[time-nuts] Crystal filters in test equipment

[John Ackermann N8UR]

One of my Sulzers on an oddball frequency seems, from the phase noise
plot, to have a crystal filter about 1 kHz wide -- see attached.

John
----

On 3/26/20 8:04 PM, Richard (Rick) Karlquist wrote:
> On 3/26/2020 3:03 PM, Perry Sandeen via time-nuts wrote:
>> Learned Gentlemen,
>> Both the HP 106 and 107 have a post oscillator crystal filter.  There
>> is also a 10 MHz crystal filter used in my Tracor 527E FDM.
>> So the question I have is there anything to be gained by adding 10 MHz
>> crystal filters to the 10811 and similar OCXO's?  They are very
>> inexpensive to purchase.
>> Regards,
>> Perrier
>> _______________________________________________
> 
> I never knew those oscillators had filters, even though
> I worked for HP.  My understanding is that the flicker
> noise of a crystal oscillator is established by the
> crystal, as opposed to the electronics.  A crystal
> filter using a similar crystal could not clean up
> flicker noise.  However, it could follow the buffer
> amplifier and clean up far out phase noise.  I am
> thinking that is why those models have filters.
> 
> The 10811 has an improved buffer that has very low
> far out phase noise, so I am thinking that there
> is no need for a post filter.  In all my time at HP,
> no one ever suggested a 10811 post filter.
> 
> "inexpensive to purchase" filters would probably not
> have good enough flicker noise and would degrade the
> close in noise.
> 
> The 8662 sig gen multiplies 10 MHz to 80 MHz and then
> has an 80 MHz crystal filter to clean up far out
> phase noise.  That makes more sense than a filter at
> the oscillator frequency.
> 
> If you really need lower far out phase noise than
> the 10811 offers, you can redesign the 2nd and 3rd
> buffer amplifier stages.  The 10811 designers knowingly
> degraded the phase noise in those stages because of
> requirements to be backward compatible with 10544
> sockets.  They made a one-off demonstration oscillator
> coded named "Barnabus" with ultra low noise.  It always
> seemed to be the proverbial "solution in search of a problem."
> 
> When I was designing the E1938A oscillator, I remember
> reading some papers about crystal oscillators that
> had a 2nd crystal that was installed in a Wheatstone
> bridge and used to servo the frequency of the oscillator
> to reduce temperature drift.  The breakthrough in the
> E19838A was to put the crystal in a bridge while
> simultaneously using it to make an oscillator.
> 
> Rick N6RK
> 
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