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
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