[time-nuts] Building a DMTD/phase noise set in the 21st century

[Gerhard Hoffmann]

Am 25.08.19 um 10:52 schrieb Hal Murray:
> attila at kinali.ch said:
>> As you have always two channels, I would recommend to use two mixers fed with
>> an LO that is 90°C out of phase to get I and Q components.
> How does that compare with running a single channel twice as fast?
>
>

At low enough frequencies, the two ways are identical in performance

and linked by the Hilbert transform.


At high enough frequencies, ADC performance starts to suffer. One gets

a smaller "effective number of bits"  ENOB  as sample rate and/or input

frequency rise.


In that region one may gain an advantage by halving the sample rate

at the cost of doubling the hardware and needing a Hilbert transformer,

also known as a wideband 90° phase shifter.  The phase shift

might come cheap, depending on the upstream processing.


One could also time-interleave 2 ADCs or even more. Some ADC chips

do that internally, and some scopes, too.

regards, Gerhard.