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