[time-nuts] Low-pass Filter for 5 and 10 MHz

[Bob Camp]

Hi

A simple way to look at filter stability is to first look at group delay at
10 MHz (or what ever your signal frequency is). The higher the group delay,
the more likely you are to get into the sort of trouble you thinking about.
Low Q / wide band filters are your friend in this case. Band pass filtering
is going to be more of an issue than a low pass. 

Unless you are playing with 1x10^-15 at tau=1 sec sort of signals, or really
crazy environments (missile launch), you can get away with some filtering on
the signal. It's likely that the harmonics you remove would have caused you
as much grief as the stability you compromise with the filter. 

Bob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Volker Esper
Sent: Friday, April 12, 2013 10:59 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Low-pass Filter for 5 and 10 MHz


...but what about the phase jitter of the filter itself? While absolute 
phase shift may not (or may?) be an issue I guess that passive filters 
do have a phase jitter, too, due to mechanical vibration, tempco, and 
what else.

Particularly at frequencies where the filter response has sharp slopes 
(resonance or corner frequency) the phase variation (d phi / d f) is 
quite big. Thus small frequency changes lead to considerable phase shift 
variation what in turn should lead to phase jitter added to our holy 
signal - what about overall ADEV?

Wouldn't it be better to not filter the 10 MHz signal when used solely 
as a frequency standard?

I understand, that a high Q filter in a PLL reduces the phase noise of 
that oscillator - until the jitter of the filter becomes important. Am I 
wrong?

Volker


Am 12.04.2013 02:31, schrieb Richard (Rick) Karlquist:
> Actually, the opposite is true.  Notches have the least phase
> shift at the frequency being passed, which is what matters.
> It is true that the phase shift at the notch frequency is
> uncontrolled, but that is not important.  The HP8662A
> had an interesting PLL synthesizer where they had 10 notch
> filters for the first 10 harmonics of the sampling frequencies.
> This minimized phase shift within the loop bandwidth that
> detracted from phase margin.  I designers of the 8662
> definitely know what they were doing.
>
> Rick Karlquist N6RK
>
> On 4/11/2013 5:02 PM, Alan Melia wrote:
>> Maybe a silly question but isnt the phase response of the filter
>> important in this application ?? notches have fairly vicious phase 
>> shifts.
>>
>> Alan
>> G3NYK
>> ----- Original Message ----- From: "Luciano Paramithiotti"
>> <timeok.it at gmail.com>
>> To: <time-nuts at febo.com>
>> Sent: Thursday, April 11, 2013 5:42 PM
>> Subject: [time-nuts] Low-pass Filter for 5 and 10 MHz
>>
>>
>>> A simple low pass filter to cut second and third harmonics from a 5 
>>> or 10
>>> MHZ signal.
>>> See the paper:
>>> http://www.timeok.it/files/5_and_10mhz_low_pass_notch_filter.pdf
>>>
>>> Luciano Timeok
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