Thursday, May 5, 2016

EMRFD Direct conversion 40m receiver in a tea caddy

I woke up one day eager to build something simple (at least I thought is was simple) and opened up the first chapter in EMRFD and decided to build the 40m direct conversion receiver. I already had most of the components in my junk box. By the way, Experimental Method in RF Design (EMRFD) is the best book about homebrewing amateur radio gear. You should get it if you don`t have it.

I know you are sitting on the edge of your chair, eager to see the end result, so I will give it to you straight away.  Here it is: My 40m Direct Conversion Tea Caddy Receiver.


Then, lets rewind back to the build process. I started off with the schematics from EMRFD.

The first chapter of EMRFD is available online, so I guess I will not go to jail by showing the circuit diagram here. 
The receiver is based on a NE602 gilbert cell mixer and the famous and old LM386 audio amplifier. The rest of the components are a few capacitors, including three tunable, two T37-6 toroids, and some resistors. 



Prototype of the mixer circuit almost done.


Prototype of the receiver is now finished. In the first version I followed the circuit diagram from EMRFD 100% and used two 75pF air tuning caps (from Russia). The receiver was very difficult to tune with those caps, and I got some oscillations. It was a very fine Theremin, however.


The above video show the performance of the receiver. It is not very sensitive, but it is working. In this version I had some problems with the preselector filter. It seemed like all the RF went straight to ground. Trying to debug this problem did not help much, so in the above video, the antenna is connected straight to the NE602.


For the second version I used just one tuning capacitor of 80pF. It was even more difficult to tune, but I got rid of the oscillations. In addition, I went ahead and used a different preselector circuit from QRPme.com Sudden receiver.


On the final version, I used a 60pF polyvaricon with a reduction drive as the tuning cap and adjusted the oscillator circuit to enable the receiver to tune from 6.9-7.3MHz. It was build using Qrpme MePads in sort of Manhattan style. I soldered the component on two PCB boards which was soldered together to fit in a oriental Chinese tea caddy.


Finally, I added a switch and a 5mm LED. The receiver is powered by 6xAA batteries. It could probably run on anything between 6-9 volts. In the datasheet of the NE602, it says that the maximum supply voltage is 9V, so I am probably pushing it a bit. I used a protective diode and a series resistor for safety reasons and to keep the voltage down a bit.


I used an SMA-connector on the back.


The final receiver. Eager to sniff RF from the ether...



The schematics for the receiver (using Fieldnotes schematics software). The series diode is not in the schematics, but I put it in for good measures. I used somewhat different component values in the colpitts oscillator than those in EMRFD.

The verdict

It was a very fun build. It is a simple circuit on the paper, but it sure teaches you a lot regarding radio functionality. Alright, the receiver works, but it does not seem very sensitive. I should be noted, firstly: that I do not have any 40m capable receiver to compare with. Secondly, my antenna is not very good on 40m. Thirdly, I have no idea what I am doing.



That being said: CW pops in very nicely, while SSB is a bit difficult to tune in. The oscillator seem suprisingly stable given that it is a VFO rather than a VXO, and the fact that I used random capacitors and no fancy NP0-ones. At least it was stable enough to receive RTTY with fldigi during a brief experiment.

In a future version I would like to add some sort of audio filtering before the LM386. Some sort of audio gain control would also be nice, since it is a bit loud on my iPhone headphones on strong signals. 

Further reading:


Saturday, April 16, 2016

Loaded dipole for 20m

My HF-project has stalled since I do not have a decent antenna. I have limited space for a full-sized antenna on my roof. My friend LA8OKA has assembled a loaded 20m dipole, and I wanted to test a similar design so I could get on the air on 20m.

First I simulated the antenna in EZNEC+ 6.0. The antenna is about 6m with 9uH loading coils halfway on each dipole segment.


Above, the parameters in EZNEC. I created two coils at about 9uH, which I calculated as 0R+791j ohm.


This is how the antenna looks like in EZNEC.



The SWR minimum is 1.34 at 14.05 MHz, meaning that the antenna should be great for WSJT work at 14.0760 MHz.

Then I created the two coils using 32mm PVC and 23 turns of 1mm2 multicore copper wire. I calculated the inductance to be 8.8uH, and a prototype coil proved to be in the ballpark of the calculated value.


Picture of the coil (in the rain).



I used a cheap 1:1 China-Balun as the center isolator.

Then, I mounted the antenna on a test location on my balcony and tuned the antenna using a NWT150 scalar network analyser and a directional coupler.


The above picture show the test setup using the SNA, a directional coupler, and a 50 Ohm dummy load for calibration of the SNA.


The first sweep gave a minimum SWR of 2.0 at about 13.5 MHz. By shortening the antenna, the minimum moved to 14.07 MHz. Minimum SWR is still 2.0. I suspect that the antenna impedance somewhat below 50 ohm due to the loading coils. In addition, there was about 50cm of wet snow on the roof just 1m below the antenna when I did the measurements. This might affect the result, which deprives me from experiencing the holy grail of 1.0 SWR. 

The verdict: Honestly, I am not sure how good the antenna is, since I do not have anything to compare with. In addition, the band conditions on 20m has been really poor and my softrock only outputs about 1W. Hence the chances to obtain great DX is limited. Nevertheless, I have made a few contacts up to 2500 km.

Wednesday, February 17, 2016

Synthicase and Softrock timelapse build video

I finally got around to throw my synths in a suitcase. It is a injection moulded case (Pelicase copy). The case consists of a x0xb0x bass synth, a Sonic Potions LXR drum machine, a Shruthi-1 synth, a MFOS noise toaster, and my DIY modular mixer consisting of mostly MFOS modules. The devices are mounted with velcro tape.



The LXR is not subjected to the best fit in the case, since I do not have right angle phono and MIDI cables.

I created a first tune with my Synthicase using the x0x as the master MIDI clock and the LXR sequencing the Shruthi. The x0x bass tune is a classic theme, probably heard before. Ok, here it goes, DJ DIYcrap in action:


By the way, the video shows me building the Softrock RXTX HF Transceiver. The video is shot using my DIY time lapse device and a Nikon D90. In other words, this video is truly DIYcrap.

Saturday, January 23, 2016

Minima #2 - Crystal filter construction

Making the crystal filter for the Minima Transceiver has been a challenging but very interesting experience. Challenging because I do not have any fancy test equipment. But I learned a lot, and it was very fun going through all this.

I bought about 50 HC-49 24MHz crystals from different ebay sources and  started out making the G3UUR colpitts oscillator tester from Experimental Methods of RF Design (EMRFD). I used 330pF capacitors for Cf (Ref EMRFD) and measured Cs to 35pF (including the switch).




All crystals were fundamental mode crystals, but they were all over the place frequency-wise. I borrowed an old Phillips PM6671 frequency counter for the characterization, as my own counter does not go all the way to 1Hz resolution.



I used boxes with small compartments to keep the crystals in order during the work.


The frequencies was jotted down in Google Sheets (with the G3UUR switch in both positions), and then I sorted the crystals by frequency.


I found seven crystals within 65 Hz for the QER crystal filter. I calculated Cm for the crystals using the updated formula from the 2015 ARRL handbook, and used Dishal to calculate the crystal parameters.


A 2.8 kHz bandpass resulted in 234pF capacitors and input/output impedances of 22.6 Ohms. I did not use this alternative, however. Instead I constructed the filter for 50 Ohm in/out, which required 109pF (I used 100pF) and an estimated bandwidth of 5.6 kHz. Probably a bit wide, but I wanted to give it a try.



Then I "characterized" a few capacitors and soldered the filter together bravely. In the above picture you see a 6dB pad at the input and a 51 ohm resistor at the output. I have no network analyzer so I had to improvise.



I used a Si5351 controlled from an Arduino. A few buttons let me step the frequency in 100Hz intervals. The Si5351 was connected to the input of the filter and the output was connected to an oscilloscope. For every 100Hz interval, I jotted down the RMS voltage at the input and output, and calculated the loss.

The response is not great, as there is some falloff. The passband ripple is about 2-3 dB. I have no idea whether this is bad or not. The Elecraft K2, for example, is supposed to have 3.2dB ripple, so my filter can not be all that bad although the QER filter is supposed to be very flat. The reason for the passband ripple can be either an error with my measurement technique, or it could be that the output impedance is not exactly 50 Ohm as estimated in Dishal (i have not measured the impedance), or it could be that the individual placement of the crystals matter (I did not care), or that the crystals are crap. The filter is about 5kHz, a bit narrower than estimated.

Anyway, the filter is good enough for initial testing, and I am satisfied. I think I need a simple Scalar Network Analyzer, however, as all these measurements were a bit tedious, and it could be interesting to do them again them with different capacitor values.




Tuesday, December 22, 2015

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.


Saturday, December 5, 2015

Homebrew SWR and power meter

First of all, this SWR meter in this article is not exactly homebrew, as I based the circuit on the schematic in the excellent book Arduino Projects of Amateur Radio. Although I bought the book and respect the copyright of the authors I will share my own schematic here. Why? First, the design in the book is not completely original and is based on similar designs from others. One example here.  Second, I changed a few things, removed some stuff,  and added some other things, to my own liking, so the design is not identical anymore.


The circuit is based on two AD8307 log amplifiers, which are connected to the forward and reflected ports on a directional coupler.  The AD8307 amplifiers gives a DC voltage of about 25mV/dB of the input signal, which is amplified using an opamp (LM324). The opamp also provides the reference voltage to the Arduino (AREF) to ensure that the full ranges of the A/D converters are used.


I used the fourth opamp in the quad LM324 to provide input voltage reading. That is why the PCB in the picture has two power jacks. One input and one output. That way I know the drive voltage for the radio. I primary plan to use this device for my Softrock.


I created the schematic and the board in Eagle, and submitted the gerbers to Elecrow. I received 10 PCBs after about 25 days.


The board is not much larger than a standard 20x4 LCD.


I choose to use an Arduino nano rather than populating a AVR328 on the board and messing with USB converters. I am glad I did. I did one mistake on the board however. The Arduino was not powered from the 5V rail. The problem was that it was an error on the Eagle footprint for the Arduino Nano. I just downloaded the footprint file uncritically from the Internet without checking it. Other than that, It was fine.


Another problem was that I got oscillations on the LM324 opamps connected to the AD8307. The oscillations were around 40KHz and about 400mVpp on top of the DC signal. Hence it was impossible to calibrate the device. The solution was to desolder the output capacitors on the LM324. I do not think they are really needed on a DC design.


This was my first SMT design, and I am fairly happy with the result. The SWR and the power measurements seems accurate. I used the Arduino software from the book, and modified it to include a larger display and some other things. As you may notice, there are no buttons on the device. Originally I planned to use a rotary encoder and a bunch of menus, but as they say in the Soldersmoke podcast, menus are for restaurants. I totally agree. Besides, programming all kinds of features to the device is totally insane. This is a simple SWR meter and thats it. 

The files are available here, should you be interested. Note that there is an error in the schematic. The Arduino is missing +5V, but it should be easy to fix.

Sunday, October 18, 2015

Nixie tube experimentation with IN-12

Introduction


I have always wanted to try out Nixie tubes in one of my projects, and here we go. Nixie tubes come in many different sizes and there are both top view and side view nixie tubes out there. I settled on some russian IN-12 top view nixie tubes as they are cheap, and should fit nicely on a front panel (for example on a radio).


The IN-12 was originally used in russian multimeters, radios, frequency counters. This R-155A Brusnika Radio Station serves as a good example on how nixie tubes were used in the cold war era.

Recently, this particular tube has been used for many hobby purposes such as clocks, geiger counters, etc. While I am mentioning it, the major page for steampunk and Nixie-fans is Bad dog designs having loads of excellent clocks using nixie tubes. Ok, enough drooling over other designs. Back to my modest attempts.

Testing a nixie



I purchased a 110-180V power supply kit from ebay. I highly recommend this kit from seller lumos-sk. It comes with an excellent build manual and was very easy to assemble.


The assembled power supply was set to 170V...



...and the IN-12 glows fine.

A PCB for four IN-12 tubes.


To control the nixies, I made a PCB in Eagle. The circuit uses two 74HC595 shift registers to drive four K155ID1 decimal decoders.


The schematic is shown above.


It is possible to cascade several of these boards to drive eight, twelve, sixteen, etc Nixies from just three pins on the microcontroller. I ordered the PCBs from OSH-park, and they were excellent.


However, I should have read Kevin Ryes blog before creating the PCB. I just took it for granted that the Eagle IN-12 part was correct, but as Kevin Rye found out, it is not. The pin numbering on the silk screen is wrong and the anode is connected to pin 5 instead of pin 1. However, if one ignores the numbering, my PCB works if the IN-12 is mounted on the back side of the board (i.e. mirrored). 


Originally I planned to use IN-12 sockets (purchased from Ukraine) but since the IN-12 part was mirrored, the mounting holes were on the wrong locations and it made no sense to use the sockets. In addition, the parts I got were used, so I had to desolder chunks of 30 year old wiring. Totally boring work. So instead, I went back on Ebay and ordered some pins from the old Soviet Union (see picture above). They were excellent for this purpose.


To test the shift registers, I copied the Arduino code made by Imperkins over at Instructables. The only change was to add support for an additional shift register.


The above film shows the Nixie PCB in action.
As mentioned, it is simple to cascade several PCBs to control more nixie tubes. My secret plan for the future is to use eight nixie tubes as a frequency display for a HF radio, such as Farhans Minima.

Further Reading


1. http://scott-bot.com/nixie-bot/
2. KiCAD library
3. IN-12 datasheet (in Russian)
4. IN-12 geiger counter (with laser cutted parts).
5. IN-12 clock (shares Eagle files and svg of front panel cutout)