CW RX Update: Band Pass Filter

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.

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CW Superhet Receiver Project PART-2

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:

Bandwidth: 100Khz

Center Frequency: 7.050Mhz

LO and Image Attenuation: -50Dbm or greater

Insertion Loss: less than 2dbm

 

Evaluation:

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.

Links:

http://vk4ffab.info/2017/01/07/cw-superhet-receiver-project-part-1/

https://martein.home.xs4all.nl/pa3ake/hmode/bpf_all.html

http://www.qsl.net/g4aon/g4aon_rx/

http://www.hfsigs.com/bitx40v3_circuit.html

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CW Superhet Receiver Project PART-1

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.

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Simple-Ceiver 40m DC Receiver

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:

sc-afamp

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.

sc-rf-jfet-mixer

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.

sc-bandpass

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.

sc-rf-preamp

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.

sc-lo-txswitch

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.

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Black Box Station

Not all black box operators have a cornflakes packet understanding of electronics, some like myself also have a rice crispy understanding HIHI. Anyway, this is my black box operators station, it consists of 3 black boxes, 1 an unpowered speaker box, the 2nd a direct conversion receiver for 40m and the 3rd a 40m 5w cw transmitter. The transmitter and tx rx switching are a work in progress, but the rest works well.

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40m DC RX Continued

So i finished of wiring the receiver up and putting power and an antenna on it and having a tune about. Well it works, I was receiving the Manly Warringah radio club doing an open night for the cubs, where the cubs were sending their names back and forth using the phonetic alphabet. The kids were having fun and enjoying the radio. Also, a number of Japan and Indonesian stations were copied as well as some VK3 and 5 stations on CW. All in all, I am happy with the radio other than the finicky tuning system.

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This is how i had the receiver setup on the bench for its initial test.

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Did not plan the internal layout all that smartly, but it did fit in there eventually.

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And it looks all pretty with its nobs on.

40m DC Rx

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40m DC Rx

So today we start on the next project in the series, a 40m direct conversion receiver to mate up with the 40m cw transmitter i built previously. I will update this post as I make progress.

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What we are starting with.

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Diodes in.

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Electrolytic capacitors in, kind of an upside down way of kit building, but i am looking to add in some landmarks because of the lack of silk screen on this kit.

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Without a silk screen it is a good idea to lay the components out on the board layout I think. HIHIimg_20161010_165811

Capacitors all in.

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There was some resistance, but i overcome it in the end.

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Most things now on the board. Next job is to wind the toriods and add wires where the power and speaker sockets go.

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40m DC Receiver

DC-rx

Among the many projects that i have neglected lately, the direct conversion receiver has probably seen the least amount of love. So feeling energetic today I gave the board layout a little love, moved somethings about and added in a few others. The main change has been the addition of some Rx Tx switching. I have a single pole single throw relay in my box of tricks, i do not know where it came from, but i thought i would use it to switch the incoming signal on and off. Not really eloquent, but it will do the job and I wont have to round RF from the back panel to the front. Other than that, reverse polarity protection diode in the 12v input like and a little tweaking of the band pass filter. After i give this all the twice over, it will be export gerbers, fire up the CNC and route a new board for this and get it boxed up.

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40m Band pass Filter

So i came across this schematic on Ebay for a 40m dc receiver kit, the front end looked nice so i copied it for my own project.

Untitled

I am assuming that the band pass is transforming impedance also 50 ohm in and 1500 ohm out into the 612 mixer. I built mine for 50 in and out, by replacing the 270 and 18pf caps with 220 and 56pf.

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Using my scope and signal gen i tuned the filter for max amplitude on 7.100mhz, and then did a sweep of the filter from 5 to 9 mhz and plotted its response on the graph above. Looked better than I expected.

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Here is the filter as built on the board. Followed by a mess of failures where I was trying to add in a preamp that just kept loading up and not doing anything. Such is life.

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Direct Conversion Receiver with DDS VFO

So not to be outdone with just one project on the go, I also get it in my head to rebuild the 2-IC DC Receiver from page 1.8 of EMRFD. I have built this simple receiver before, it was one of my first projects ever and it worked first time out of the box, well kind of.

My original was built Manhattan style, but at the time, being rather newbie, i was not able to make the LC oscillator circuit with sufficient Q to make the thing work. It was yanked out and initially replaced with an xtal, making it rock bound and of limited use, then the xtal was removed and replaced with an Arduino DDS VFO and my love of DDS was born LOL.

My first project that gave excellent results, I was receiving stations from all across Australia, New Zealand, Japan and the USA. It was thrilling. I was now hooked on homebrew.

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Above is the schematic of the circuit, i have changed out the local oscillator for a DDS VFO, and i have made a couple of minor changed to the audio amp, mainly double the gain it will now put out maximum gain, the only issue no gain control on the audio amp.

 

dc-rx

Board was laid out in Sprint Layout, with extra holes drilled for mounting the DDS board onto it.

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Its a nice simple project, wont take long to solder it up and get it on the air and tuning the 40m band.

UPDATE:

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So it is all together and it works ok. I did have to change a few things, mainly making the audio single ended rather than differential, this gave much more audio at the speaker. Also the tank, while it functions, does not seem to do all that much, tuning it by the trimmer, yields no change what so ever. Its something i will need to look at.

While its working, it could do with some improvements, especially to the front end. Here is a video showing it working.

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