[time-nuts] Thunderbolt Harmonics
csteinmetz at yandex.com
Wed Jan 18 05:21:41 EST 2017
> 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
Even harmonics (which make the carrier asymmetrical) can cause phase
errors that are harmful in high-precision systems , 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.
 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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