This card above is an interesting one. I did read somewhere about how it is sometimes polite to let the dx know you can hear them but with a report that is not workable. So while trying to work some DX on 15m I had JH3RGD coming back to my call, i was getting bits and pieces of his call and not much more. So i sent back SRI OM NO COPY UR 319 DE VK4FFAB and got an SWL report back, very nice indeed.
Yes sometimes things make no sense at all. Typically because you stuffed up in the first place. In this post haste modern world I got my Ying mixed up with my Yang and well things go bonkers when you do that.
So I am still trying to work out why my inductors are not right, but I have finally started to get some results that make sense and fit with that I am seeing with my measurements. So my dilemma has been my low pass filter designed for 7mhz had a center frequency closer to 20mhz, obviously something is wrong. I checked all the cap values as I was building it, they were all correct, I wound the inductors with the correct number of turns for the inductance needed, check them with an LCR meter and they seemed to corroborate. There were no solder bridge’s or cold joints. So the obvious place to look was the inductors.
So i knocked up this test jig, the inductor is in series with a known value resistor. V1 is a signal generator, the frequency is changed until the voltage at V_2 is exactly 50% of the voltage at V_1 and the frequency of the signal generator noted.
So I did this 2 times, the first time above using a 99.7R resistor, that was the measured value of the resistance used and the above was measured on the oscilloscope. The blue trace is the input voltage V_1 and the yellow trace the 50% measured voltage V_2. The frequency on the signal generator was 6.3mhz. This scope if not very accurate.
I repeated the process using 199R resistor and noted its frequency, the measured inductance was 4.3uH and 4.5uH certainly near enough to the method was working as it is meant to and well within the margin of error for this type of ball park measurement technique.
So with the frequency of the 50% voltage we can use this formula to find the inductance. L= R*sqrt(3)/(2*pi*f) which works out to be 4.3uH or near enough
So with my new found inductance value, I simulated the filter this time using 4.4uH for the inductance value mid way between my 2 measured values, and the simulation plot started to look a lot like what I was I was seeing with the bode plots i was making of the filter. So it looks like I have found my culprit. The actual inductance was lower than what would have been expected for the number of turns I had on the toriod. A few more turns and it should be right to go.
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.
This guy is keeping one eye on things, just to make sure I do things right.
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.
You realize the yellow inductors you bought off ebay are not iron cores but probably ferite because 40 turns is equal to + 70uH. Or when you realize your LCR meter is a giant pile of gimp shit and will not measure a value of inductance lower than 10uH. It has been one of THOSE days in homebrewing and so now I am making a test jig to use with the signal generator and oscilloscope to measure the value of the inductors I am winding with some level of accuracy and then getting confused about how accurate the measures will be because inductance changes with frequency and and all that. HIHI. The life of the unwitting, the unknowing and the down right confused.
EDIT: the yellow inductors are most likely type 26, yellow and white. The inductance values I am getting for a given number of turns would suggest this is the case. Type 26 is not much use in RF, good to 1mhz.
Band Pass Filters
If we refer back to part 1 of this project and look at the block diagram that outlines our project, the first block to evaluate is the band pass filter. Its purpose as the name suggests is to pass a band of frequencies while rejecting all others. In this case, i want the filter to pass the 40m band and everything else. A good question to ask at this point are what frequencies we want to pass, and what frequencies require special attention in rejection. Being that the overall design of this receiver is as a CW receiver to be mated with an existing CW transmitter, passing a 100khz range from 7.000Mhz through to 7.100Mhz would cover the part of the band in VK where most of the CW activity is. So, we now have our first design criterion, 100khz band width, 7.050 center frequency.
What about rejection, well the greater the attenuation the better of course, but there are 2 frequencies that special attention need to be given to. The first one is the IF frequency, in our case the IF is 12Mhz, so we need good attenuation there and the other is the mirror image of the mixing products. Our desired frequency is 7.000Mhz and our LO is 5.000Mhz, so 7 + 5 = 12, but also you have the image of that as well, so 7 – 5 = 2Mhz. Another good question is how greater attenuation is needed at those 2 points? Well, to be honest I am not all that certain at this point, but for argument sake lets through a number out there and see how we go, -50Dbm. So if we have a 0Dbm 7.000 mhz signal in the filter, both 2 and 12Mhz will be -50Dbm down or greater.
Now where did I get the -50Dbm number from, well its the legal requirement of transmitters to have its harmonic content -50Dbm down on the fundamental so if it is good enough for that, it seems like a good enough number to start with to keep the crap out for the specifications of a band pass filter. I might be wrong, I really dont know. We will see as the project progresses. One last thing to consider is insertion loss. All filter designs will have some loss in their design, there is nothing you can do about that its the nature of LC filters. And remembering that insertion loss is attenuation of our desired signals, the less we lose at the beginning, the less we need to make up for later on in the gain stages. -3db is a halving of our desired signal, so lets shoot for less than 2Dbm. It sounds like an achievable value without being overly critical on component tolerances.
Desired Filter Parameters:
Center Frequency: 7.050Mhz
LO and Image Attenuation: -50Dbm or greater
Insertion Loss: less than 2dbm
First up is a design i have used a couple of times in different projects. The topology is one we see often in internet designs. I have never simulated it in LT Spice before now, so i never knew how good or bad it was. I have tweaked the values a little, but the filter does fit all the design parameters. Bandwidth is 100khz, IF is -49Dbm down and the Image is greater than 100Dbm down.
Next up is the PA3AKE filter for which the design can be easily found and plenty of people have copied and used. You can buy parts kits for them and board even. It has an extra inductor over the previous design, but its levels of attenuation is far superior. Bandwidth of the design is for the full 40m band, I can narrow up the bandpass easily enough b changing some of the cap values. Both the IF and the Image are both over -70Dbm down. So that is a substantial improvement.
I include this filter as a bit of a novelty, you see similar on many direct conversion designs using NE602 mixers, hence the 1500 ohm termination, I have used it a few times in different designs myself, so it is about time to see how it actually performs. Well it was safe to assume this would never meet the design criterion, but it is better than expected.
Mostly the filters you find in schematics on the net kind of fit in with the above 3 designs. The only difference is the starting value taken for the inductors. The BITx uses the same topology as the PA2AKE filter above but starts with 6uH inductors and standard value capacitors for almost the same level of attenuation, as does the Universal RX project by Dave G4AON, who started with 13.8uH inductors.
There is also software like ELSIE and AADE which will spit out values for all sorts of filter designs. I have been playing with both and to be honest i cannot get a better looking filter than the better two above. And in the end I will probably use the same values used in the BITx, we all have 100, 470 and 1000pf caps in our parts bins and 6uH inductors are not all that burdensome to wind and it meets all design specs by some.
ADD BITx 40 simulation here:
In the next part we will build and test the bandpass filter to see if the simulation and the actualization meet somewhere close to each other.
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.
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.
So initially when i was thinking about building a receiver to mate up with the 10w CW transmitter project, I was thinking SimpleCeiver by Pete N6QW would be the go as a direct conversion receiver. I even did a bunch of work on laying out boards i was going to route up and populate. Now its all good and proper to copy someone else’s design, I mean that is why we publish build details and blogs and Pete’s project is awesome, but there is not a lot of design learning in monkey see monkey do. If you have not read all 20 something blog posts for the SimpleCeiver project, here is a link to the beginning. http://n6qw.blogspot.com.au/2015/09/moving-on-to-simple-ceiver-project.html
Rather than build a cut and paste copy of the SimpleCeiver, I am going to copy Pete’s design methodology of Simulate, Evaluate, Compare, Prototype and Modify, and roll my own design from the best designs i can find.
Simulate: Starting at the front end of the receiver, I will simulate each stage in turn using LT Spice.
Evaluate: Using the tools within LT Spice evaluate the performance of the circuit.
Compare: Using LT Spice, simulate a number of different circuit typologies for each stage selecting the best performing one.
Prototype: Build on perf board or Manhattan style on copper clad board a prototype and document its performance against the simulation.
Modify: Make modifications to the prototype if needed to improve its performance if the actualized circuit does not measure up to expectations.
Now we have the methodology sorted, lets set of some specifications for the design. There are number of limitations in the design based on things I already have in the parts box. This project is not about reinventing the wheel, it is about using what I have on hand and making the most of those things.
Design Limitation 1: The mixers will be Mini Circuits TUF-1 double balanced mixers. I have had these mixers in the parts box for a long time now doing nothing. This seems like the right project to pull them out for and put to good use.
Design Limitation 2: The LO and BFO functions will be fulfilled by a SI5351a driven by an Arduino Nano. Now there is a lot to be said by designing and building analog oscillators and for those that have the patience and the skill to make such things I really do take my hat off to you, its just not my thing. I am more inclined to use things that are easy to use and work well out the box. Also writing and modifying software is well within my skill set, and i find Arduino C as easy to use as others find making ceramic resonator VFO’s.
Design Limitation 3: 12Mhz IF I have a couple hundred of these crystals so i should be able to match up enough to make quite a few IF filters with these. At this point I am thinking 6th order, 600Hz wide Cohn Min Loss. But i might also get excited and categorize the motional parameters of the crystals and design another topology using Dishal.
Design Limitation 4: Use what I have in the parts box. I have a bunch of things I have collected over time that might have been used hear or there for learning, but never put to use in a project. I have some TDA1905 5w AF IC’s MC1350 IF Amps, NE5534 Op Amps as an example of parts I have in the box that really should get a run and used in a project. But, I could also go with popcorn variety circuits also and just use J310 or 2n3904’s
Above we have a block diagram of what I am envisioning at this moment. Nothing revolutionary, nothing extraordinary, just a very typical single conversion superhet design. So in the next part of this series of posts, we will start looking at band pass filter designs and simulations.
So a few weeks back I got making this nice looking lab power supply, but other than running my bench LED lights, its not been used. And thus begins my world of hurt, because the other day I had a project that needed some power and after connecting it up, things just were not doing what you would expect them to do.
A circuit that should give a nice sine wave looked a mess on the scope, a receiver I powered up had this god awful switch mode racket on just about everything. So get get the pixie wrangling gear out to take a look at what was going on and it was nice a pretty picture.
As you can see from the scope output, the 12v DC was not pretty and it was making my projects not happy. So i figure, It needs a filter and I do 30 seconds of math and figure 10 to 100uh of inductance with 100uf of capacitance should be close to enough to the business.
So I make a nice looking filter and connect it up to the power and yes it attenuated the noise and ripple but it really did nothing else, all the problems were still there.
When you know you are chasing your own tail, start talking to the smart people. In my case this is Brenton, and we got talking about all aspects of the design, what modules I used what SMPS i used and a bunch of other specifics.
Well it turns out, had a fundamental flaw in my PSU design, I had left the 0v DC floating and this is a trap for young players. Because in the words of someone smarter than me “Without that connection the output is magnetically isolated and noise leaks out via capacitive coupling in the output transformer.” So after checking that 0v DC was not tied to Mains Earth, they were quickly coupled together and all my problems went away.
And as you can see from the first scope output above, the power is now rather clean, and in the 2nd scope output, is the DC after my new filter. The filter will be installed permanently in the next few days and will remain a permanent part of my lab power supply. And with that fixed, we can get onto building a receiver now for the 10w CW transmitter.