Hi > On Jan 19, 2017, at 11:48 PM, Rhys D <heyr...@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 <t...@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 >> >>> >> >>> >> >>> >> >>> >> >>> _________________________________________________ >> >>> time-nuts mailing list -- time-nuts@febo.com >> >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >> >> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to https://www.febo.com/cgi-bin/ >> mailman/listinfo/time-nuts >> and follow the instructions there. >> > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.