The oscillator transistor and buffer amplifier are basically the same as the HP 10811, except for the absence of mode suppressors. The difference here is that the oscillator self limits in the oscillator transistor, whereas the 10811 has ALC. The discontinuous operation of the transistor, as explained by Driscoll some 45 years ago, is undesirable because it increases the load resistance the crystal sees. The 2 transistor "Driscoll oscillator" fixes this problem by using an additional stage that limits instead of the oscillator transistor. This has been widely used for decades. It is interesting to note that the 10811 ALC works by varying the DC bias current in the oscillator transistor. This is in contrast to the elaborate DC bias current stabilization here.
I have demonstrated that the close in phase noise in the 10811 is entirely due to the flicker noise of the crystal. The only place where the 10811 circuit comes into play is beyond 1 kHz from the carrier, where the Burgoon patent circuit (which apparently has prior art from Ulrich Rhode) reduces the phase noise floor. I have built two different oscillator circuits for 10811 crystals and have measured the flicker noise as being the same as the intrinsic noise of the crystal. Thus, obsessing over noise in oscillators circuits may be overkill, unless you are planning to use a much better crystal (BVA, etc). OTOH, it might be advantageous to improve the reverse isolation by adding additional grounded base buffer stages. There are various NBS/NIST papers where several grounded base stages are cascaded. I did this in the HP 10816 rubidium standard. It is good to see time-nuts learning about oscillator circuit by building them. Rick Karlquist N6RK _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
