[time-nuts] Thunderbolt Harmonics

Rhys D heyrhys at gmail.com
Thu Jan 19 23:48:58 EST 2017


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?

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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