[time-nuts] Thunderbolt Harmonics

[Charles Steinmetz]

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).