Measure Once, Cut Twice

An Experimental Method Of Receiver Design

Ok, so this could end up being a post of epic proportions. So here goes it. Actually most of what I am going to write about as been the culmination of a long process of learning and knowledge gathering, where I am going to attempt to bring all those threads together into a whole. At this point, the jury is still out on whether I can actually do it, I have not yet even completed the build, but rather, am going to try and document the entire process from somewhere in the middle to the finish.

To go from the start I would have to explain in detail every little design decision I have made and why. Suffice to say, I have all those details buried deep in my brain, but I am not here to argue with internet experts, I am just trying to work this shit out for myself and understand the process better so that I might eventually build something worth using more than one and perhaps something that could replace the Icom as my daily drive.

So lets start this in the middle. Here is the board I designed, laid out and had manufactured. It is nothing revolutionary, it is a single conversion superhet. The mixer IC’s are gilbert cell types used in cordless phones. The IF amps are SMD version of the 2n2222 and the crystal filer is a 4 pole Cohn Min Loss design using 12mhz computer grade crystals.

The reason for the rubbish level crystal filter is that this is not going to be my forever receiver build and design, it is first and foremost an experimental board whose purpose is to learn as much about superhet design as I can and to try out some methods test and measurement based around using the nanoVNA for much of the measurement, especially for impedance.

So first things first, fire up the crystal oscillator and find some crystals that are close enough to each other to make the filter with. Don’t you love my oscillator, yeah even I used to homebrew like its 1932 and do Manhattan construction on copper clad board like some kind of peasant who could not design a PCB in some ecad. HAHA,. Actually, seeing this board has been getting a workout of late, i have designed one up and will get it made next time i send of boards. Might as well make something pretty looking 🙂

So here are my selection of crystals. The numbers are the last 4 digits of the frequency readout, so the split between the highest and lowest crystal is 13hz, which is a pretty tight grouping, typically i am looking for a spread no more than about 50hz. Crystal filter building can be hit and miss, but I am trying to work on a simple and yet repeatable method of making filters even with the rubbish computer grade crystals we can get now.

Having a fairly accurate frequency counter really helps here to get crystals as closely matched as you can. This one was rejected because it was 7 hertz different from the rest.

Now I know a 4 crystal filter is going to be kind of rubbish, but, when all you are trying to achieve is learning and a method of design, test and measure, it will be good enough for that. So the above image is the Z plot and 21 gain plot for the filter terminated so of course it looks rubbish.

The 2 markers show kind of where the passband is and if you look at the Z lot you can see that the impedance is all over the place with a minimum of 75 ohms and a maximum of 230 ohms across the passband. This is not going to be very easy to get a match with, but like i has been doing with the other crystal filters I have been attempting to build, I will split the difference and attempt to match the output of the IF amp to maybe something like 150 ohms and just see how the losses look. If its less than 10dB i will be happy enough.


The design goals for the IF amps was to have 20dB of gain. They were designed in LTspice because that hurts no parts LOL. Ignore the transformer ratios because they will need to be measured to see just where things are at. But, as for the gain of the amp, well its pretty damn close to having 20dB gain.

The input signal was 53mV RMS. Ignore the p-p voltage its actually displaying wrong. Shit scope what can i say.

With the output of 520mV RMS and a little math that looks something like L = 20 × log (voltage ratio V2 / V1) and you get 20dB gain. Bloody dead nuts on.

Measured the input impedance and its about 85ohms.

Measured the output impedance and its about 40 ohms.

And this is where we are currently at, needing to wind some transformers to see if all this measuring, testing and thinking like a big boy actually means anything in reality. But for now, things are looking ok and we have all the numbers we need.

RF IN is 50 ohms.
Mixer Input and output is 1500 ohms.
IF amp is 84ohms in and 40 ohms out.
IF Filter is 150ohms in and out.

And this post is 900+ words and 11 images HAHA and we are not even ready to feed some signal into it and see if it actually works. But hey, this is what learning is all about. Measure once, cut twice HAHA. Only time will tell now if i have my shit together.

Peace out.



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.





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.


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.


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


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.


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.


The Quarterly Essay

How To Be An Elmer

Usually by now I have posted something of a state of the union address outlining what I did last year and what I plan to do this year. Most of it follow much the same outline, i set out a grand addenda and then 30 seconds later get sidetracked into other thoughts and get sucked into many other rabbit holes and I never get much done that I planned. So rather than do that this year, I thought i would take a very different approach to my first blog post of the year. I am going to take a topic, an action or something I have found interesting and write a whole lot of shit about it.

Even the title hints at this. The quarterly essay is an Australian long form journalism journal that comes out 4 times a year. There are no sound bites, no click bait titles, just a really good look at a specific topic in depth. And in podcast form Radio Lab from WNYC Studios kind of does the same thing. The whole idea of long form journalism has kind of lost its way in favor of click bait on social media to sell adverts because no one wants to pay to read real quality reporting, they want some fuckwit pundit to tell them how to think so they can go perform how woke they are on any given subject.

Take the youtube stars as an example, so much of what gets posted is essentially click bait. When I see a video posted with the guy pulling a face pointing at something in his card, I think, fuck that. Do not treat me as an imbecile, or as a means to and ends, I am not here for you to earn ad dollars, so stop with the stupid shill videos, just get to the point and show me either nothing, or something relevant, you pointing at something like a fucking idiot is not going to get me to click on your video, the content will.

So anyway, I have been struggling to a cohesive narrative for this post. Its been more rambling on than anything thus far, but all along I have wanted to talk about elmering. I have been sitting on this post for over 2 weeks not sure of where I wanted to take it, but now I know, and the reason why i know will soon become clear.

For a while now I have been helping a couple of people who have been struggling with focus and understanding concepts and via email i have been answering their questions by mostly asking them questions. I think this is the best way to gain understanding, though, my wife hates it when I do it to her and I have been helping her with essay writing for the university course she is doing for a promotion at work.

As far as elmers go, I have been lucky, I have a EE lecturer at a uni who confuses me more than most, but there is a lot of great technical knowledge coming from him and there are a couple of community leaders who i look up to and respect and while I do not ask them lots of questions, I like to share with them my progress, just so they know that their efforts in writing articles, blog posts and the like are not wasted and that there are some who really appreciate the effort they go to to share their time, skills and knowledge to a wide audience.

And it saddened me, while speaking with one of these wizened leaders of the home brew community that he said he was retiring from home brewing. And lets face it, he has certainly earned the right to retire and enjoy the fruits of his dedication to home brew and construction. But that is not why he is taking a break, he is taking a break because of emails saying things like I don’t like your designs and I disagree how you treat the radio art.

Seriously, what the fuck is wrong with people. If someone emailed me something like that, they would get told to go fuck themselves with a hammer, their email would be black listed and their IP added to IPTABLES to DROP all connections from them. See, I just do not have time for these kinds of gutless fucks. Which is why i just focus in doing what I do. People can come along on the journey with me and offer support or they can go to hell.

Its a rather sad indictment on our hobby that people who mostly do nothing, contribute nothing and build nothing are experts on everything that others are doing and building. As some of you are aware of, I moderate a couple of home brew related groups on fakebook and I see this all the time there also, people who really never build anything join these kinds of groups just to tell others how expert they are and to post stupid things that have nothing to do with home brewing. They do not last of course because I take a zero tolerance view to moderating and just delete their posts and or remove them from the group.

I have had a few people over the years tell me that my blog or my videos are not up to their standard. People who have never posted a single thing in their own lives, not even to their QRZ pages. Its tough tits to them hey? And I have others asking for X Y and Z, to which I reply, I am not doing any of this as a service to anyone, I am doing this because I want to keep some sort of documentation of what I have done. This blog for me is not a teaching tool for anyone, its my diary that documents my journey. I can look back in time and see where I was and how far I have come. I am also not really a teacher, I am more an encourager and hopefully and inspirer. If one person reads stuff on here and goes, well if this idiot can do all this, I am sure I can. Or if someone goes OHHH thats a good idea let me improve on it, then for me I have one and fulfilled my duty as a ham to pass it on. For me its not knowledge, but passion that I wish to pass on.

So, how to be a good elmer? Just get out there and do what you love and do it to the best of your ability and show the world just how passionate you are about your thing. It really does not matter what your thing is, be it home brew, portable operation, contests or heavens forbid bloody rag chewing. Do it with love and passion and inspire others with your addiction. You see, the people who I look upto and respect in the hobby and the guys who actually get off their asses and do something. Their actions inspire me to work harder, understand more and to be better.

Ultimately and elmer is someone who leads by example. And leaders are something sorely lacking in the world right now, lets not make ham radio less than what it already is, get out there and lead and show people there is more to the hobby than pissing and moaning on fakebook about who touched you on the frequency, who does not radio like you do, what the idiots at “Insert You National Organization” are doing. Be number 13 on the DX code of conduct: I will respect my fellow hams and conduct myself so as to earn their respect. And be the best you that you can be and avoid the arseholes.

To my friends and the people who have inspired me, thank you.

Baxter Out.


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.