[time-nuts] Re: Seems like the new version PN2060C is available for sale US$680

[Ed Marciniak]

Yeah, USB2.0 is awful with 12/24/48/480 MHz clocks locally. And then you’re bringing in the signal from the host as well. And people usually don’t have USB isolation either so there’s also ground loop potentially in play.

With an SDR using a 6.144/12.288/24.576 clock for Nx48KHz sample rates, you either end up with DC offsets or the 12MHz at Nyquist nulls depending on sample rates.

Some of the TI ADCs use a 250x decimation instead of 256x decimation to avoid that problem.
Intel HD audio codecs take a similar approach.

With USB3.0, we have a 5GHz clock, with 8B/10B line coding for a 4Gbit/sec payload, spread spectrum clocked with 33KHz modulation. The bit scrambler LFSR gets reset under specific conditions. Inevitably, the USB3.0 will have PLLs and/or synthesizers involved.

While I'd expect the PN2060A/B/C doesn't use sigma delta ADCs, it's worth pointing out that in multibit sigma delta systems, that correlation might be different than expected.

How exactly all of the possible sources of trash can be excluded is a sticky problem.
________________________________
From: Poul-Henning Kamp via time-nuts <time-nuts at lists.febo.com>
Sent: Saturday, October 7, 2023 9:07:08 AM
To: Discussion of precise time and frequency measurement <time-nuts at lists.febo.com>
Cc: Poul-Henning Kamp <phk at phk.freebsd.dk>
Subject: [time-nuts] Re: Seems like the new version PN2060C is available for sale US$680

Bob Camp via time-nuts writes:

> One way or the other, the spurs are from an internal source in the device. [...]

That is why I was surprised to see that they retained the USB interface ?

As a first order approximation, anything related to USB is cost
optimized, and therefore EMI/EMC efforts terminate the moment the
spurs are 3dB under the template.

They do have a comment about stationary vs. laptop giving different
spurs, so I guess they are aware of it.

At that level of ambition I would have reached for batteries +
linear PSU + fiber-optics, even though it would be both more
cumbersome and costly.

--
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
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