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 > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there.
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