Roll Your Own SDR SSB Transceiver
N6QW Moves Totally to the Dark Side of the Hobby!
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