What's the loaded Q of such a notch filter? 50 dB 100 Hz off of 10 MHz sounds like a pretty lousy Q.
On Sunday, 2 October 2016, Bob Camp <[email protected]> wrote: > Hi > > The issue is *not* about notch filters and if they are useful. The gotcha > is > that they are mainly useful far removed from carrier rather than close in. > The > statement “works 100 Hz off carrier” requires a *lot* of qualifiers to > make it > apply in real measurements. Without getting into the limitations, it is > very difficult > to determine just how close to carrier you can go with a notch. That is not > implementation specific it applies to all notches. You *do* need to get > into the > details. > > The stuff we have gone over so far is hardly an exhaustive list. There are > many issues. > We have yet to get into the amount of power being delivered to the crystal > in the > notch filter and the behavior of crystals when driven with a lot of power > …. > > Bob > > > On Oct 2, 2016, at 12:10 PM, Adrian Rus <[email protected] > <javascript:;>> wrote: > > > > Bob, > > There is no issue about using or not using notch filters. We know (?) > all of these. It is about a damn simple topology. Yes, this topology has > its shortcomings (impedance, variation with freq and the so). > > The math is close, but not exact. > > I gave up here. If the topology is of no use, this is it. For me it was > funy to _discover_ that simple topology doing a notch. > > Best, > > Adrian > > > > Sent from my BlackBerry 10 smartphone on the Orange network. > > Original Message > > From: Bob Camp > > Sent: Sunday, October 2, 2016 18:35 > > To: Discussion of precise time and frequency measurement > > Reply To: Discussion of precise time and frequency measurement > > Subject: Re: [time-nuts] notch filter for close in phase noise > measurement > > > > > > Hi > > > > Ok, the next issue with the notch filter technique is the termination of > the oscillator > > it’s self. The notch may (or more likely not) provide a proper 50 ohm > load at the > > carrier frequency. Even if it is correct at the carrier, it will go off > impedance as it > > moves away from carrier. You either need a pad in series with the > oscillator (which > > drops sensitivity) or something similar (like an isolator). The gotcha > here is that the > > phase noise of the device may not be the same when it is incorrectly > terminated. The > > issue is more significant in minimum stage devices or when the output > stage contributes > > to the total noise of the device. > > > > A bit of math: > > > > A good 10 MHz oscillator will be in the -155 to -165 dbc / Hz range at > 100 Hz off carrier. > > If you have lost 20 db of energy due to the notch width, that is now > -175 to -185 dbc / Hz. > > If the oscillator is putting out +10 dbm, that would be -165 to -175 dbm > / Hz. The lower > > number is at the KTB level without any loss in the bridge, a the > attenuator, or noise figure in the > > post amplifier. The higher number is only 10 db away. If the notch has a > bit more loss, things > > get even tighter. This is more than just a theoretical issue. > > > > After that you do get into the AM + PM thing. The notch is normally > proposed for use on > > floor measurements. Details are in the FCS paper by Stone back in the > 1970’s. There the argument > > is that the noise process *must* be producing equal amounts of AM and PM > noise. That makes > > the conversion of “what I measured” to phase noise fairly easy. Close > in, you can indeed have > > processes that produce unequal amounts of AM and PM noise. Without a way > to separate the > > two, you toss a fairly large bit of doubt into the measurement. > > > > Bob > > > > > >> On Oct 2, 2016, at 11:03 AM, Adrian Rus <[email protected] > <javascript:;>> wrote: > >> > >> Yes. It can be used for offsets starting some 100-200Hz. Plus, the > measured noise is PN+AN. Again, the only reason I wanted to share this > topology is its outrageous simplicity. All pluses and minuses of notch > filer measurement methode, remain. > >> Sooner (or later) I shall share with you (after the real life > validation) an (again, very simple) interderometric methode. > >> Adrian > >> > >> Sent from my BlackBerry 10 smartphone on the Orange network. > >> Original Message > >> From: Bob Camp > >> Sent: Sunday, October 2, 2016 17:54 > >> To: Discussion of precise time and frequency measurement > >> Reply To: Discussion of precise time and frequency measurement > >> Subject: Re: [time-nuts] notch filter for close in phase noise > measurement > >> > >> > >> Hi > >> > >> The notch is (say) 60 db deep at the carrier frequency. At 100 Hz off > the carrier frequency, > >> it still has some depth. It might be 50 db deep, it could be 10 db > deep. A lot depends on the > >> crystal you have. Even if it’s only 10 db deep, the phase noise you > measure at 100 Hz off > >> carrier will be “off” by 10 db. > >> > >> Bob > >> > >>> On Oct 2, 2016, at 10:46 AM, Adrian Rus <[email protected] > <javascript:;>> wrote: > >>> > >>> Hello, > >>> The _generator_ is a reference 10MHz oscillator and the only > calibration of the notch is to equal the oscillator freq. > >>> The basic idea of the message is its simplicity (as compared to other > notch approaches). > >>> Best regards, > >>> Adrian > >>> > >>> Sent from my BlackBerry 10 smartphone on the Orange network. > >>> Original Message > >>> From: Bob Camp > >>> Sent: Sunday, October 2, 2016 17:06 > >>> To: Discussion of precise time and frequency measurement > >>> Reply To: Discussion of precise time and frequency measurement > >>> Subject: Re: [time-nuts] notch filter for close in phase noise > measurement > >>> > >>> > >>> Hi > >>> > >>> Getting close to carrier with a notch filter involves a bit of > calibration of the notch. It’s not > >>> imposible to do, but it is a needed step. The generator you use to do > the measurement has > >>> to be pretty clean to get adequate data at low offsets. > >>> > >>> Bob > >>> > >>>> On Oct 2, 2016, at 3:56 AM, Adrian Rus <[email protected] > <javascript:;>> wrote: > >>>> > >>>> Hello list, > >>>> For those of you interested in phase noise measurement without using > fancy/dedicated gear, here you are the way I have got. Disclaimer: as far > as I am concerned, all phase noise measurements use a technique to get rid > of carrier: quadrature mixing, interferometric [more on that, later] and > notch filters. > >>>> > >>>> The simplest way use notch filters, and the simplest notch filter can > be arranged with just 3 elements: > >>>> - one return loss bridge > >>>> - one quartz crystal > >>>> - one resistor > >>>> Hook the crystal on DUT port, the oscillator to be measured on IN > port, the SA [spectrum analyzer] on OUT port and the resistor on REF port. > The resistor have to be determined by trial and error to equal the series > resistence of the crystal at series resonance. From some -50dB up, can hook > a potentiometer in parallel to the resistor[s] and fine tune for the > deepest notch. > >>>> It is easy to get notches as deep as -85-90dB. The filter is useful > in close in measurements not closer than 100-200Hz from carrier. Yes, > between the notch and SA you should insert a 40-60dB amplifier. The > amplifier will not degrade the flicker noise [as there is practical no > carrier - see Rubiola papers], but will set the noise floor. > >>>> The series resonance freq have to be selected from multiple crystals; > I have experienced series resonance in 10MHz crystals ranging from -300Hz > to +100Hz against 10MHz sharp, and have selected a crystal resonating at > +25Hz at room temperature. For exact fit you can either tune the oscillator > @+25Hz, or better, thermostat the crystal; thermostating the crystal will > also tune the notch to the desired freq. > >>>> My selected crystal was equilibrated by a series resistance of > 14.7ohm. Please note, the series resistance of other 11 crystals I have > tested range from 14ohm to tens of ohm. > >>>> Regards, > >>>> Adrian > >>>> _______________________________________________ > >>>> time-nuts mailing list -- [email protected] <javascript:;> > >>>> 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] <javascript:;> > >>> 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] <javascript:;> > >>> 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] <javascript:;> > >> 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] <javascript:;> > >> 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] <javascript:;> > > 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] <javascript:;> > > 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] <javascript:;> > 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.
