Felix and I had been thinking about similar things; we've been more thinking of using the extra DAC channel to generate a low frequency oscillator with a limited frequency range, which one could use with a programmable PLL/synth chip to get a sufficiently clean / low-harmonic LO at frequencies that we care about. Regarding cost of IQ modulators: yep, not cheap, but let's face it: this would give one several dozen MHz of baseband bandwidth, and even if we spent a Euro for every MHz of bandwidth we get to make this a viable quadrature mixer architecture, we'd still be off relatively cheap – and would have something to let students measure, calibrate, improve etc for months. Best regards, Marcus On Thu, 2018-04-26 at 07:44 +0200, Sylvain Munaut wrote:

...

can the USB to VGA adapter output quadrature I and Q pairs via the red and blue channels? with maybe using green as a configurable local oscillator.

Sure, we thought about that. But :

- IQ modulator chips are not cheap and you'd need to low pass I/Q before injecting them there. - The LO range would be quite restricted because for a LO you want a clean signal (no hamonics, or as litte as possible, and as little jitter as possible) so you'd have to use a freq way lower than the sampling rate and filter it. But then you have like ... 0 - 35M LO, not super useful. Using an IQ mod with built in LO and control it via I2C is much more viable ... but more expensive (several times the price of the vga dongle in the first place)

Cheers,

Sylvain