So i got some more boards made in China. Board one has a PLL Synth DDS VFO, Tx Rx switching and an AF Amp. Board two is a 3 band Bandpass or Lowpass filter board and Board three is a shield, this boards are designed to be stacked together using double height headers with SMA patch leads joining blocks together.
Ok, so i could not get the variable gain to work as desired the last time I updated this. Now it worked, I was getting Oh i dont know -3db of gain attenuation, but that was not anywhere close to where I wanted to be, IE to have 0 to -20db attenuation. So I have given up the ghost on that idea and put it back in the sometime when i know more basket.
Ok, so I still need to be able to vary the gain of the RF Preamp, well, rather than vary the gain, lets attenuate the crap out of it. So after a lot of googling about trying to find solutions, i came across a circuit that is a variable attenuator. It should attenuate things by a minimum of -3 to -20db, so that’s in the ball park, its just a handful of resistors, caps and diodes, and by varying a supply voltage with a pot, you can change how much signal passes through the circuit.
Its simple enough, that even I can do it. About the only thing it requires are PIN Diodes, which if you are a popcorn kind of guy like me you wont have, but i have plenty of signal diodes, so off to LT Spice to build and evaluate a the circuit to see if it will do the job I require and then, if it works ok, build a prototype on the breadboard to make sure its going to work for reals.
UPDATE: Well I am thinking that LT Spice might not be the best tool for this job, by their nature diodes are non linear and a pin diode is linear and it is a non trivial matter to do linear artistry in LT Spice. The outputs i am seeing are quite wrong the higher the attenuation becomes as can be seen by the following image, however, when i ramp up the voltage, the output trace does track in input trace perfectly, so i think i might need to just breadboard this one and see for reals what is going on.
Will update again after I have breadboarded the circuit.
So i got it all built, for some reason the gain adjust is not working and I need to investigate that to see why but other than that, it seems to be working and the gain is about what was predicted in the simulation as can be seen by the scope output below. So I will fix the few bits that need fixing and then move onto working on the IF filter. I think this will do the job required quite nicely.
Oh and I have worked out whats going on with the variable gain, Q2 is a current source, so it supplies more current to the amp above, not less, so what I need to do is redesign the amp to have the MIN amount of gain required for my circuit, then use Q2 as a gain boost. Now that I understand what is going on, I can redesign the circuit to have 3 to 12db gain swing. More than enough for the kinds of girls I go out with.
So i have merged the separate simulations together into the one file now to get a better understanding of how they will work together.
And the simulation looks promising, they seem to be happy working together.
So now I have built the RF Preamp onto the perf board with the bandpass filter. The only testing I have done so far has been checking for continuaty as i go along to make sure A goes to B and does not short to ground hehe. I still have to add in the pot for setting the amount of gain in the amp and to wind the 200 ohm to 50 ohm transformer that goes on the output of Q1. Its actually nice to see this moving along, lets just hope it works. HIHI.
RF Pre Amp
So I am back feeling a little better and it is time to move this project alone a little. Some say that for a 40m band receiver an RF Preamp is overkill. In some places it probably is, but where I live most of my contacts are over 1000km away, are weak at the best of times so I think having a little gain ahead of the mixer is a good thing, but it needs to be variable.
My initial thought was to replace an emitter resistor with a potentiometer but that would mean having rf running though cables to the front panel and the chances of things going bonkers increases when you do things like that. Emitter degeneration was still going to be the way to do this, but by using a second transistor as current controller, control the current and control the gain and that can be done with voltage and a pot. So no RF being shipped about the place.
So now for the actual circuit, this was one given to me by Pete N6QW homebrewing legend and all round good guy. Its one of his GOTO circuits, nothing fancy just a BJT amp optimized for bandwidth and as an IF Amp. So I took that and modified it slightly to fit my needs as a 7mhz rf preamp. And like batman off to the batcave, we hit the simulator in LT Spice to confirm that things are close to spec and things will work well within our design parameters.
Ultimate gain is about 9db, and with 3v on the base of Q2 the gain is like 3db or there abouts. Should be good enough for what I require. Now I just have to build it and see if the simulation meets up with reality. Will post an update when I have it built.
Now i also should mention that I was going to compare and evaluate a number of designs for each stage. I did pull some designs out of different schematics on the web and put them through their paces, mostly they were crap. There were some that were better, but they were not popcorn enough for this project. A very notable mention is the front end of the simple ceiver with its dual gate mosfet made from a pair of J310. While i have J310 here in the crap box I am saving them for another project. If I can keep this one to BJT only i will be happy. Anyways, time to melt some solder and see if it works.
I was digging about in my pile of crap the other day and came across these IF filters i bought from MiniKits. 455khz IF with 2k -6db bandwidth. So seeing I have not been well for the last couple of weeks, I decided to knock up a schematic using these parts and a bunch of other bits i have sitting here, with the idea of eventually building it.
The audio stages are what I will use in the CW Superhet project, so it was not a total waste of time, same with the RF preamp. Anyway here is the schematic, its unbuilt and untested, so it might be a big pile of crap. You be the judge.
So i rewound the inductors, after lots of stuffing about I got them ball park.
Ran the filter up in the bode analyzer and well the filter part looks close enough, but it still needs more inductance, maybe another 2 turns per toriod to get the center frequency up to where I want it. Now, that second hump in the plot at 21mhz, I got no idea why that is there, when you look at the next plot where i inject 20mhz into the filter and look at the output on the scope, you will see it does not exist in reality, but shows in the bode analyzer.
Filter output at design frequency.
Frequency output where that hump is on the bode plot. Its obviously an aberration in the analyzer and for now I am not sure why its there.
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.
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, now that I have a working transmitter that I am happy with (See Here) and wish to actually use it, because it is not just popcorn fun, but actually very usable and of good quality, it is time to move on to the next issue and that is having a receiver to go with it. With that in mind, I have thought about what exactly I would like and came up with a small list of needs, modular was top of the list, so that I can modify and expand it over time as i develop greater understanding and skills.
So, as I have been listening a lot of late to the Solder Smoke pod cast, I became aware of a project by Pete Juliano that was created from the ground up with modularity and expandability in mind called the Simple-Ceiver, Here is a link to the actual project http://n6qw.blogspot.com.au/2015/09/moving-on-to-simple-ceiver-project.html, there are 20 odd blog entries to read chock full of information, the design decisions and the criterion used in the development of the board. I would recommend reading them all, Pete is not only a good home brewer but also a very good writer.
So in keeping with the idea of modularity, I have laid out my boards which I will route on the CNC Router with one stage per board. I have a pile of 100x65mm boards I bought off ebay for this very type of project, so they will be getting a workout now.
Now, something i also learned from reading the blog entries is that I would seem to be a really good idea to build your projects in reverse order, starting with the AF Amp and working towards the antenna. i can see the benefit in doing things this way, rather than my usual gun ho method of get out the flame thrower and start weldering up everything in sight 🙂 So in following Pete’s advice i will start with the AF Board and work my way forward. I will not be departing much from the published design, other than using a bandpass filter I have used before with success and to bring the only part missing from the original project and that is the Local Oscillator. I will upload the gerbers for each board as i build them to ensure they are working etc.
Here are by board layouts:
The AF Amp Board: nothing extraordinary here, 2n3904 preamp into an LM386, yes they sound like farts, but they do the job. I will likely use much less gain in the 386 than in the original schematic.
Dual Gate Mosfet Mixer: Now i actually like the simplicity of this circuit, yes I could use a NE602 but where is the fun in that. The 2 Jfets are J310’s which are cheap enough to get, though now they are not being manufactured anymore so they will soon become extinct, so stock up on them now while you can.
The bandpass filter is one i have used in a number of projects and performs well. Losses are low, out of band attenuation is good. I have modified it here for 50 ohms in and out.
Something I have not typically added in on any other DC receiver i have made is an RF Pre-Amp, the few times i have tried and built and added them into my designs have ended in dismal failure. Such is life when you are trying to shoe horn other peoples designs into things they are not really meant for. This design uses J310’s again to give 15db of gain before the bandpas filter. I am not sure why it is before the filter and not after, but this is something I will email Pete and find out the reasoning behind it.
This board is 150×100, and other than having the Arduino and SI5351 synth on it, it also has the TX and RX switching on it, and yes, if you look you will see a mistake on the layout, that I have just now fixed. HIHI. Anyway that is all from me for now, tomorrow I will start on making the boards and slowly building up this receiver project.