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 -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
