[time-nuts] Crystal filters in test equipment

[Richard (Rick) Karlquist]

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