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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