OpAmpia: The Op-Amp Receiver

They say a picture is worth 1000 words. Well A video must be worth 1000 pictures then. I have to admit, i really do not enjoy making videos for youtube. I do it reluctantly and only when I really have something worth showing off. I really do not care about building an audience and being famous, I just like to do what I want, when I want and not feel compelled to make anyone else happy other than myself. I am not entertainment, I am just a guy who is documenting his journey, nothing more. Even this blog, its not a how to, its not a guide, its often not even correct, does not work or is fundamentally flawed. That is what happens when you home brew. You make mistakes, things do not work and you have to trouble shoot and trouble shooting means learning something. A good day for me, is a day I learn something.

Well, this is the PCB. I have named this receiver OpAmpia. Because its just a bunch of op-amps. There are 3 high speed op-amps on this board, acting as RF Amp and IF Amps and then on the other board another 5 op-amps doing audio agc and preamp and audio filtering. So all up, we have 8 op-amps, hence the name.

This is a revision of what I actually built. I have redesigned the bandpass filter and will be using TOKO styled canned inductors. The higher Q of the inductors should see much less loss than the SMD inductors I used on the test board.

Same goes with the IF strip, the IF filter is now not plugin, but will use all SMD components, I have a bazillion SMD crystals and so its time to start using them. Actually in everything I am moving away from through hole components where I can. SMD is just so easy to use you can always find a useful part that is cheap. Take the Gainsil op-amps I used, they cost like 30 cents each. Try and find a highspeed op-amp in though hole for that price.

For alignment I used the function generator set to 7mhz and with both VFO’s showing no offsets tuned things until they sounded great. Having 2 VFO’s really helps here. Now i can just do some simple math and remove the offest and display the actual RX frequency. A little bit of coding and we will be in business.

That’s all from me. Another month and I might have this as a complete and working transceiver. LOL Who are we kidding 😉

 

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More Receiver Work

So with the VFO and Audio board working i turn my attention back to working on the receiver. Now the first version of this was an unmitigated disaster. Clever me, always throwing caution to the wind used MMIC gain blocks for IF amps and had nothing but trouble with them oscillating and going bonkers. Well, this time I threw caution to the wind again and used high speed op-amps for the RF and IF amps. Everyone has done a 2n3904 IF amp and i could have done the same, but, lets try something different. And i could not be more pleased. Actually using op-amps might just become my thing LOL they are actually surprisingly easy to work with once you know what you are doing.

 

I have posted about this before somewhere on the blog, but this is the circuit I used after reading some app notes from one of the big manufactures. Non inverting, 50 ohms in and out impedance and thats about it really. Unity gain bandwidth of the op-amps i used is 380MHz and they are a 20 cent Chinese part.

Doing the initial alignment of VFO and BFO frequencies I had more probes up its cackler than an abductee at a aliens convention.

 

The mighty RF Explorer spec-an for the win getting the VFO frequency mixing with the RF input to be right smack center in the passband of the 500hz wide IF filter. Yeah its a CW rig this one.

VFO A is the IF frequency and VFO B is the BFO frequency.

And this is the 600hz tone out the AF port. Which will eventually get routed the audio amp that I actually short circuited while fixing it until it was broken. Actually, the AGC and the Audio filter circuits work just fine, its the AF amp i killed. Its just a matter of de-soldering the amp module and replacing it with one that works. Every day I am getting closer and closer to something that actually works good enough to use more than once.

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Its Working Again :)

So I pulled everything off the shelf to give this a test. Its working. What I have done in the pictures below was to use an DDS vfo as the incoming 7mhz signal. It run into as much attentuator as I have here, giving a final signal out of about -95dbm or there abouts according to the spec-an. This number is rather uncertain because that is right on the noise floor of the device and by my calculations, the actual signal is closer to -100dbm. Now I am still hearing a signal out of the speaker at that level. So its not an Icom, but its not totally deaf either. I do think i could use a little more gain in the audio stages. But, we will see how things pan out with some loudish signals on 40m this afternoon.

The receiver setup with the attenuators.
DDS VFO used as the signal source.
Function Generator being used as the VFO and BFO.

 

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Its a Superhet Thing

 

A few days back I slapped together an superhet, it kind of works, certainly nothing to rave home about that is for sure. A few take outs from it, using MMICs as IF amps is not a clever option as you really cannot control the gain all that easily. And the other thing is audio. You really gotta make sure you use an audio amp with sufficient enough power rating to drive a speaker.

After playing around with things, I did notice that I had some rather obvious distortion. After changing out the audio amp for something sturdier and testing the audio stages by themselves, I came to the conclusion that the distortion was coming from the IF amps. I will see if i can tame the gain in them somewhat and see if i can get this working better.

I am still not sure about the audio sound, the SSB filter is somewhat thin and the audio AGC is really not AGCing. I am not sure what is going on there. But I have an AGC ic here on a board that i might try and see if that works better than the discrete version that I built.

I configured the superhet with both CW and SSB IF filters. Not sure how good my filters really are, but once i sort out the distortion, it should be able to see how the filters perform.

 

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Power Inductor LPF

More things being tested for the hand held project. These are 1A power inductors. I need something much more low profile than a toroid to use in the low pass filter and also something not through hole. So i grabbed these power inductors to test to see how much loss they are going to induce. Turns out they are not all that bad and at least as good or better than the hand wound toriods i used on another board.

Other than the corner frequency being in the wrong place because i used the wrong cap values, the loss is less than 2db and that will be good enough for the kinds of girls I go out with. Oh and the hand wound type 2 iron cores? 3db loss. Go figure.

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HT Receiver

 

Ok, the long and the short of it. It actually works well. And by well, i mean it can hear me talking on the home station, it can make a tone from injected signals yadda yadda yadda.

So what was the problem, well ended up being a dry solder joint, the most basic of all problems. I had pulled it apart, cut traces, did eleventeen other things to it trying to work out why it was ringing its head off and making no sounds, and it turns out the LO input pin was dry and not making contact.

So what next? Well i start to integrate things onto one board now and start actually assembling something I can use to make a QSO with. Wish me luck.

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