Hi > On Oct 27, 2015, at 12:15 AM, Bruce Griffiths <bruce.griffi...@xtra.co.nz> > wrote: > > The 10811A ocxo uses an oscillator of this type albeit with a lower crystal > current, an overtone crystal. However the output stages spoil the PN > floor..Cascaded transformer coupled CB stages are somewhat quieter.
….. and since this is the region that the circuit really *is* the issue, lowering noise there is well worth doing. The target is basically signal to noise. That makes it much easier to analyze than some of the non-linear stuff that can impact the close in noise. Bob > Bruce > > > > On Tuesday, 27 October 2015 2:31 PM, Bruce Griffiths > <bruce.griffi...@xtra.co.nz> wrote: > > > Various versions of this oscillator circuit have been employed as high > stability OCXOs eg: > > http://ri.search.yahoo.com/_ylt=AwrTcaxMzS5WgJIAMwk3QIpQ;_ylu=X3oDMTBzbW1zYXBzBHNlYwNzcgRwb3MDMjEEY29sbwNncTEEdnRpZAM-/RV=2/RE=1445936589/RO=10/RU=https%3a%2f%2fescies.org%2fdownload%2fwebDocumentFile%3fid%3d60902/RK=0/RS=.Rmksavr9Ui3TZ8D1XyZ06TpeDY- > An AGC circuit can be employed to adjust the dc current of the oscillator > transistor to stabilise the crystal current. > The circuit as given was merely intended to show an alternative to the > corresponding Wenzel version which employs a high input impedance buffer. The > Wenzel version has a relatively low oscillator transistor Vcb which is > perhaps somewhat undesirable. > Driscoll developed various high frequency crystal oscillators employing MMICs > RF splitters together with a crystal, various matching circuits and a diode > limiter. > > Bruce > > > On Tuesday, 27 October 2015 2:01 PM, Bruce Griffiths > <bruce.griffi...@xtra.co.nz> wrote: > > > On Tuesday, October 27, 2015 12:03:49 AM Attila Kinali wrote: >> Hi, >> >> I've been trying to read up on low noise crystal oscillators and had >> a closer look at the design by Bruce Griffiths[1]. There are explanations >> to how the circuit works, but I have some questions on the details. >> I would appreciate if someone could answer these questions. >> >> [1] http://www.ko4bb.com/~bruce/CrystalOscillators.html >> >> >> I will do a short recap how the circuit works, just to make sure I >> haven't misunderstood it. >> >> The oscillator core is the colpitts oscillator build around Q104, >> C107/108 form the driving/feedback path to form a negative resistance >> over the quartz crystal. The resistors R112 and R113 are there only >> to keep the crystal bias free and prevent charges from building up. >> >> The output is formed using the crystal as filter to get rid of >> harmonics and noise outside the crystal bandwidth. The "ground" point >> of the crystal is formed using the low input impedance of the common >> base amplifier formed by Q102. The output is coupled using a > transformer >> to make it DC free and for impedance transformation. >> >> Q103, LED102 and R116 form a constant current source for the collector >> of Q104, using the base of Q104 as control input. >> >> >> Q105 acts as a series voltage regulator, using multiple LM329's as >> reference, which are averaged for lower noise and Q106 to compensate >> for Q105's B-E voltage drop. >> >> Q101 is the input power supply filter. >> >> >> Now my questions: >> >> Doesn't the non-zero input impedance of Q102 dampen the >> crystal unnecessarily? > The effect is relatively insignificant provided the crystal esr is > significantly > larger than the CB stage input R. > With an overtone crystal this is readily achieved. >> >> Why use a colpitts oscillator when using the crystal as output filter? >> Wouldn't a Butler oscillator make more sense? Or is there some >> disadvantage of Butler oscillator that I am not aware of? >> >> > Avoiding doubling resistance in series with the crystal due to the 2 > transistors of the butler configuration. >> Why are LEDs used as voltage references? Don't they have a horrible >> temperature coefficient and bad aging characteristics? >> My guess would be that LED101 is not that critical as it will only >> result in a slight change of the collector current and thus only >> a slight change in the input impedance common base amplifier Q102. >> > In both cases the LED forward voltage tempco is approximately matched > by the Vbe tempco of a transistor so that the resultant dc current is > nominally temperature independent. > LEDs have relatively low noise however they are somewhat photosensitive. > Using low noise dc bias circuits like these can significantly reduce the > close in phase noise of RF amplifiers significantly compared to a bias > circuit using a voltage divider from the power supply. >> >> Does the constant current source (Q103, LED 102, R116) sufficiently >> stabilize the power inside the crystal, and thus the output power? >> My guess would be that changes in h_fe of Q104 will result in >> different biasing of Q104 and thus in changes of the power within the >> crystal, which then affects frequency and aging. >> > The colpitts oscillator transistor in this circuit operates in a > discontinuous > mode. > >> Can the noise induced by Q103 be further decreased by increasing > C109? >> Or is there a reason why C109 is just 10nF? Stability maybe? >> If stability is the problem, how about using an RC low pass filter? > the noise contribution by Q103 isnt significant. > Yes bias loop stability is an issue you cant just insert arbitrary low pass > RC > filters some design effort is required. >> >> If one would want to make this circuit tunable, where would the >> varicap get connected to? My guess would be on the right side of >> the crystal, between the crystal and C105, going to ground >> The bias voltage would be then applied directly at the > crystal/C105/varicap >> node. Is this correct or is there a better way? >> > In series with C105 is a far better location. > You may then need to increase the value of C105. >> >> What are the criteria to choose the transistors? >> > Low flicker noise and sufficient RF gain at the crystal frequency. > >> Thanks in advance >> >> Attila Kinali > Bruce > _______________________________________________ > 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. > > > > > > _______________________________________________ > 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. _______________________________________________ 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.
