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 <[email protected]> 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 <[email protected]> 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 -- [email protected] > > > > >> > > > > >>> To unsubscribe, go to > > > > >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > > > >>> and follow the instructions there. > > > > >> > > > > >> > > > > >> _________________________________________________ > > > > >> time-nuts mailing list -- [email protected] > > > > >> To unsubscribe, go to https://www.febo.com/cgi-bin/ > > > > >> mailman/listinfo/time-nuts > > > > >> and follow the instructions there. > > > > >> > > > > > _________________________________________________ > > > > > time-nuts mailing list -- [email protected] > > > > > To unsubscribe, go to > > > > > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > > > > and follow the instructions there. > > > > > > _______________________________________________ > > time-nuts mailing list -- [email protected] > > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > and follow the instructions there. > > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
