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@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. >

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@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. >

----- Original Message ----- From: "jimlux" <jimlux@earthlink.net> To: <time-nuts@febo.com> Sent: Friday, January 20, 2017 8:34 AM Subject: Re: [time-nuts] Thunderbolt Harmonics > 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 > If you want to see the levels of the harmonics you should notch out the fundamental. Regards, Tom

Hi > On Jan 19, 2017, at 11:48 PM, Rhys D <heyrhys@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 <time@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.

Hi > On Jan 20, 2017, at 11:22 AM, Tom Miller <tmiller11147@verizon.net> wrote: > > > ----- Original Message ----- From: "jimlux" <jimlux@earthlink.net> > To: <time-nuts@febo.com> > Sent: Friday, January 20, 2017 8:34 AM > Subject: Re: [time-nuts] Thunderbolt Harmonics > > >> 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 >> > > If you want to see the levels of the harmonics you should notch out the fundamental. ….. and use a notch filter that does not miss-terminate the output amplifier (as most notch filters do …) Bob > > Regards, > Tom > > _______________________________________________ > 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.

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 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@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.

 caused by

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@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 > > > > > > > > > > > > > > > > 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@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. > >

hello to all, to put things right into perspective please note that the famous OCXO hp10811 is "only" specified with harmonics <25 dB (see page 11 of manual https://www.febo.com/pages/hp10811/HP10811AB-Manual.pdf). This device is but still often felt to be useful. Goetz Am 22.01.2017 um 09:21 schrieb Rhys D: > 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 > >