preamp i use on just about everything, this should make for a good combo.
Well its bound to happen eventually, the need to use SMD components. I have been collecting some now for a while and ammassed a tidy little assortment of these beasts. Best of all, they will all fit into 3 tiny ring binders. One for Caps, one for Resistors and one for Inductors and Semi Conductors. Might as well get in now and make a collection before all the large sized SMD vanish and only the grains of sand remain. I currently only have the one binder, but have 2 more on the way from China just to be sure to be sure.