[time-nuts] Thunderbolt Harmonics

Sun Jan 22 08:21:16 UTC 2017

Bill,

No offence taken at all. On the contrary, I really appreciate a great deal
the effort you have gone to straightening out my misconceptions and
blunders!

This forum is such a great learning opportunity. I usually work with
digital systems, so my RF and analog knowledge is sketchy at best. Half the
reason I bought a spectrum analyser is to have an excuse to learn a bit
about RF in a hands on way.

It's funny you mentioned the overly precise numbers. After I wrote them I
thought, well that's ridiculous, but I left them as is. That'll teach me!

Cheers,

Rhys

On Sat, 21 Jan 2017 at 2:37 PM, Bill Byrom <time at radio.sent.com> wrote:

> I'm trying to be gentle, Rhys. :)  I work with these issues every day at
>
> work. Here are a few more comments. I assume you have the preamplifier
>
> in the spectrum analyzer turned off.
>
>
>
>
>
> The term "X harmonic" (such as 2nd or 3rd harmonic) means a
>
> multiplication of the fundamental signal by the given factor. So the
>
> term "1st harmonic" isn't used -- that's the fundamental. The 2nd
>
> harmonic is 2X the fundamental, and the 3rd harmonic is 3X the
>
> fundamental. So in your examples you should have said "2nd and 3rd
>
> harmonics):
>
>
>
>
>
> 15 dB attenuation: 2nd harmonic is (-49.13 - +11.40)= -60.53 dBc
>
>
>
> 20 dB attenuation: 2nd harmonic is (-48.84 - +11.40)= -60.24 dBc
>
>
>
> 25 dB attenuation: 2nd harmonic is (-48.32 - +11.28)= -59.60 dBc
>
>
>
>
>
>
>
> In nearly all cases it's silly to compare RF powers to 0.01 dB
>
> resolution.  The uncertainty of the signal powers being measured,
>
> cable/connector loss, and instrumentation errors is in nearly all cases
>
> larger than 0.1 dB. Your spectrum analyzer doesn't have separate
>
> amplitude log linearly error specifications, but the total amplitude
>
> error with 20 dB of attenuation is specified as +/- 0.7 dB. So the 2nd
>
> harmonic values are not significantly changing as you change the
>
> attenuation, so the source you are measuring probably has about -60 dBc
>
> 2nd harmonic output.
>
>
>
>
>
> The 3rd harmonic results are going to cause me to wave my hands and make
>
> uncomfortable assumptions. The 20 dB 3rd harmonic level seems to be an
>
> outlier, but there is a possibility that a small amount of instrument
>
> distortion is out of phase with the source signal so that they partially
>
> null. RF measurements ARE magic in some cases. <LOL>
>
>
>
>
>
> The use of the external 20 dB attenuator means that the spectrum
>
> analyzer return loss is isolated from the signal source. What does that
>
> mean? Any RF signal traveling down a cable is slightly reflected by
>
> cable defects, connectors, filters, mixers, and imperfect attenuators or
>
> terminators. The reflected signal is called "return loss" and in some
>
> cases "VSWR" or just "SWR". If you had a perfect 50 ohm termination
>
> (load) at the end of a perfect 50 ohm cable, all of the power sent into
>
> the cable would be absorbed by the load and the return loss would be
>
> infinite. The phase of the reflected signal at the source output
>
> connector depends on the round-trip electrical length of the cable and
>
> the nature of the reflection. The reflection from a short is 180 degrees
>
> different from an open, and other types of load can produce different
>
> reflected phases. By the time the reflection gets back to the source
>
> connector, the phase of the reflected signal can cause the impedance to
>
> appear to be nearly anything (greater or less than 50 ohms and probably
>
> capacitive or inductive). If you change the source frequency there is a
>
> different phase round-trip delay due to the wavelength changing, so in
>
> general the RMS voltage at the source will have some ripple vs
>
> frequency. If you place that 20 dB attenuator directly on the source
>
> output connector, the return loss that the source "sees" is nearly
>
> completely controlled by the quality of the attenuator. Even if the
>
> cable had an open or short at the end, the signal passes both ways
>
> though the attenuator so the return loss must be >40 dB (assuming a very
>
> high quality attenuator). This is the same as saying that the VSWR
>
> (Voltage Standing Wave Ratio) is close to 1. A 40 dB return loss
>
> corresponds to a VSWR of 1.02. If an RF filter doesn't see a low VSWR
>
> load, it may not produce the desired filtering behavior.
>
> --
>
>
>
> Bill Byrom N5BB
>
>
>
>
>
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>
>
> On Thu, Jan 19, 2017, at 10:48 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?
>
>
>
> >
>
>
>
> > 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.
>
>
>
> >>
>
>
>
> >>>
>
>
>
> >>
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>
>
> >>> R
>
>
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> >>
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>
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> >>>
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> >>
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> >>>
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> >>
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> >>>
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> >>
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> >>>
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>
>
> >>
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>
>
> >>> ___________________________________________________
>
>
>
> >>
>
>
>
> >>> time-nuts mailing list -- time-nuts at febo.com
>
>
>
> >>
>
>
>
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>
>
>
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>
>
>
> >>> and follow the instructions there.
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>
>
> >>
>
>
>
> >>
>
>
>
> >> _________________________________________________
>
>
>
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> >>
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>
>
> > _________________________________________________
>
>
>
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