On Dec 30, 2007, at 19:03, Grant Hodgson wrote: > Henk > > Two things will dominate if you want such a low phase noise spec.:- > the > loaded Q of the oscillator circuit, and the flicker corner frequency > of the sustaining amplifier transistor. To get a high loaded Q you > need > a crystal with a high unloaded Q - maybe 100 000 or more - this isn't > difficult to achieve from a good crystal manufacturer, but you can't > expect any old crystal to work. And the rest of the oscillator > circuit > should not load the crystal too much, otherwise the loaded Q, and thus > phase noise, will suffer. Good crystal manufacturers will provide the > necessary measurements of series resistance, motional inductance (or > capacitance, or unloaded Q - doesn't matter which) and static > capacitance. Lesser crystal manufacturers - don't.
It showed to be very difficult to come lower than what I have now. If can be the crystal. How can I decide? > Also the flicker corner frequency of the transistor needs to be as low > as possible. Generally speaking, at offsets below the flicker corner > frequency you will get 30dB/decade, above the flicker corner frequency > you should get 20dB/decade, or flat, depending on the level of the > phase > noise floor. If you can find a transistor with a lower corner > frequency, the flicker noise will be reduced. In fact, this is one of > the dominant parameters when choosing a transistor as an oscillator - > any old transistor can be made to oscillate, but to do so with a low > flicker corner frequency is not so easy, and the corner frequency is > usually a function of bias current. I used the BC375 for the low Rbb' and assume that the noise corner must be low as a result of that. Is this not true? > At 11MHz, most crystal oscillators use parallel resonant crystals, > although some are series resonant, such as the excellent Driscoll > oscillator which is capable of the performance you desire with a > suitable crystal. I was aware that most lower frequency circuits are parallel resonant. I used series in class A because I thought is was better, it is easier to use the current though the xtal. Is there a fundamental difference between parallel ore series w.r.t performance? > Then you have the added problem of the FSUP. It's a superb > instrument, > but it has it's limitations. The FSUP data sheet states a phase noise > spec. of -130dBc at 10Hz offset for a 10MHz signal, which gives a > resulting sensitivity of -127dBc - 3dB worse than what you are > trying to > achieve. You would need option B60 (cross correlation) to > significantly > reduce the effect of the internal source by (say) 15db or so. I do normally not have access to a FSUP but borrowed the instrument for two weeks. To my luck it has the B60 option and I used this of coarse. There must be a reason for my employer to buy this fantastic tool. Henk _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.