[time-nuts] Thunderbolt Harmonics

Bill Byrom time at radio.sent.com
Thu Jan 19 18:26:31 EST 2017

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:


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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