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

[Richard (Rick) Karlquist]

On 4/12/2013 7:58 AM, Volker Esper wrote:
>
> ...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?

Most of the time, you don't have to worry about jitter in passive
filters unless you are in a high vibration environment.  The other
common cause of jitter is permeability modulation of magnetic cores
in inductors from power line fields.  However, in an atomic clock,
the clock stability is so high that phase tempco becomes significant.
A temperature ramp becomes a phase ramp which becomes a frequency 
offset.  In the 5071A, I multiply the 10811 crystal oscillator to 80 
MHz.  This 80 MHz is used for both the microwave synthesizer chain and 
also to generate a stepped approximation to a 10 MHz sine wave.  This 
greatly reduces harmonics up to 60 MHz, which allows a much less severe
output filter to be used.  This alleviates the effects noted above.
The multiplier to 80 MHz, another possible source of phase drift,
is common moded out in this architecture.

For further details, see my FCS paper.

Rick



>
> 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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>
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