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Monday, October 26, 2020

VK3YE: Solving the Direct Conversion RX -- Double Sideband TX Incompatibility Problem


You have long been one of the leading gurus on DSB.  I remember absorbing all the info I could from your website when I was getting started in DSB back in 2001.   

It's great that you found  the article about DSB with inverted audio.   It would be very cool to build a transmitter with the inverted audio, then confirm that it could be received with a direct conversion receiver without distortion.  

The incompatibility of DSB TXs and DC RXs seems like a very cruel trick of nature.  There are only a few people in the world who think about this, and most of them are in the comments section of your YouTube video!  An elite group indeed.  

Back in 2015 your review of a DSB rig got me thinking about this incompatibility: https://soldersmoke.blogspot.com/2015/07/peter-parker-reviews-dsb-kit-and.html   

It is easy to see how a slight frequency difference between TX VFO and RX VFO would cause a lot of distortion, but similar distortion would be caused by a phase difference between the two VFOs.   AM SW Broadcast receivers try to minimize the effects of fading by using an internal oscillator to replace the wavering carrier -- but they have to have it exactly on frequency and locked in phase with the distant station's carrier. I have a little Sony portable that has this "synchronous detection" circuitry.   It is a complicated task and I don't think you could do it with the highly suppressed carriers of our rigs.  Inverted sidebands to the rescue!     

Thanks for the great video and all the tribal knowledge.  

73  Bill N2CQR

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