[time-nuts] Thunderbolt Harmonics
kb8tq at n1k.org
Fri Jan 20 12:50:07 EST 2017
> On Jan 20, 2017, at 11:22 AM, Tom Miller <tmiller11147 at verizon.net> wrote:
> ----- 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.
….. and use a notch filter that does not miss-terminate the output amplifier (as most notch filters do …)
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