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

Volker Esper ailer2 at t-online.de
Fri Apr 12 17:35:26 UTC 2013


Am 12.04.2013 18:24, schrieb Bob Camp:
> A simple way to look at filter stability is to first look at group delay at
> 10 MHz...
That's what I meant (d phi / d f), I just wrote it in a roundabout way...
> The higher the group delay,
> the more likely you are to get into the sort of trouble you thinking about.
>    
Yep, my apprehension...
> 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.
>    
I haven't launched missiles, yet :-), but am playing with GPSDO signals, 
and planning to build a distribution amp - so I am a bit afraid about 
using filters, at least high Q ones.

Thank you, Bob, for your comment.

> -----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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