[time-nuts] notch filter for close in phase noise measurement

Sun Oct 2 12:27:39 EDT 2016

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 <kb8tq at n1k.org> 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 <adrian.rus at broadhurst.ro
> <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 <adrian.rus at broadhurst.ro
> <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 <adrian.rus at broadhurst.ro
> <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 <adrian.rus at broadhurst.ro
> <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
> >>>> _______________________________________________
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