Overtone ultra-low phase noise crystal oscillator

[cbsirio]

ultra low phase noise crystal oscillator -vco

Hi all,
I was searching for a ultra low phase noise overtone crystal oscillator at 150 MHz with -150 dBc @ 10KHz or hints to achive this goal.

I have made several tests on various circuits I have found on Internet and on various books, but no one reaches my goal of -150dBc @ 10 KHz...
I have used several attentions, like limiting the rf across the coil of inductor, take the output with a buffer from the base of the transistor, or using a jfet (u310)...

I have used a battery to power the circuit, to avoid effects of poor regulation, but with no success.

The only circuit I have found wich promise this goal is a circuit used by Rohde-Schwartz found on a post by an edaboard member (sorry, I don't remember the name) on page 18 (of the file I will attach) It looks "theoric", but it could be a good starting point...

Can anyone help me, please?

Sirio.

 

[flatulent]

One common trick is to follow the oscillator by a crystal filter. Use a crystal just like the one in the oscillator. Many older receiver books from around 1930 or 1940 will have circuits of a one crystal filter used for morse code reception. Basically, it is a center tapped transformer with the crystal in one end and a nulling capacitor in the other end both connected to a common output point.

 

[cbsirio]

Hi Flatulent!

Can you check the schematics arttached here, please? Is what you suggest to do?

I am not very familiar with this filters!

Thank you very much!

Sirio.

 

[flatulent]

You have the general idea. The capacitor should be a variable to null out the crystal holder capacitance. Then you may need more elements to pull the crystal frequency a bit to get the peak on your signal.

Try measuring the circuit on a network analyzer to check it out.