[time-nuts] Building a DMTD/phase noise set in the 21st century
Gerhard Hoffmann
dk4xp at arcor.de
Sun Aug 25 11:59:15 UTC 2019
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.
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