One Small Step For Ham Kind

I have built projects before with SMD components on them, but this is the first time that I have included SMD components on my own projects. Yeah i know, its nothing extraordinary, its a 5V regulator, but we all gotta start somewhere. Which got me to thinking, perhaps I should do more with larger SMD components and be able to fit more on a board. We will see LOL 🙂


Oh and for anyone who is interested, this is a respin of my band pass filter board using these variable inductors i got. Anyway, all i have done at the moment is build one filter for testing. Testing which still needs to be done, but I will get to that soon enough and post up some results.

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IF Amps and Mixers

So I am sitting here this morning drinking a massive cup of espresso coffee and thought i really should post up where I am at with the receiver project. Its getting closer now to final testing and making noise. In fact, the final board will arrive today from china and as long as it does what its meant to, i might only be days away from hearing beeps

So anyway, onto the IF amps and mixers. I find the whole amp design, input and output impedance matching and how one can effect the other to be slightly confusing. So I went with an IF amp design that eliminated that confusion for me, the TIA amps. Termination Insensitive Amplifier by Wes Hayward

The mixers will be ADE-1 Mini Circuits jobs as I have a bunch of them here doing nothing and figured why not. I do have a board here I am designing with a number of different mixer topologies on it that I will eventually have made and test out homebrew mixer designs more fully, but for now I will use the commercial product.

I built the 2nd IF amp first, so I can have a direct conversion receiver happening in short order once I have a working audio board.

Well it works as designed and does the business obviously. There is a missing trace on the board, so I will eventually have to respin them and I want to fix the layout somewhat also and change the transistor footprints to wide not narrow. I do not like those small pads to solder to. LOL

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Switchable Crystal Filters

I am going to redesign this board. The layout is sub-optimal. But for now, I am going to assemble it and see if I can make 2 filters worthy of use.

The one thing that has always frustrated me with many SSB/CW homebrew rigs is that you get one filter and one filter only. I have seen some use varactor diodes to solve this problem, by varying a voltage on the diodes you can vary the capacitance and this the width of the filter. I thought about going down that route myself for a while, but in the end, settled on having 2 filters as I have lots of relays already.

The plan is for an SSB filter that is about 2200hz wide and a CW filter about 500hz wide. I am also using the good crystals for these also. 4.91520Mhz xtals sorted to within 10hz of each other.

Each crystal was loaded up into my you beaute xtal tester and the frequency measured to within an inch of its life.

My function gen also has a frequency counter that counts down to 1hz resolution. So I measured my crystals and sorted them into piles within 10hz of each other. If i cannot make acceptable filters with such tightly grouped crystals, then there is no hope in hell i can ever make a good filter ;

Out of this pile of crystals I have 6 groups of 8 crystals all within 10hz of each other and a few more groups all within 20hz of each other. Buying these crystals was certainly well worth it. The pile in the bottom right corner are the outliers. It seems even good quality crystals have some in each batch that should be rejected.

Got the parts all sorted, just gotta build it now. Will update this post once i have built and tested each filter.

Part 2: Well i kind of expected that to not take forever, but anyway here is the update.

The build is pretty straightforward and all went to plan

Other than the width being a little wide in both cases and a bit to much ripple, I will say that I am happy with the shape of both filters. I can narrow both up easy enough, just need to change the cap values and the ripple, well it might be a bit of work, I will need to play with transformer values. I used 4:1 impedance transform, but 3:1 might actually be closer to the mark. I will have to play there and see what i can come up with, but a better match will remove some of that ripple.

For now though, I can keep on building the rest of the project as these will be acceptable to get things working. I can make them perfect later. Next job is IF Amps and mixers.

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Radio Bebek Xtal Filter

Radio Bebek Xtal Filter

I thought before i added in the IF Amps i would build the xtal filter first and see how it looked. The values of 33pf seemed very small and would end up with quite a wide filter. Typical values for a 2K wide filter are usually around the 100pf mark, so i started there.

Obviously i was wrong, but i am thinking 50pf might be closer to the mark after looking at the bode plot. Its not going to break any performance records and the skirts are kind of garbage for a 5 xtal filter. But, I will see how it goes on receive once i have the rest of the radio built.

Radio Bebek Xtal Filter Bode Plot

As far as xtal filters go, this is one of the better looking ones I have built HEHE, so I guess the others have been really crap. It is kind of symmetrical looking and only 3db-ish of ripple. Might be ok. We will see.

2200hz IF Filter

Ok, so after playing with cap values i got the filter about the right width, but there is a lot of variance in the passband. 8db to be exact. Way to much. But as there is no way to impedance match this, there is not a lot i can do. It is what it is as they say and as I am only building this as a receiver, I can live with it.

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Receiver RF Front End

VK4FFAB Receiver Front End

Ok, I am posting this mostly because I can, but I wont publish the gerbers yet because I actually want to layout the board better, fix some footprints and have the boards remade. Not because they are bad or do not work, but because I know i can make them so much better

So this it my receiver front end. RF comes in from the antenna, through the TX-RX relay into a 0-31dB attenuator module, into a 5 to 15dB RF Preamp.

The RF amp is a pair of J310 as a dual gate mosfet as designed by Pete Juliano here: SimpleCeiver

Variable RF Preamp Schematic
Prototype Built and Tested
PCB as originally laid out.

I might add some more info here later, or will just post anew when i have redesigned the board. The main thing i want to do is to remove those wire links joining the attenuator to the rest of the circuit and replace that with some right angled SMA connectors as the attenuator board comes with SMA connectors on it already. I will probably change the relay also as I now have some better RF relays, not just chinesium grade floor sweepings.

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4 Band Bandpass Filter Board

The relays are double pole double throw in a standard 18x9mm package size. The transistors are anything you have in CBE footprint, I used 2n3904 and the resistor is there for current limiting, just pick a typical value, i think i used 4.8K. The trimmer caps I use are 3 leg 100pf max value to tune and align each filter.

SMA edge connectors bring RF in on the left side of the board. By default all relays are in the OFF position and no RF can pass through. The 6 pin header brings control signals and power to the board. To turn a relay on, a 5v control control signal is placed on one of the first 4 pins to turn 1 pair of relays on. The last 2 pins are ground and VCC of the relays you used.

Aligning each filter can be a bit tricky, it is best to use a spectrum analyzer and tracking generator if you have one, or a noise source and spectrum analyzer or even in a pinch if you slow down the seconds/division on the oscilloscope so you can see the filter shape. The reason being, if your filters are not quite tuned right, you will end up with 2 peaks and some attenuation in the bandpass you just do not want.

The filters themselves are Double Tuned circuits. To design the values needed for each band I used some software called DTC.exe which can be found on a CD in the book Experimental Methods In RF Design. I have linked it here for download, but I am not sure if I am allowed to and i will pull it down if asked by the authors: DTC DOWNLOAD

The gerber files for the board can be downloaded from here: Bandpass Filters

Here is a schematic of the filters with values for 80, 40, 20 and 15m.


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4 Band Lowpass Filter Board

Board Design 

Ok, in the spirit of sharing and making things easy for people, I am going to start releasing some gerbers for the various boards I have designed for my various projects, that work HAHAHA, because, yes, not everything i do works.

I am not into this for making money, I do everything that i do simply for the love of it and for the learning experience. So selling boards and things is not high on my agenda. And besides, you can get 10 boards made and send to you from JLCpcb in China for not much more than it would cost me to send you one from Australia.


First cab off the ranks 7th order low pass filter boards. So you will notice there are no part values on the boards. Why? Well because i do not know what bands you might like to build for. The relays are double pole double throw in a standard 18x9mm package size. The transistors are anything you have in CBE footprint, I used 2n3904 and the resistor is there for current limiting, just pick a typical value, i think i used 4.8K.

SMA edge connectors bring RF in on the left side of the board. By default all relays are in the OFF position and no RF can pass through. The 6 pin header brings control signals and power to the board. To turn a relay on, a 5v control control signal is placed on one of the first 4 pins to turn 1 pair of relays on. The last 2 pins are ground and VCC of the relays you used.

For designing the filters I use an online calculator like this one at Calculators Edge: http://www.calculatoredge.com/electronics/bw%20pi%20low%20pass.htm Or a software tool like Elsie.

Lowpass Filter Board

Gerber Files can be downloaded from HERE:  http://robs-blog.net/Files/LowPass_Filter.zip


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Control Board

This is the 2nd time that i have rolled my own control board using the SI5351A. The first one was much simpler than this and had a few other bits on the board. This time around I have gone to town on features that i will incorporate into the firmware. This is why i have used the Mega, i am using just about all the IO pins on offer. 38 pins are being used for the TFT LCD display. I would have preferred to use an I2C or SPI display, or perhaps even a nextion, but they are either 2 times the price or in the case of the nextion, i would have to learn how to program it first.

I have a total of 8 push buttons or switch control lines, 5 are currently going to be used for things like band change, mode change, tx/rx switching etc. The bandpass and lowpass filters take up another 4 lines each. Yes, this is a 4 band receiver/transceiver. The IF filters will also be switchable between 2 filters, one for SSB and the other a narrow CW filter.

7 of the analog pins will be used to control an attenuator that will give the radio between 0 and -31db of attenuation and finally, we have a rotary encoder for the VFO a keyboard and real time clock. I plan to incorporate my arduino logging software into the radio to have everything in the one box.

The plan at the moment is to have 80, 40, 20 and 15m SSB and CW receiver with a CW transmitter for the same bands, with the option of using the same boards to make a SSB/CW version at a latter date.

Boards were designed and laid out by me and manufactured in China at JLCPCB This is the 2nd time i have used them for prototype boards. The first order was great, nothing wrong with the boards at all. This last lot have a lot of pads with solder mask covering parts of the pads. Nothing that cannot be fixed with a little sandpaper, but very annoying. I think the issue was due to them being rushed because of a public holiday in China. I will give them another go and if the same thing happens I will go back to using PCBWay.

I have been planing ahead for this project for sometime. I had ordered these 40pin ribbon cables on the understanding that it was row compatible for use on LCD displays. Its not, it reverses inside row to outside somehow. Anyway I got Ebay China’d on that part LOL. So I had to make one using using a bit of superglue and some female to female dupont jumpers. Its rather stiff, but will work till a new part arrives.

The new desoldering vacuum made light work of removing the female headers off the Mega, being replaced with pin headers.

 

Board layout is a skill in and of itself. Its nothing easy and something that i struggle with. But i am getting better everytime i do a board. Above you can see i added parts to the other side of the board. LOL this is a first for me and I did not stuff it up. The 5v regulator is orientated the right way. 🙂

The fun part is about to start. And that is developing the firmware to make this thing do what it is meant to do. And no, i will not be sharing my firmware. The main reason is because I do not want to have the issue of supporting it or helping people who cannot get it to work. Most of the code is already on the net. There are a dozen VFO codes already on the web, the Arduino Logger code is on my site, things like push buttons etc are easy enough to do, so anyone who is keen enough can very easily integrate these things together and make a functioning vfo, logging and rig control board.

Here is the PCB layout as it currently stands. All the connectors are 2.54mm JST-XH connectors. The SI5351A is from QRP LABS   I know you can get these also off ebay and simular places and the footprint is probably a little more friendly, but I choose to support QRP LABS when ever i can. They are a good grass roots company that has never let me down.

Gerber Files: Control_Board Use these at your own risk. At the time of posting the board was not 100% checked to see if all functions worked as designed or the layout to be suitable for any particular task. By downloading these files, you agree that you will only use them for your own personal projects and not to sell bare boards, kits or to make a commercial product with them. I call this, the don’t be an arsehole license. If you cannot agree to not be an arsehole, do not download my files.Facebooktwittergoogle_plusredditpinterestlinkedin

Rob’s Receiver Part 3: IF Amp and Xtal Filter

First IF amp and the Xtal filter is build. IF amp is perfect and works exactly as designed. The Xtal filter is a bit rough as you can see from the plots below. It needs to be a bit narrower, closer to 500hz would be nice. Its a bit ripply which is probably due to imperfect terminations and because the xtals are not very well matched, being more like + or – 300hz not 50hz. I have a better frequency counter and a couple hundred more xtals coming slow boat from China, so I should be able to improve things quite a lot when I can measure down to 10’s of hz.

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Rob’s Receiver Part 2: Band Pass Filter

Bandpass filter as designed in Elsie. In practice was a little low in frequency when built. I changed the end caps to to make them a little lower to bring the frequency up. The bandpass is rather ripply and if i remember latter I will Bode Plot it on the red pitaya.

Here is the schematic. It works but does not have the smoothest bandpass. I will see how it goes for now and if its not good enough, redesign it.

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