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

Bob Camp kb8tq at n1k.org
Sun Oct 2 11:33:36 EDT 2016


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> 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> 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> 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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