[time-nuts] Thunderbolt Harmonics

Fri Jan 20 16:42:23 UTC 2017

Hi

> On Jan 19, 2017, at 11:48 PM, Rhys D <heyrhys at gmail.com> 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?

Like any instrument, there are limits to it’s performance. There are ways to 
check if you are at these limits. It’s not a matter of trust. It’s a matter of 
trust but verify ….

Bob

> 
> By the way, I should just you know that I am not trying to solve a specific
> timing problem here, I'm more using it as learning opportunity and making
> sure that my setup is the best it can be.
> 
> Thanks again for the input.
> 
> On 20 January 2017 at 12:26, Bill Byrom <time at radio.sent.com> wrote:
> 
>> You can't trust such low harmonic spurious measurements from a  spectrum
>> analyzer unless you know how the spurs change with input level. The
>> second harmonic spur created in an amplifier or mixer inside the
>> spectrum analyzer input will typically increase by 2 dB for every 1 dB
>> of input level increase. Anytime you see a frequency converting RF
>> component (such as the mixer in the input of a spectrum analyzer), it is
>> nonlinear and will generate harmonics and intermodulation products. All
>> you need to do is to keep the input level low enough so that the
>> distortion products generated in the analyzer are below the signals you
>> are measuring. The best and easiest technique is to increase the input
>> attenuation by the smallest step available (such as 5 dB or 10 dB) and
>> checking how the spurious components change.
>> ** If the harmonic or other spurious signal is coming from an external
>> source, it should not change as the input attenuation changes.
>> ** If the harmonic or other spurious signal is generated inside the
>> analyzer, it should change relative to the fundamental signal as the
>> input attenuation changes.
>> ** I'm talking about the harmonics or other spurious signals relative to
>> the fundamental frequency being displayed. If you remove the input
>> signal and still see the spur, it's a residual spur created inside the
>> analyzer unrelated to the input signal.
>> 
>> 
>> If you graph fundamental signal displayed amplitude vs changing input
>> level, you will typically see the following for spurious signals created
>> by most mixers or amplifiers:
>> (1) Fundamental signal = slope of 1
>> 
>> (2) Second harmonic signal = slope of 2
>> 
>> (3) Third order intermodulation (sum or different frequencies caused by
>>    mixing of two signals) = slope of 3
>> 
>> 
>> For more background, see:
>> 
>> https://en.wikipedia.org/wiki/Third-order_intercept_point
>> 
>> 
>> 
>> If that is a SiglentSSA3000X series analyzer, here are the spurious
>> specifications from the datasheet:
>> ** Second harmonic distortion: -65 dBc (above 50 MHz input with
>> preamplifier off)
>> 
>> 
>> Note that the second harmonic distortion is only specified at 50 MHz
>> input and above and at a -30 dBm input power level with the preamplifier
>> off. For comparison, here are the specifications of a Tektronix RSA507A
>> portable spectrum analyzer. Disclosure: I work for Tektronix.
>> ** Second harmonic distortion: - 75 dBc (above 40 MHz input,
>> preamplifier OFF)
>> ** Second harmonic distortion: - 60 dBc (above 40 MHz input,
>> preamplifier ON)
>> 
>> 
>> I'm sure that the reason for a lower limit on the second harmonic
>> specification is that the results are worse at lower frequencies. So
>> it's quite possible that the harmonics you see are mainly coming from
>> the spectrum analyzer input mixer or preamplifier. As I suggested
>> earlier, try lowering the input level by 5 or 10 dB  and see if the
>> harmonics go down linearly.
>> --
>> 
>> Bill Byrom N5BB
>> 
>> 
>> 
>> 
>> 
>> On Tue, Jan 17, 2017, at 08:40 PM, Rhys D wrote:
>> 
>>> Hi all,
>> 
>>> 
>> 
>>> Before I start, let me say I'm rather a newbie at this sort of
>>> stuff so
>>> please be gentle.
>> 
>>> 
>> 
>>> 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.
>> 
>>> 
>> 
>>> Can anyone here shed some light on what I am seeing here?
>> 
>>> Surely this isn't what it is supposed to look like? Should I be
>>> trying to
>>> filter these before going to my distribution amplifier?
>> 
>>> 
>> 
>>> Thanks for any light you can shed.
>> 
>>> 
>> 
>>> R
>> 
>>> 
>> 
>>> 
>> 
>>> 
>> 
>>> 
>> 
>>> _________________________________________________
>> 
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>> 
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