Building Farhans Minima Transceiver Part #1

Introduction

I have started my most ambitious electronics project so far. After building my Softrock RXTX I was eager to learn more about RF designs, and HF radios in particular. I came across Farhan, VU2ESE, via Soldiersmoke and decided to build a transceiver based on his Minima.

There are three published versions, and many variations in between that are produced by others. All versions are based on a Si570 PLL Local oscillator and a discrete component BFO.

The first version uses mostly discrete components, KISS-mixer and discrete component audio amplifier, The IF is 20 MHz and it switches between two filters to cover the entire HF band.

The second version uses a FST3253 mixer and a TDA2822 audio amplifier

The third version uses a standard diode ring mixer and a TDA2822 audio amplifier and an IF of 24MHz. I have seen a few versions of the first Minima verison around the web, but I have not seen any of the other two. 

Building my own version

I plan to base my design on the third vesion. It seems to be the simplest of the three, although it does not cover the whole HF spectrum. Since the IF is 24 MHz and it uses only one LPF it is limited to the frequencies below 24 MHz.

To make things a bit more interesting, I have planned to make a few alterations to the Minima 3. First, I will use a Si5351 as the local oscillator instead of the Si570. I used the Si570 in the Softrock RXTX so this gives me the opportunity to try something new. In addition, it contains three oscillators so one can serve as the LO while the other can serve as the BFO. N6QW has advocated for this several times, so why not give it a try.

Further, I will use ADE-1 double balanced mixers for both mixing stages. I will also use a standard LM386 as audio amplifier as I have many of those in the junk box.

Display and front panel

The Minima design and the software from Farhan is based on a 16x2 display.

I have used the standard 16x2 and 20x4 LCD displays in several projects previously: My 3D-printer, my timelapse device, a Shruthi-1 synth, a LXR drum machine, and more recently, my SWR-meter all use this display technology. This time I want to try something different, something more in the spirit of homebrew oldschool radios, and something more difficult. We are talking, Nixie tubes, Dekatrons, tuning eyes and neon indicators. No freakin LCD display or even a single LEDs on this rig.

My initial front panel is made in Front panel express. I have printed a test version on paper and glued it to a piece of cardboard. The purpose is to verify that all the bits and pieces fits to the panel before I order the final panel from Schaeffer.
In the above picture, the eight nixie tubes, one dekatron and one tuning eye is mounted temporary.

I think it will look great when the front panel is all finished. For the time being, we have to use our imagination, but nothing beats the warm glow from nixie tubes and dekatrons. The nixie-board from the above picture is one of my previous creations.

Audio

Moving to the radio itself, I started out building a LM386 audio amplifier. Homebrew hero N6QW always recommends to get the audio part going first, so here we go. The circuit has only 20dB gain at the moment, and I will have to increase the gain to at least 40dB later on, but it is enough to get started.

The sound from the little chip is not bad, and I hooked it up to my iPhone for testing. The speaker is a simple 8 ohm 8cm speaker from Tayda.com.

Low pass filter

The next stage I built was the low pass filter.

As there are no build manuals for homebrew projects, I decided to simulate some of the circuit modules. This is simple in LTSpice and gives valuable insight.

The simulated frequency response seem reasonable for a sub 24MHz receiver.

I used 0805 surface mount capacitors for the LP filter. I made some room on the PCB for a second LP or BP filter, should I have the urge to go above 24 MHz in the future. I hooked up the ADE-1 to the Si5351 via a 6dB pad. The output is terminated to a 51 Ohm resistor for testing. I am not sure whether the 6dB pad is necessary, since it is possible to adjust the Si5351 output power to drive the required 7dBm to drive ADE-1 from the Arduino software.

Future work

There are many parallel projects going on in this build, there are dekatrons and neon tubes that should glow, there are front panels and mechanics to resolve, and there are amplifiers to solder, and crystals to characterize. Hence, I did not want to collect everything in one big and totally confusing posting at the end (will it ever end?). The next stage is to finish the IF amplifiers and the crystal filters. I hope that others will take on building the third version of the Minima as well. If you are one of them, I would like to know. Stay tuned.

WSJT-X and HDSDR with Softrock RXTX

Introduction - A Raspberry failure

My first (and, as it turns out, rather ambitious) plan was to use a Raspberry Pi 2 as SDR computer for my SoftRock, with Quisk, Fldigi and WSJT-X. The plan was to use VNC to connect to the Raspberry, so I could operate digital modes from all my devices; iPad, iPhone, Mac and so on.

I made all of the above almost work. Quisk was easy to get going, but I struggled to compile WSJT-X. However, after a very time consuming upgrade from Wheezy to Jesse, I managed to make it run. But due to a conflict between Qt (the GUI framework in WSJT-X) and TightVNC, I could not make it run without a display connected. Bummer.

Second bummer: I had a hard time getting my Behringer UCA-202 sound card work correctly,  It seems like that the left channel is delayed by exactly one sample relative the right channel on this card. Reading this, I gave up, and ended up purchasing a Asus Xonar U7. I made the Asus work with the Softrock RXTX, but only on 192KHz, which seemed to be to CPU intensive for the Raspberry. The sound quality was really poor. 

All in all, I gave up on the Raspberry (for now), as I was eager to get the Softrock on the air and make some QSOs.

Softrock RXTX and Asus Xonar U7 in a "Pelicase". Plenty of room for Power supply, LP-filter and SWR meter.

Going all in for Windows

Being a Mac and Linux user for many years, I finally took the plunge and purchased a Windows 7 based Lenovo machine. It was a bit painful, but I came over it.

I now use HDSDR as the SDR program communicating with the Softrock RXTX with I/Q using the Asus Zonar U7. I use 48KHz 24bit at the moment (both directions) although the Asus can go all the way to 192KHz. I do not see the wide spectrum given by 192KHz being all that necessary for digimodes, but is certainly very handy to view the band in a glance panadapter-vise if you are searching for SSB ragchewers or whatever.

To connect HDSDR to digital-mode programs I purchased "Virtual Audio Cable". It was dead simple to set up, and worked with WSJT-X straight away. I also use com0com as a virtual serial port to let WSJT-X enable TX-mode on the Softrock through HDSDR. I have not made full CAT-control working yet, so I have to dial the tuning frequency on both WSJT-X and HDSDR.

I do not know anything about logging QSOs online or shipping QSL cards, but it has came to my understanding that logging as such is important and after a quick search I installed DXKeeper. It is free and it seems to work.

DXKeeper interoperates seamlessly with JTAlert which is a must-have add-on for WSJT-X.

To make all the above interoperate, the applications have to be started in the correct order. Virtual Audio Cable and com0com seems to always work, and can be set and forget. To make HDSDR, WSJT-X, JTAlert and DXKeeper play nicely, I created a small script:

echo off

echo "Starting ham radio software"

echo "Starting DXKeeper"

start C:\DXLab\DXKeeper\DXKeeper.exe

timeout 20

echo "Starting HDSDR"

start "" "C:\Program Files (x86)\HDSDR\HDSDR.exe"

timeout 10

echo "Starting WSJT-X"

start C:\WSJT\wsjtx\bin\wsjtx.exe

timeout 15

echo "Starting JTAlertX"

"C:\Program Files (x86)\HamApps\JTAlert\JTAlert.exe" /wsjtx

I saved it as a *.bat on my desktop. By clicking the bat-file the applications starts in the correct order, and ensures that JTAlert can talk to the DXKeeper database.

HDSDR, WSJT-X and JTAlert. The radio was absent when the picture was taken.

First QSO

With all the above running on my fresh Windows computer I threw out a simple dipole antenna and tried to make some QSOs with JT9 and JT65. M0LMI, Lubomir, from England, answered one the very first CQs sent by my Softrock and minutes later I had made my first QSO! I checked the power with my scope later and it turns out I was only running about 0.5 Watts. After cranking up the power to 1 Watt I made a few more QSOs, and I have to say I am happy with the setup. I have only been running on 20m so far.

JT9 and JT65 may be an easy mode compared to CW, but with limited power and a compromise antenna (I have no idea about the SWR and resonance and all that), you sure have to be patient.

Future work

I still have not given up on Raspberry Pi2, but before working on that, I will build a low-pass filter so I can operate on 30m. I think I also need a decent power meter so I can have an idea on what is going on. It is probably going to be a power meter DIYcrap style.

Softrock RXTX completed

After finishing up the receiver part of my Softrock Ensemble RXTX SDR transceiver with great success, I got really motivated to get the transmitter going. The last weeks I have followed AB4UG's progress building his own RXTX with great interest, and I got even more motivated to finish my own kit when I saw that he got the last component soldered on his RXTX.

Soldering the last component on the RXTX

Building the TX part was fairly easy after having built the RX part. The first thing after melting the last piece of solder on the board was to make a 50 ohm dummy load using a few resistors, and then to measure the output power with my scope. After that, I balanced the I/Q signals by playing with the TX image rejection settings in WSPR and by watching the transmitted signal on a RTL-SDR receiver. The RTL-SDR operated in a software direct sampling mode to make it work on the 20m band. This setup is not the best tool for this calibration task, but I think I came pretty close in my effort. I might check it up more thoroughly use a spectrum analyzer later.

RXTX is running WSPR

WSPR is an excellent choice for testing out a freshly built transceiver since it gives immediate feedback through Internet if anyone out there receives my signals. After a bit of fiddling, I got the RX mode running directly on I/Q in WSPR, and prayed to the radio gods that the TX would work as well. After verifying that the transmitter did "something" when connected to WSPR (it went hot), I left the softrock running for 24h with about 1W output. Then I just hoped that the black suited government frequency authorities would not kick down my door to revoke my amateur licence due to RF harmonics or for causing QRM on the image frequency.

Both RX and TX is now working in I/Q mode

Luckily the frequency authorities have not been kicking down my door (yet). On the contrary, during one night of operation I have gotten WSPR reports that my signals have reached most of Europe and even across the pond to America!

I can enjoy my working transceiver from my iPhone

I am truly amazed that my 1W transmitter (which I bravely soldered myself) can reach more than 7000 km. Notice that my "antenna" is just an indoor wire dipole (at about 5m length).

If you are listening on 20m and observe that my Softrock emits energy in the wrong parts of the frequency spectrum, please be kind to me. Please. I am a fresh amateur with very limited RF self esteem.

Summary of the build experience

I followed the excellent build guide of WB5RVZ step by step and it was really helpful. The most challenging part of the build was in fact soldering the through hole components on the awfully small soldering pads on the PCB. The SMT parts were mostly SOICs and 1205, which were easy. The Si570 (QFN) were the most challenging. I measured every resistor before soldering and took great care not to make any mistakes along the way. All in all, it was really an enjoyable build and the entire process building the transceiver took about 10-12 hours in total. The only mistake I did was to solder an opamp in the wrong orientation, but that was easy to fix.

Future work

The next step is to test out other WSJT digital modes such as JT65 and JT9 and to make some real QSOs. The goal is to make the Softrock operate stand alone on my Raspberry Pi2. 

Building a SoftRock Ensemble RXTX SDR

Introduction

The last couple of months I have been listening a lot to the soldersmoke podcast. In case you do not know, it is two guys, Bill N2CQR and Pete N6QW babbling about building homebrew radios. I am very happy I found this podcast. It is highly inspiring, and by listening to the two experienced homebrewers I immediately wanted to melt some solder and build radios myself. Hence, I had to get myself a licence (got it last week: LB0MG) and start soldering radios.

As much as  I would like to be in the possession of a totally homebrew fundamentalist discrete component ham radio station (like Bill N2CQR), I concluded that I rather should start out with a kit. (I guess some homebrew fundamentalists threw up a bit now since I mentioned the word "kit").

Building the Softrock RXTX

Anyway, choosing a kit I might as well do it the modern way and build a SDR. I initially considered the PeaBerry SDR, but ended up with the Softrock Ensemble RXTX SDR Transceiver kit since I have the required sound card capabilities laying around after my previous synth projects.

The kit is nicely packaged and contains about 250 parts. Some through hole and some SMT.
I am pretty sure I will not do much contesting in any perceivable future so I choose to build the Softrock for the 17m, 20m, and 30m bands (two contest free bands).

There is an excellent build manual on the pages of WB5RVZ.  I addition I have had great use of some of the videos from W2AEW to understand the concepts of SDRs and to improve my toroid winding technique. He has some of the best engineering videos on the entire Youtube.

Progress

So far I have built the entire RX part of the kit and it has come along quite nicely. The SMT parts are rather easy to solder. Surprisingly enough, the through hole parts are a bit more challenging since there are very little space around the components, and the pads are really small and not gold plated. A lot of flux really helps.

The only mistake I did was that i lost the LT6231 SMT RX opamp (probably in the vacuum cleaner). Straight away I ordered two new chips from DigiKey and when they arrived at my door merely 36 hours later, I was so eager to get the RX working, that I accidentally soldered the small SMT chip in the wrong orientation. Hence I had to use the second one (glad I ordered two of them). But it worked! I got a small Image rejection problem, but traced it down to a setting in the sound card I used (Behringer UCA-202).

In the above picture you can notice the excellent indoor dipole "antenna" connected to the Softrock. The length of the dipole is about 5m, so It is not the best choice for receiving either of the bands. I guess some you antenna fundamentalists threw up watching this picture, but hey, the antenna sort of works. 

To please the homebrew fundamentalists out there, I used my 22 year old homebrew LM317 bench power supply to provide clean and pure 12V DC.

I connected the receiver to HDSDR, grabbing the I/Q signals. I have received SSB, RTTY, WSPR and JT65 on 17, 20, 30 and 40m bands. In the above picture you can see some digital signals popping in at 20m. I tried to use WSPR software directly on the I/Q signals from the radio but with little success. I had to output USB audio from HDSDR via a second sound card and then into WSPR.

From my perspective the RX test was a great success. Even with the indoor "antenna" I received WSPR on 30m from 60 different transmitters over a 24h period, and even across the pond. You might see LB0MG in the above map: Yeah, that's my Softrock sniffing RF from the ether, placed in an attic in Norway. 

I even got reports coming in from K1JT (yes, the Nobel laureate, Joe Taylor, that created WSPR) himself and got a bit starstruck. It is funny to think about the fact that he was transmitting his modest 5 Watts from New Jersey, and that my home soldered Softrock received his RF vibrations all 6000km away, even with a crappy piece of wire as antenna. The radio gods are undoubtedly on my side on this one.  

Now, I am eager to generate some RF myself and will get on finishing the TX part.

Receiving JT65 with RTL-SDR

A couple of weeks ago I bought a RTL-SDR (software defined radio receiver) dongle with the 820T2 tuner chip. You just cant beat the fun/price ratio on this one. For 11$ (with free shipping) it is a bargain, and it is a great way to learn about radios (at least the reception part).

A bargain on AliExpress

Initially I used Gqrx on my Mac to communicate with the RTL-SDR, but eventually I had to resort to SDR# on Windows 7, as it has a bit more features. I have the impression that most radio (and ham-stuff) is made for Windows rather than Mac or Linux.

Apparently not much activity on 10m, but JT65 lurks there in the noise

Even with the standard antenna I had no problem receiving FM broadcast between 88-108 MHz, DAB around 200 MHz, some public transport transmitting FM on 151 MHz and some sporadic narrow FM around 140 MHz. I could also see that there was a lot of activity on 434 MHz and 868MHz bands from car keys, weather transmitters, baby calls and so on.

A few JT65 signals in the WSJT-X waterfall

However, what I really wanted to receive was amateur radio digital modes JT65 and PSK31 on the HF bands (preferably on 10m and 20m). I made a half-wave dipole for the 10m band (28MHz), and soldered the coax directly on the board. The RTL-SDR seem very noisy on HF and all I got initially was a weak RTTY signal on 10 m (which I was unable to decode), and a brief SSB-reception. I guess that one reason for the bad performance is that the strong FM broadcast signals on 88-108MHz interferes with the lower frequency bands.

Receiving JT65 in WSJT-X

However, after a bit of tweaking with the audio levels in SDR# and WSJT-X, I was able to receive some JT65 messages. It was incredibly fun to receive signals from Spain, Russia, South Africa and Brazil (among others) on this 11$ stick and my home-made dipole antenna. (Now I finally understand what the amateur radio people are so intrigued about.)

Received JT65 on 10 m from ZS6KMD in South Africa (9715km)

JT65 on 10m from PY2RED in Brazil (10550km)

Obviously, there are some improvements that can be done with the RTL-SDR to help HF reception. A FM-broadcast bandpass filter would eliminate the interference from the strong FM broadcast transmiters. As I am mainly interested in HF, and do not care about the performance above 30MHz, a simple low-pass filter at 30MHz will probably also do the trick.  Better shielding and filtering on USB can also help. Many folks are also using an upconverter, for example the Ham-it-up, which is undoubtedly the way to go to improve HF sensitivity.

Although I might try to improve HF reception on the RTL-SDR later, I will rather go building a Softrock RXTX SDR Tranceiver. The said kit is already on my workbench, and the soldering iron is hot. More to come...