How Hans G0UPL Cracked the Code for Stable I and Q from the Si5351

L to R: Farhan, Roy Lewallen, Eric Guth and Hans Summers

Back in 2018, I remember Farhan telling me that "Hans has cracked the code" getting stable I and Q from the Si5351.  But I never really appreciated the importance of this breakthough until I recently started looking at the receiver from Macedonia.  Then I asked AI (Google Gemini) for a readout on who was responsible for the I and Q innovation.  Here is what I got yesterday from Gemini: 

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The credit for solving this problem and unlocking stable, single-chip I/Q quadrature generation from the Si5351 goes to Hans Summers, the British QRP developer and founder of QRP Labs.

While other talented developers in the amateur radio community—most notably Jason Mildrum (NT7S)—wrote the definitive, widely used standard open-source Arduino libraries for basic Si5351 tuning, it was Hans Summers who fundamentally cracked the math required to force the chip into a stable 90 degree phase shift over a wide frequency range without breaking lock.

The Breakthrough Discovery

Around 2014 to 2015, while designing highly compact, low-cost transceivers like the QCX and ultimate VFO kits, Summers realized that standard programming libraries were completely incompatible with generating stable quadrature signals.

He pioneered a completely reversed architectural method to control the chip, which became the blueprint for homebrew SDR builders everywhere:

• Flipping the Fractional Math: Standard libraries fixed the primary internal Voltage Controlled Oscillator (VCO/PLL) at an integer value and used the second stage (the MultiSynth fractional dividers) to dial in the exact frequency. Summers realized this broke the phase shift. He discovered that you must lock the MultiSynth divider to a fixed even integer and force the first stage (the PLL VCO) to do the fractional math instead.

• The "No Reset" Discovery: Standard practices issued a soft PLL reset command to the chip every time the user turned the tuning dial to ensure clean frequencies. This reset caused the phase relationship to drop out or randomize. Summers proved that after an initial startup reset to align the phases, you can smoothly glide the VCO frequency up and down via the I2C bus without resetting the PLL, keeping the I and Q signals locked in a perfect mathematical embrace.

Thanks to Hans Summers publishing his detailed engineering application notes and open-sourcing his mathematical methods, developers around the world were able to write the code that modern microcontrollers (like the ESP32 in your receiver) use today to keep the I/Q phase rock-solid as you dial across the bands.