A Lesson In Inductance

This is all kind of Schrodinger’s cat, but it turns out not all 6uH is made equally. 6uH can be 6uH, 6uH can be 12uH and 6uH can be a capacitor and the only thing different between all these 6uH’s is the frequency they have been measured at.

So tonight i got well and truly schooled on inductance and why my bandpass filter was looking rather bonkers. I built it with the right capacitors, i wound inductors with the right number of turns for the required inductance i assembled a really nice looking filter. But when it came to measuring the bandpass it was orders of magnitude wrong.

This is how my filter should have looked, well, at least something like that.

This was the bonkers mess I was measuring in the Bode Analyser.

So after much discussion with a much smarter man than me, and working out everything i was doing wrong, I ended up with this testy jig, 200R in series with the inductor, feeding one side with the signal generator while measuring both the input voltage and the voltage across the inductor. Changing frequency until my output voltage is 50% of the input and then using this following formula to calculate the inductance. L= R*sqrt(3)/(2*pi*f) that at least gets me in the ball park, and it turns out I was orders of magnitude off with the inductors i wound and were most likely acting like capacitors at 20mhz where my filter was peaking.


Easy Grips

No No No, I have not abandoned the receiver project, I have just hit what is likely the first of many road blocks, and I have not yet worked out what is going on or why for that matter. So in the mean time, I am going to turn out some bits and pieces that I have been meaning to do for a while, while I contemplate what is going on with the bandpass filter and why its center frequency is bonkers off where it should be. More on that to come.

Anyway, I always need to connect A to B and B to C while i am building things, so i made up some double ended easy grips for just such an occasion. Hardly rocket surgery and defiantly not brain doctoring, but handy to have none the less. Next up is a test jig for measuring inductors on the oscilloscope.


Red Pitaya Stem Lab

The Red Pitaya Stem Lab is a multi function piece of test equipment no bigger than the palm of your hand. For the most part, my needs for test equipment are not that demanding, I just need simple devices with simple controls to give me the kinds of information I need for my homebrew projects. For my needs there are 5 basic functions listed below that I will find immediate use for, but there are other uses it has also, like being turned into an SRD Transceiver or being used as an LCR meter among others. Total bandwidth is 50Mhz

1. Oscilloscope: doing probe compensation.

2. Function Generator: is part of the oscilloscope interface upto 1v p-p output in Sqr, Sin etc.

3. Frequency Analyzer: A usable 50Mhx of bandwidth is good enough for the 80 and 40m band projects that I build things for.

4. Bode Analyzer: I have spent a fair amount of time today playing with the bode analyzer and trying to get to grips with it, I am unsure about things like what probes to use, what input settings and what voltages to use on the sweep generator. Well i got this far and have a plot that looks like it should. One thing that i did not do was terminate the double tuned band pass filter with 50 ohms, this will have an effect on this plot and how it looks. But, it does actually look like a thing, even if it looks rather wonky and the high side attenuation is rising at the edge where it should still be falling.

5, Logic Analyzer: Not something that I use very often, but when you have some Arduino widget that is not playing ball, its nice to have the right tool for the job on hand.

At 3mhz a square wave still looks square, by 5mhz its starting to look sinusoidal, nothing fancy, but good enough for my needs. All in all, i am happy with how well this thing works, I am no power user with high demands, I just want to be able to see a waveform, produce a waveform and see spectral content of RF i am producing with the highest frequency of interest being 7mhz, I look forward to putting the Red Pitaya to use in the CW Receiver project i am just starting.


My initial thoughts after using it for some time are still positive, there are some things that are a little funny in how they work, but for the most part, this is going to be a good tool to have on my bench, that will do just about everything that i could want.


4 Channel Variable Lab Power Supply.

A good lab power supply can cost a lot, even a basic one is not all that cheep and what you get is not all that convenient. 1 variable supply and that’s about it. But with off the shelf components and some time you can make a good lab supply for about $100.

When I am experimenting, i will often build circuits on separate boards and wired them together into a functioning whole, and this means needing 3 or more power supply rails. Most of the things i build do not have huge current demands, so as long as each rail can supply 2A it will do all that I need and supply voltages from 3 to 30v. So, with that as my design criteria, the basic specs for the lab supply became as follows.

  • 4 channels
  • 10A maximum current
  • 2 variable supplies 3-12v and 12-30v
  • 2 fixed supplies


I am not a fan of building things that can kill me, so for the power supply I used this 10amp 12v switch mode PSU and for the variable supplies I used a Buck and Boost converter to get the 3->30v coverage. Actually it will work down to 2.5v and up to 34.5v, a nice huge swing.


Next was to mark out, drill, cut and mount the voltage display modules, the pots and binding posts. The 3-12v supply has a 10 turn pot and the 12 to 30v a single turn pot.


The integrated power jack and switch was then installed on the rear panel.


10amp house wire was used to wire the switch and 240v power, I soldered and heat shrinked this side.


Spade connectors were used to wire into the psu.


The buck and boost converters were mounted onto the top of the psu.


The rest of the 12v wiring was then installed, crimps being used on the psu side and everything else soldered.


Wiring complete, liberal use of zip ties to make it tidy inside.


The inside of the front panel now it is all wired up.


And now with 240v applied. Works a treat, the voltages are stable under load and for under $100 I have a pretty good lab power supply that will keep up with my home brewing needs.




Lab power Supply

A lab power supply can cost a lot for a good multi channel one. I dont want to spend $300 + dollars on a 3 channel psu, so I am making my own out of an ATX PSU and a handful of buck and boost converter modules. The good thing about doing things this way is that you do not have to play with 240v.

The specs will be 3.3v, 5v and 12v fixed supplies, 2 times 3-12v variable supplies and 12-30v variable supply. A cornucopia of supply rails with the 12v rail able to supply a maximum of 20 amps, i will not want for power when I am home brewing circuits and need multiple supply rails.

The buck and boost converters are off ebay, they also have current limiting, which is nice I will have them turned down to less than an amp most of the time, and the most they can supply is 5a, i will never get close to hitting the max on the psu even if i make something that tries to draw a lot of amps.


Mocking up the layout after making the base of the case.


The entire case will be framed in alloy angle as i have no bending facilities.


Cutout and mounted the psu to the back panel.

Layout of the front panel making sure there is enough space around the pots and banana sockets.

Front panel is as crooked as a hillbillies tooth thanks to not owning a real drill press and using a dremel drill press for doing the pilot holes. I will probably keep it and not have a do-over, now its on to the rats nest of wiring and building in the rest of the case.



VK3AQZ Kits Attenuators


I ordered these kits on Monday and had them in my hands Tuesday, nice work overnight express delivery. Bought from VK3AQZ Kits for $20 each plus post. I would be hard pressed to Ebay the parts for these and come in under that cost and then they would not look as professional and well made as these with their machined cases and chromed labels.


The 2 kits i bought give a total of 10 and 50 db of attenuation, the first gives 1, 2, 3 and 4 db of attenuation.


The 2nd gives 10, 20, 20 db of attenuation. They are rated to 250mw and 50mhz. But, with paralleling the shunt capacitors you can make these 1w, either way, for just about every application one might use these for in their homebrew projects, they are more than adequate for the job at hand. Form 3db points on filters and resonators, to 10 and 20db points to align the rf meter, to noise testing and the like. Attenuators are handy kits to have.


Kits go together very easy.


RFPM1 RF Power Meter Part 2

RFPM1 RF Meter Kit by vk3aqz Kits.

Ok, so a quick update on the RF power meter, I have got it complete, I did not take as many pictures as i should have. The kit is super easy, the instructions are brilliant, the packaging brilliant and the service from Lou the kit maker, just awesome. I cannot fault any aspect of this build, it has all been super clear and easy. The videos of the build process show everything you need to know, and the alignment videos are superb and make things so simple and clear.

All i have left to do is smoke test and align the meters and power head. To do that I need attenuators, and for that, I will write another blog post, as i bought 2 attenuator kits also from VK3AQZ kits.


SMD part in the RF power head are easy enough to solder, not a huge fan of doing it, but nothing a bit of liquid flux wont make easy.


RF power head assembly is rather straight forward. The video instructions make things very clear here.


Now i did not take any pictures of this as it went together, totally forgot, but the build is super simple and the use of JST XH connectors throughout makes for a nice clean wiring job.


The unit fully assembled, ready to smoke test and align.



Only job remaining after alignment is to add the handles, and feet to the case and then use it often in projects. All in all, very happy with the quality of the build. I cannot wait to get it aligned now and to use it on my projects.


Homebrew 455khz Ceramic IF Filter

So today after reading some more of SolderSmoke – Global Adventures in Wireless Electronics” and feeling somewhat enthusiastic about advancing some projects i went out to my work bench and soldered up a test rig used to determine the motional parameters of crystals and in my case, ceramics.


Circuit went together rather uneventfully, built in what i call my Ugly Manhattan style more slum lord millionaire than trump towers LOL. But it works and it does the job it was designed to do. And thats all that matters. Schematic can be seen in the above picture, but there is a better one to follow.


So i plugged in the first ceramic and took a measurement with the switch open and as expected, the 455khz resonator gave a nice readout of 455.68khz. About what you would expect.


Next we hit the switch, or in my case, pushed the bit of solder wire onto the groundplane with a pen and got the second reading. 448.12khz, now with these 2 frequencies in hand i could plug them into some software and get the values for Lm and Fo which are needed to design the filter.


With the two values i plugged them into Ladder Crystal Filter Design and got the values for Lm = 128.6mh and Fo = 447.2085khz which i then plugged into Dishal to design the filter as i like its interface that little bit better.


Immediately i saw a huge issue, the centre freq is all wrong, its way to low, if this was going into a homebrew rig, that would not be so much of an issue, just change the frequency of the LO. But, dropping this into my ICYAWOOD is not going to work, the IF there is right on 455khz, and so that is also where i need my centre frequency. Not sure what to do at this point. I do have some resonators that are closer to 458khz with their resonate frequency, they might have a better chance, but im thinking the low Q of these ceramics is the real issue and even if i come up a few khz, that is still not going to get me closer to 455khz. Back to the drawing board for now.


Determining Motional Parameters of Ceramic and Crystal Resonators

Ok, let me just say I am no expert on these things. But this has been my experience thus far. What got me on this bandwagon was the need for a CW filter for my IC-718. As you most likely already know this is a low spec, low cost radio and a narrow CW filter for this radio costs 50% of the value of the radio itself. In local money, that is close to $300 for the crystal filter from Icom, or you can buy Collins mechanical filters for about $230. Hardly cheap at all, and for a radio I intend to upgrade very soon, it would be a waste of money. So i decided to attempt to home brew a CW filter in the mean time. I mean what is the worst thing that can happen here, the filter is crap and i wasted 10 bucks and learned a whole lot in the mean time.

So before i started on this, i needed to know some parameters the filter uses in the radio. The IC-718 uses a 2nd IF of 455khz, with an input and output impedance of 1500 ohm. Ok, so we know the basics, now i went and scoured the net for as much information as i could find about how to go about homebrewing a filter.

Next on my list of things to do, what kind of filter am i going to be building. Ceramic ladder is very common in home brew rigs, especially in the 9mhz and 10mhz ranges where crystals are plentiful and cheap. But my radio uses an IF of 455khz, and while you can get crystals for this frequency, they cost $30 US each way too expensive for a home brew experiment, but, 455khz ceramic resonators are plentiful and cheap but are low Q and Q is everything when it comes to filter building. A typical crystal might have a Q of 10,000 to 100,000, where as a ceramic might be lucky to have a Q of 3000. So, things were looking iffy at this stage.

So i started to play with some computer simulation and filter design programs, Ladder Filter Programm “Dishal” and Ladder Crystal Filter Design (easy to find on google) and I start plugging in various bits of information to see if it is even possible to make a low Q filter from ceramics and it would seem that making something usable might actually happen, in theory at least.

With a pile of cheep Chinese ceramics in hand i began the task of finding 8 closely matched resonators. Now these things have an accuracy of + or -10% ir something nuts like that. First job was to find the resonate freq of each resonator. Out of 100 i did not even get a group of 8 that were within 100hz of each other. But you can see in the picture above that i did manage a couple of groups of 7 and a few of 4 and 5. Well, enough to make a couple of groups of 8 anyway. Why 8? well i intend to make an 8th order filter, the higher the order the steeper the sides of the filter is and the greater the attenuation.

Now to design a filter you need to know some of the motional parameters of the crystals or ceramics, namely you need Fs series frequency and Ls series capacitance. There are a number of ways to do this. I started with the above schematic as described HERE  and found no matter what i did, i could not get a good enough reading using my scope and signal generator to make any usable data.


Which leads me to where i am currently at with this project. The above schematic is an oscillator and buffer amp, and by using the values of capacitance in the circuit, you can determine the upper and lower frequency of the resonator and with those numbers can determine Fs and Ls for the resonators under test. I have not built this rig just yet, but plan to over the next few days to get to it, then measure my group of 8 resonators and average the Fs and Cs values and then use the averaged value to design my filter. I will update my blog once we have gotten to that stage. Anyway, failure is always an option 🙂