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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
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.
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.
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.