[time-nuts] Thunderbolt Harmonics

[Scott Stobbe]

Harmonic traps are another avenue to explore since the frequency is "fixed"
at well below ppm. Which leaves the fundamental untouched.

On Wed, Jan 18, 2017 at 5:21 AM, Charles Steinmetz <csteinmetz at yandex.com>
wrote:

> Rhys wrote:
>
> I was looking at the output of my Trimble Thunderbolt GPSDO and was rather
>> surprised to see really "loud" harmonics in there. ~ 60dB down from the
>> 10Mhz signal.
>>
>
> Welcome to the world of RF.  Loudest harmonic at ~ -60dBc (dB with respect
> to carrier) is actually pretty good for a commercial product. Very few
> distribution amplifiers do this well.  For that matter, many good
> laboratory RF generators are specified with harmonics only below -35 to -45
> dBc.  We do not generally expect RF sources or amplifiers to get down to
> the -80 to -90 dBc range (although amplifiers with harmonics < -80dBc at
> 10MHz/1Vrms/50 ohms are possible), and certainly not the -100 to -120dBc
> that we expect from high fidelity audio sources and amplifiers.
>
> Even harmonics (which make the carrier asymmetrical) can cause phase
> errors that are harmful in high-precision systems [1], so I am a vocal
> supporter of distribution amplifiers with harmonics < -80dBc.
>
> Note that cleaning up the Tbolt output to < -80dBc would probably require
> a crystal filter (a filter with a sharp corner very close to 10MHz, in any
> case), which means its phase response changes very rapidly with the filter
> frequency.  Sharp filters shift frequency with temperature, which causes
> temperature-dependent phase shifts.  Unless the filter is maintained in an
> isothermal environment (like a good oven), this can cause problems in
> sensitive applications.
>
> Best regards,
>
> Charles
>
>
> [1]   F.L. Walls (NIST), F.G. Ascarrunz (SpectraDynamics), The Effect of
> Harmonic Distortion on Phase Errors in Frequency Distribution and Synthesis
> (year unknown, probably late '90s).
>
>
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