[time-nuts] Thunderbolt Harmonics

Tom Miller tmiller11147 at verizon.net
Fri Jan 20 11:22:27 EST 2017


----- Original Message ----- 
From: "jimlux" <jimlux at earthlink.net>
To: <time-nuts at 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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