can we do Quadrature pairs?

Müller, Marcus (CEL) mueller at kit.edu
Thu Apr 26 08:09:10 UTC 2018


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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