More Protoboard Madness

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.

 

 

 

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Splitting Hairs With Crystal Filters

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.

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Crystal Filter Project Files

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

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A Story Of Two Crystal Filters

While waiting for parts and boards to arrive for the main things I am working on, I thought I would turn my attention to the age old problem of crystal filters. Its not like you can easily buy filters off the shelf these days and even finding suitable crystals can be a problem. The low profile half cut crystals can be hit and miss and full size crystals can be expensive. Hayward wrote a few papers not long back on using 9Mhz crystals in simplified designs and pretty much if you buy 9Mhz 30pf crystals you should be able to duplicate his work with minimum effort. Problem is,  9Mhz crystals are non existent in Australia even in the low profile cases and they are $1 or more each in quantity on mouser. Screw that Jacko.

I have had mixed results making home brew filters, but this one looks the business. Other than being 4K wide, it looks like a filter should look. Fixing the width is just a matter of changing the capacitor values. The second filter is 600Hz wide for CW, thats just about where I like it, but its got -10dB loss, not sure the issue just yet but my gut feeling is that its one of the crystal is off from the others. I did measure them all, but you know, failure is always an option. I am going to suck all the crystals out and start with a fresh batch of 8 and see if that is the problem and then change the caps on the SSB filter to narrow it up some. I hope to have 2 very usable filters.

Typically i fix things until they are broken, but hey this actually looks alright. I need to narrow up the caps some, but now its not all loss and no stop band. -60dB rejection is more and good enough for the kinds of girls I go out with.

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More OpAmpia Receiver

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.

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Opampia Revisited

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.

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