Category: QCX

SolderSmoke Podcast #225: Mars, uSDX, G-QRP, HP8640B, DX-390, Rotary Tools, Walla Walla SDR, MAILBAG

SolderSmoke Podcast #225 is available

http://soldersmoke.com/soldersmoke225.mp3

Mars, West Coast smoke.

Pete’s Activities:
— DC receivers.
— CW offset
— GQRP talk
— The uSDX project

Bill’s Bench
— Sliding into the Vintage Test Gear Cult: HP8640B .
— Fixing up and figuring out Radio Shack DX-390 receivers.
— 220 to 110 on a few remaining devices.
— Got myself a Dremel-like rotary device.

Tech News:
— ARRL/TAPR Convention: SDR project from Walla Walla University students. Intuitive explanation for why desired and image freqs in a mixer come out with very useful phase differences.

— Chuck Adams’ Amazing Lab Notebook. Includes a simple circuit to measure resistance and Q in crystals. FB.

MAILBAG:

— Dino KL0S SITSing in his shack, homebrewing 9 MHz filters FB Dino. Airborne!
— Dave NT1U sent us the famous 1968 QST Article by W7ZOI re DC RX.
— Ron K0EIA listening to SWBC staions with uBITX.
— Ted AJ8T Korguntubes making a 12AX7 equivalent.
— Joel N6ALT sent me a nice DX-390 manual. Thanks Joel
— Bob KD8CGH alerted us to the uSDX project — story on the blog.
— Craig AA0ZZ Sent a great message with insights on computer code — I will put up on the blog.
–Tracy KN4FHX reports on optimistic prognosis for SolarCycle 25. Some chickens may have to be sacrificed.
— Stephen M0OMO Thanks SolderSmoke for rekindling interest in this hobby.
— Paul VK3HN has a cool new rig — The Prowler — check it out
— Steve N8NM working on his Sunbeam car — Pete already knew about the carburetor synch problem. N6QW knows everything.



Mixer Insights using Propellers and Cameras — From Walla Walla University. And SDR Design Info.

Pete Eaton sent us this video from the 2020 ARRL/TAPR Communications Conference. I have the portion of interest cued up (above). (The portion of interest begins at 6:59:46.)

There is a lot of really cool SDR design info in this video and in the associated paper (the TAPR site says you have to pay the ARRL $9 for the paper, but in the comments someone says the papers will be available free after the conference).

What caught my attention was the students’ discussion of mixer action. They use an analogy with a spinning propeller (the incoming RF) and a camera (triggered by the local oscillator) that samples the incoming signal at a specific rate. This is analogous to a Quadrature Sampling Detector.

The really interesting part for me was how this analogy allows us to see how phase differences between the desired signal and the image signal arise. These phase differences permit an SDR receiver (or indeed an old fashioned phasing Direct Conversion receiver) to reject the image while allowing the desired signal to pass.

This is a key point in understanding mixers, and is really quite amazing. Before I saw this video, I had just come to accept (without understanding WHY) that the desired signal and the image signal would have phase differences, EVEN IF THEY WERE COMING OUT OF THE MIXER AT THE SAME FREQUENCY. It is this phase difference that allows us to knock one down while allowing the other to pass. The propellers and cameras of Walla Walla University gave me insight as to how and why these phase differences exist.

In their paper, the Walla Wall group mention uSDX, the project that is currently generating so much excitement around the world:

Low-cost is not the only reason SDRs have become more popular among the amateur radio
community. More recently, Guido Ten Dolle’s μSDX open source transceiver has generated
increasing interest in quadrature sampling down-conversion SDRs in the homebrew QRP
community. Guido, PE1NNZ, was able to modify the QCX, QRP transceiver for SSB operation
with an efficient class-E amplifier, using only an ATMEGA328 and Arduino code to run the QSD
SDR. This groundbreaking work in this type of SDR has inspired various renditions of Guido’s
radio, fostering a lively groups.io group that can be followed at https://groups.io/g/ucx.

Kudos to Caleb Froelich, Dr. Rob Frohne KL7NA, Konrad McClure, Joshua Silver, and
Jordyn Watkins KN6FFS, all of Walla Walla University, for some really impressive work. (BTW: Rob tells me that back in the mid-90s he too built one of Rick Campbell’s phasing receivers and wrote a QST article about it (probably the first SDR article published by QST). Details on the project are here: http://fweb.wallawalla.edu/~frohro/R2_DSP/R2-DSP.html

Global Collaboration: The uSDX — A Multi-Mode QCX

Bill:
There is a new open source, home brew, multi band, multi mode QRP transceiver that grew out of the QRP Labs QCX. Through some serious magic it retains an efficient class E RF amplifier for sideband and digital modes. It crams impressive SDR capabilities into an Arduino. More info at https://groups.io/g/ucx/topics

The basic work appears to have been accomplished by Guido Ten Dolle PE1NNZ. It uses pulse width modulation of the PA supply voltage to transmit modes other than CW while retaining class E efficiency and uses a direct conversion SDR receiver.
https://github.com/threeme3/QCX-SSB .
There are several variants by different developers. I built a variant designed by Barbaros Asuroglu WB2CBA
https://antrak.org.tr/blog/projeler/usdx-an-arduino-based-sdr-all-mode-hf-transceiver-pcb-iteration-v1-02/ that uses through hole components (mostly) and I’m pleased with it’s performance. I also designed and 3D printed a case.

This has an interesting development process with contributions by many, including the usual gang of suspects: Hans Summers, Ashhar Farhan, Manuel DL2MAN, Kees K2BCQ, Allison KB1GMX and Miguel Angelo Bartie PY2OHH. I apologize to the many others whose names I didn’t list.

The band switch multiband version by DL2MAN is even smaller, but with SMD components which I wasn’t ready to tackle yet.

BTW – your podcast encouraged me to go in this direction. I built a BITX 40, a uBITX (sent you a pix of it in an old Heathkit Twoer case), U3S, QCX and now my first step from kits to built from plans.

73

Bob KD8CGH

https://antrak.org.tr/blog/projeler/usdx-an-arduino-based-sdr-all-mode-hf-transceiver-pcb-iteration-v1-02/