----- Original Message -----
From: "jimlux" <jim...@earthlink.net>
To: <time-nuts@febo.com>
Sent: Friday, January 20, 2017 8:34 AM
Subject: Re: [time-nuts] Thunderbolt Harmonics
On 1/19/17 8:48 PM, Rhys D wrote:
Thanks for the detailed post Bill,
I'm learning a lot here!
So the spectrum analyser is indeed a "trap for young players"
As you guessed, it is a Siglent SSA3000X series analyzer.
I just looked at the same signal again with varied attenuations dialed in
on the instrument (I am using an external 20dB attenuator from
minicircuits
as well)
Here is what I saw:
Attenuation - Fundamental - 1st Harmonic - 2nd Harmonic
15 dB - 11.40 dB - 49.13 dB - 51.12 dB
20 dB - 11.40 dB - 48.84 dB - 56.48 dB
25 dB - 11.28 dB - 48.32 dB - 49.15 dB
I guess these numbers mean I can't really trust what I can see on the
instrument screen?
Actually, that's fairly good. Most spectrum analyzers are good to about
1/2 dB with a moderate level signal (your fundamental).
The variation you're seeing is probably some combination of:
1) the mismatch between the source impedance and the spectrum analyzer
input impedance - the latter of which almost certainly changes with
attenuation setting
2) The calibration of the step attenuator.
3) maybe some change in harmonic production in the SA front end... in your
case, though the harmonic levels go DOWN as the attenuation is decreased,
which is the opposite of what happens with harmonics
If you want to see the levels of the harmonics you should notch out the
fundamental.
Regards,
Tom
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