Sometimes, just sometimes, all your ducks align in a row and you build something that is dead set on the money. I think today I have done just that with this crystal filter.

Firstly I have redesigned the PCB board and test jig setup I am using, making construction just a little bit easier and allows me to easily use the slightly larger binocular core.

But secondly, I think i hit the nail squarely on the head, with this filter. I mean its almost perfect. 5dB loss, great, good shape factor, great, -55db stop band, great, exactly 2200Hz passband PRICELESS. I honestly could not have done better even if I tried. Now to build the receiver board and pray to Jebus that it works as good as this filter looks.

As a bonus, here is his friend the CW filter, 600Hz wide. A little more lossy but still good stop band and shape. I was shooting for 500Hz, bit I need maybe another 15pF per node. Good enough for me though, that is 2 filters for a receiver, 2 filters that should perform reasonably well.

Final EDIT:

So I was going to upload the gerbers and create a great big post about how to use them. But, i went looking for the files and it looks like I have deleted them by mistake when cleaning up my project files. I got no idea how I did that, but they are gone and its a lot of work to recreate them, so there will be no release of these PCB files. Lucky for me I can reorder them easily and I also have enough board here to make 20 filters, that should be a life times supply for me. Rather sad really, because they work so well.

Rob.

I do not like winding inductors and there are 25 of them on this board. God I am glad that nightmare is over.

I have spend the last couple of days writing code for this thing. I am finally making inroads and next job is to actually build the receiver IF board and test it out. This current iteration of the PCB is so much better to work with, much more user friendly. We currently have working audio amps, band switching, variable rf gain, band pass and low pass filters, TX/RX switching and the like all happening.

 

 

 

So currently I have more crystal filters than you can poke a stick at. Continuing on from my last blog post, I have spent hours soldering and de-soldering the transformers on the CW crystal filter, both trying to improve it somewhat and to also understand what I am seeing and how it reacts to different impedance transforms. Bear with me, this could end up being long and boring.

So for those playing along at home, you might recall my rather average looking plot for the CW filter, not the best stop band, quite a lot of loss and I was never happy with it. Now the theory goes, that the input impedance of the Cohn Min Loss filter is in the ballpark of 200 ohms, so throw a 4 to 1 impedance transform at it and rock on Johnny, it will be ok. Not so with this filter.

What i did with this plot was terminate one side of the filter with 50 ohms and then ran an S11 on both ends of the filter. You can see by the plot and the one below this that maker one is around 75 ohms and it close to being within the bandpass. However, you have these other 2 peaks either side of it at around 12 ohms. I did not know what to do here, how do you raise the low impedance while lowering the high impedance? This goes against the laws of physics of course, as the impedances are going to track with what the transformers are doing.

The first thing that I tried was to use averaging. I figured that if I raised both the 12 ohm peaks to 50 ohms, the 75 ohms would go to 300 ohms, but the average of the passband would be closer to 50 ohms than not. This it turns out was a bad idea and introduced other complex impedance’s in the passband and thus greater loss and reduced stop band.

 

So i changed tack. Perhaps its what is within the passband that is the most critical here. That peak at close to 75 ohms might be the key as this is a rather narrow filter. So for the 100th time i pulled off the transformers and wound them this time with a turns ratio of 4 turns primary and 5 turns secondary, and you can see with the above plot marker 2 went from 75 ohms to 30 ohms, all i did was split the difference. HAHAHA. The other problem is, it gets rather hard to wind transformers for odd ball values of impedance transform.

But i think that this has put me on the right track. If i pick a frequency that is right smack in the middle of the filter and try and match for its impedance, I should be able to improve the losses somewhat while also retaining the much improved stop band. That extra -10db in the stop band is a massive improvement for no extra loss. However, if I can improve the pass band losses by a couple of db, without degrading the stop band, then that would be better still.

One last thing before i sign off, using the NanoVNA for measuring impedance, now thats a game changer.

These files are copyright Rob Powell, VK4HAT. By downloading and using these files you agree to the following license restrictions.

1. These files are for PERSONAL USE ONLY

2. You cannot use these files for any product that is for sale, trade, barter, exchange for sexual favors or any other form of personal gain.

3. You agree to leave all silkscreens as is within this files.

4. If you break any of this license agreement, you agree that you are a  goat fucker with a tiny penis.

5. If you do not like this license agreement, by all means write your own  code, design your own boards and contribute your own work back to the  community, so they can steal your stuff and sell it on ebay like a pack of  thieves.

6. DON’T BE A DOUCHE.

There are 4 filters per board plus a test jig. For $2 you can get 20 filter boards and 5 test jigs made. Cheap as chips yeah.

The filter is G3UUR Cohn Min Loss, the caps are all the same value and set the width of the passband.

The outer area of the solder mask are pulled back to allow for a shielding cap to be soldered over the top.

GERBER FILES:: filters_test_jig_gerber

I did a little fiddling with this again tonight, bypassed the crystal filter with a much narrower one that has less loss and probably sounds better, to boot.

It still needs more gain in the IF but there is not much I can do about that other than redesign it with regular old BJT amps or get faster, higher slew rate opamps to replace these, and well I am not spending $5 plus on an opamp to see if that  fixes the problem. So this one as hit the end of the road and is as far as I can take it. But, its working and now sounding sort of OK.

You can hear the receiver copying some CW in the above video, the 2 guys were the only signals on the band and they were not much weaker on my homebrew than they were on the Icom. So the receiver is sensitive enough, but there is a lot of room for improvement.

OpAmpia: The Op-Amp Receiver

Opampia Receiver Board

 

I have build 4 receivers in as many weeks, none of them anything i would write home to mum about. They all worked, some better than others, but they have all been rubbish in one way or another.

This is OPAMPIA, something i started a while back but put in the too hard basket when I did not have a reasonable audio amp to use with it, the problem is that its got a sore tooth, and I am not sure why yet. The output of the opamps look nice, but the audio out the final mixer has a bad dose of pythagorus’s theorem. I am going to knock some gain out of the IF amps and see if that helps. I think one of the IF amps is being slew limited and hence this horrible output.

I think this receiver could actually be quite ok once i sort out the awful sounding audio. The signal in the video is really strong and loud. The sender is about 1000km away, this was on 40m.

Ok so i built the RX part of the uSDX, just wanted to see how it works and all that yadda yadda yadda and also because the TX mostly sucks. I have plenty of transmitters anyway. It works, I can hear a tone when i inject a signal and now I need to sort out some audio amp and give it some antenna.

 

You know that one project you have had that never worked. Welcome to my nightmare, the QRP Labs receiver and polyphase network. Nothing wrong with the design, nothing wrong with the kit, Hans makes good stuff, even if he packs the parts in too closely together for the sane among us. But that said, let me say OH FUCK YEAH. I finally got this working and the problems were all mine. Of Course!!

Issue one, the bandpass filter was not working, I am not a fan of Han’s filter boards, to tiny, to fiddly, total pain in the arse to build, again my fault, will need to look at that again sometime later and find out where it went wrong. Issue number 2, I dumped a solder bridge over the two power pins, and guess what, one side was dry and so the flip flop was not getting power, thus no quadrature signal.

So i fixed both of those issues and set the signal generator to 7Mhz and 28Mhz plus offset and BANG it fires into life and I have a 600hz tone coming out of the headphones. Winner winner, chicken dinner. I finally have this thing working and its not got me thinking about polyphase receiver design again. Doing something similar to this one, but using a 7 or 8 stage polyphase network and getting better opposite sideband rejection. I had started drawing one up the other day in diptrace. While waiting on parts for the next build, i might just give this a try.