I have bought and measured the hp10811 at about -174dBc/Hz. The interesting thing is the feedback capacitor from collector to base which changes Rin=1/gm. Unless the circuit has a hidden Q mulitplier the PN (SSB) can never be better then 177 (kT) in dBm + Pout in dBm - NF of the oscillator transistor. Many of the GB stages are potentially unstable , so the "hopeful' best PN (SSB) is 177dbm + Pout ! AT 100 Mhz the leaing values are -146/100Hz offset and - 183 far out and high crystal dissipation, 2mW or so Ulrich In a message dated 10/27/2015 4:17:16 P.M. W. Europe Standard Time, [email protected] writes:
As Rick has pointed out numerous times when the output signal is extracted via the crystal by a CB stage (or cascade thereof) the PN floor is determined by the ratio of the amplifier equivalent input noise current to the crystal current. That is the amplifier equivalent input noise current at frequencies for which the crystal impedance is high. If one neglects this crucial point one comes to the conclusion (e.g. see Eq 4.-1 page 274 of Ulrich Rohde's: Microwave and Wireless Synthesisers Theory and Design.) that with a crystal current of 1.4mA rms and a crystal esr of 50 ohms that the XO PN floor cannot be lower than -154dBc/Hz. Even the XO circuit in the ARRL handbook (attributed to Ulrich) using this method of signal extraction has a measured PN floor of -168dBc/Hz. Many other XO's (including the 10811A which uses a crystal current of 1mA ) have an actual PN significantly lower than this. One would have thought that this glaring discrepancy between "theory" and practice would have been noticed and corrected by now. Bruce On Tuesday, 27 October 2015 6:01 PM, Richard (Rick) Karlquist <[email protected]> wrote: 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. _______________________________________________ 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. _______________________________________________ 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.
