[time-nuts] Thunderbolt Harmonics

Fri Jan 20 08:34:43 EST 2017

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

>
> 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.

You can have a lot of fun with a couple signal generators, or a sig gen 
and a discrete oscillator, a box of filters and pads ,a mixer or two, 
and an amplifier. You can look at mixer output spurs, compression in an 
amplifier, spurs created inside the test equipment, etc.

An inexpensive clock oscillator (like used everywhere..get one of those 
16 Mhz ones for an arduino or something)  and a signal generator some 
resistors and a T connector is fascinating.  You can see what happens 
when a signal goes "in" on the oscillator output and causes issues with 
the oscillator output buffer, or when you put some RF on the power supply.

>
> 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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