[time-nuts] Spec An for phase noise measurements
bruce.griffiths at xtra.co.nz
Tue Jan 22 23:29:19 EST 2008
David I. Emery wrote:
> It is quite surprising that the 859xE series is spec'd about
> 10-15 db worse on phase noise than the comparable 856xE instruments. I
> wonder if this difference exists in the performance or just marketing
> domains and what the actual differences are in the implementation that
> accounts for it.
Most of the phase noise originates in the local oscillator, the cost can
be reduced by using a lower performance oscillator.
> I might also add that many of the cheap 24 bit 96/192 Kbs USB
> sound input devices actually use codec chips which are DC coupled
> internally all the way to the input pin though that is usually
> referenced to half the analog supply voltage (3.3V/2 typically) rather
> than 0. So going all the way to DC with one of these chips is not
> impossible with external non-audio signal processing feeding the analog
> input pin, thus removing the 20 Hz high pass from the audio preamps
> ahead of the codec chip.
As long as any internal digital high pass filter is bypassed and the
idle tone performance is satisfactory (more likely with a multibit
modulator) with a dc input.
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