Roll Your Own SDR SSB Transceiver
N6QW Moves Totally to the Dark Side of the Hobby!
11/27/2017 ~ Observations and 1st DX Contact
Today I wanted to spend a bit of time comparing the new SDR homebrew transceiver to "filter" type rigs I have built. First though I do want to share that I worked my very 1st DX station with the SDR rig and that was XF1IM who is in lower Baja California. The distance was 1600 miles running 100 watts. So I am beaming from ear to ear.
Firstly I do want to acknowledge once again the pioneering work of Charlie, ZL2CTM and his several you tube videos that got me kick started into rolling my own SDR rig. Notwithstanding I do have several SDR kit radios including several Softrocks and the Omnia based on the Peaberry --and even one that uses a Softrock with a Raspberry Pi. So I am not brand new to SDR, but I am new to rolling my own.
Above all there seems to be a clarity to the signals and perhaps that is because the bandwidth may be a bit wider. It was even suggested to me (by an appliance operator no less) that I should open up the bandwidth to 3.5 kHz. Well friend that may be problematic. But the sound is fuller. It does have presence and brightness with no "yellowy sounds" in the mid-range.
I also note that signals seem to pop out of nowhere. Boom no signal and very quiet then suddenly a rock crushing signal. There was an occasional you are 20 hertz low and I disregarded that since I rechecked --and that station was 20 Hertz high.
All in all there is very little hardware as such and in going over the cost --about $100 with half of that in the Teensy 3.5 and Codec board. So it does cost almost 2X the Bitx and you do have to build it yourself; but there are opportunities beyond the basic radio from India. Those are mainly in adding software functionality like variable filtering (you can decide how many filters and bandwidths you would like). Charlie has developed some software so you can watch the FFT display of Frequency Spectrum -- that would be hard to do with the Bitx. So think about it -- a far greater capability for about $41 more. There is a wave coming and we are the beneficiaries of the low cost technological wonders.
11/26/2017 ~ More Refinements & More Contacts.
That is the pure beauty of our hobby -- you make contacts and you get feedback on your signal. I did get some feedback about signal quality that appeared to be related to signal distortion on large voice peaks. It was not RF feedback; but more in line of things running "too hot". [Not hot in the sense of heat but rather too much gain".] It also appears there was somewhat of a restricted voice in that the signal sounded a bit narrow.
To address these issues I went back to Charlie's original low pass filter (as I would have to design a new Band Pass filter that was a bit wider) and a clue from one of the QSO's was what happened when I reduced the gain on the J310's in the combo Rx Tx amp stage. The report back was that the distortion on peaks had almost disappeared. Big Clue. But that also reduced the Rx RF gain significantly. So I had to strike some sort of balance on the gain settings between receive and transmit.
In the EMRFD stage the input had a 5.5 dB T Type attenuator pad as I found over several earlier builds this tremendously helped the stability of the stage. Thus a plan was formulated to increase the size of the pad from the 5.5 dB to 10db and this enabled me to increase the gain on the Rx RF Amp side by about 5 dB overall. Thus we had best of all worlds. So the balance was struck. Next I went into the software and reduced the microphone gain which was set at 40 down to 30. The full gain setting is 60 --so we are at half gain versus 2/3. As a final adjustment I tweaked the bias level of the IRF 510 to about 4 volts --it was running about 3.6 VDC.
In listening to the signal on an outboard receiver and then looking at the scope pattern I could tell that the signal quality had greatly improved -- and I still was getting 100 watts out with the outboard amp. Three contacts, one in Colorado and two local contacts including one with my friend Ben AI6YR verified that all was good in the hood. I also took the time to "clean up" some of the wiring including running some leads under the base PC Board. That clean can be seen in the photo below.
The next goal is to finalize the switching circuits and get everything into an enclosure. I then want to turn my attention to learning more about the software and some upgrades in functionality like adding the 160X128 Color TFT display, switchable USB/LSB and a TONE for Tune Up whish is pretty much standard in all N6QW Homebrew Rigs. Yet another upgrade in the software is switchable filters for SSB and CW.
The next goal is to get this in a box and finalize the switching and control ciruits
11/22/2017 ~ Adding the RF Power Amplifier Stage
DRUM ROLL ~ 1st Contact with K8NG
As luck would have it I heard K8NG calling CQ on 7.213 MHz at 1510 PST and gave him a shout. Boom he came back to me with a report of 5X3, 5X4. Mac, K8NG is located in Duck Creek Utah and interestingly enough one of the video made with the Simpleceiver Plus SSB was made with Chris, KF6FZY -- also in Duck Creek. There must be a pipeline. Mac reported the signal sounded fine albeit with a bit of restricted audio. That reflects the narrow band pass filter software that I installed. I was running 5 watts with the IRF510 and no amps. the antenna was my usual droopy dipole. A great day. It doesn't get any better.
I went over to the Simpleceiver Plus Prototype and temporarily liberated the Driver, Final and Low Pass Filter stages and connected them to the Teensy SDR. I got about 6 watts out and was pleased to see this work.
I am lacking a relay to make this a fully functional Transceiver so for these test all was hardwired. I am overcome by getting ready for the gang that is coming in for Turkey Day--so I will have to curtail any activity for a few days. You can also visit my cooking website as some of the recipes on the site will be the fare for Thursday. www.pastapete.com
11/21/2017 ~ Relays added to the Front End Board
If you look closely at the J310's RF Amplifier you will see a couple of 12 VDC SPDT Communications relays that I purchased from All Electronics.
Look closely as these are listed as micro-miniature relays. These are made by Fujitsu and are the BRD series. These bought in quantities can be had for 35 cents each. The contacts are good for 2 amps. A little super glue and they are solidly mounted to the PC Board. I tested this configuration and it works FB. The relays can be seen along the upper right hand corner of the board.
Charlie, ZL2CTM shared with me how to change the Microphone Gain and I have been able to do that successfully. Stay tuned --we may be not too far off from some on the air tests.
11/20/2017 ~ First Transmit Test and More Listening Tests
The transmit concept works with my front end board. The Modem Transformers are also now a proven concept on transmit. I need to find a way to adjust the Microphone gain and to look in more detail the output SSB Envelope. But this is getting exciting.
11/19/2017 ~ Block Diagram of the SDR Front End
11/18/2017 ~ We are 98% there ... Listen to the Video
This video is a great leap up from where we started. I think we have some positive trends going here. My next task is to get it t work on Transmit and then we can see if we can make a Transceiver from these two boards.
11/17/2017 ~ The Problem Appears to be Fixed
Aside from installing a new Codec board and taking a tip from Hans Summers g0upl, today the 600 Ohm to 600 Ohm Isolation transformers arrived and were installed. That cured the hum problem and I will make another video to demonstrate the improvement. The Trick is to separate the grounds. Thanks Hans!
Shown above are the two Isolation transformers (sold as Modem Transformers from Jameco Electronics -- Triad Transformer is the Manufacturer-- about $6 each). Phasing is important and that is why you see the dots (as supplied) on the transformers. Hard to believe that what you see above is the front end, the quadrature LO, phase splitter and the I & Q Detectors. The audio amp (2N3904/LM-386-3) is also on this board.
In passing I do know that the I & Q is working very well. After modifying this board and the Teeny 3.5 board to add the isolation transformers I powered up the unit --and all I was receiving was USB -- Inadvertently I had the I & Q reversed.
The two black wires going to the ADE-1's is actually a chunk of shielded cable from a pair of defunct Sony Walkman earbuds. I made it extra long just in case I did that. I will now trim the wires and tidy things up. On either side of the J310's RF amp stage I will be adding a couple of relays on the RF Amp to use this circuit as the Rx RF Amp and as the Tx RF Pre-Amp just like in the Simpleceiver Plus V2.0. I purposely left space on this board to do that. You still need to have the isolator installed on the audio output out of the Teensy 3.5.
Can't wait to get this working as a transceiver.
11/16/2017 ~ Some Progress on Fixing the Whine Burble Noise
Part of the problem was the Teensy Codec Board. I had a second board and the above video shows the improvement. We are not out of the woods yet. There is still a bit of background hum which may be resolved with the addition of the 600 Ohm to 600 Ohm Isolating transformers on the front end output similar to what Hans Summers has done with his QRP Labs receiver board. I already have a small fortune in hardware so a few more bucks almost seems like pocket change. The flies in the face of my 1st station which cost me $20 in 1959. I had to mow a lot of lawns to amass $20.
With a large Tip Of The Cap to Charlie Morris ZL2CTM who has done some pioneering work in building SDR SSB Transceivers as evidenced by his superb videos on You Tube --I decided to try my hand at replicating Charlie's work. ZL2CTM has most kindly provided me a great deal of assistance and supplied the all important sketch code and many photos and schematics that he used.
Charlie has been successful with his rig. But I have not been so lucky. I simply cannot put my finger on the root cause but in hopes of those more skilled at this SDR stuff will immediately know the answer. View the video and "Tell Me What You Think".
The main issue is the terrible background noise that is present on the audio output. If the "whine and burbling background noise" could be resolved --this would be one heck of a rig.
By way of background the rig consists of a front end comprised of a pair of J310's configured as a DGM RF amplifier, a 40M Band Pass Filter, a homebrew ferrite core balun signal splitter, two ADE-1's as the I & Q Detectors and a SN74AC74 that is used as the divide by 4 quadrature LO. Also on the main board is an audio amplifier using the 2N3904 and LM386-3.
The SDR board has the Teensy 3.5, the Audio Codec Board, the Si5351 and some relay switching so the Line In and Line Out I & Q can be routed to/from the board. For a Display I am using the 1 inch square OLED.
After connecting everything up and listening -- it was awful and a terrible whine and burbling noise was evident which called for a disciplined trouble shooting process. The first thing I did was hook up the front end as a Direct Conversion Receiver and routed the output of the I & Q Detectors to the Audio amp (1 channel at a time). The sound was crisp, clear and no evidence of any problems. While the SDR board was bypassed, I was using the LO signal from the Si5351 and the OLED for reading the frequency. Thus the clean signal was not impacted by any OLED noise and what also was evident -- nothing was coming through the Si5351. The outlier was the Teensy 3.5 and the Codec Board.
Steve Hartley g0fuw sent me an email about an event in the UK called YOTA which also had a link to a video about the event. (YOTA = Youngsters On The Air). After seeing what these youngsters were doing -- they probably have the answer. The problem is this "oldster" doesn't! Help????
I did pose the problem on the PJRC Forum (Teensy gang) and even contacted the head guru at Teensy -- aside from one response from the forum which was not helpful nothing else has been heard other than to sell me another piece of hardware while helpful did not totally resolve the issue. Help????
This project has been a frustration to me personally as I just don't know enough about the Teensy 3.5 to lay a finger on the problem. Perhaps it is not a bad device --but I upgraded to a Teensy 3.6 and the results were worse --an important clue that the problem is a Teensy hardware issue.
Has anyone reading this blog been successful with the Teensy 3.5 -- ZL2CTM has had good luck but he too is scratching his head.
From the Dark Side