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.

Rob

Facebooktwitterredditpinterestlinkedin