Hi > On Oct 26, 2015, at 7:03 PM, Attila Kinali <att...@kinali.ch> 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?
Yes > > Why use a colpitts oscillator when using the crystal as output filter? > Wouldn't a Butler oscillator make more sense? No > Or is there some > disadvantage of Butler oscillator that I am not aware of? Build it and see... > > > Why are LEDs used as voltage references? Don't they have a horrible > temperature coefficient and bad aging characteristics? but the “right” tempco > 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. > > > Does the constant current source (Q103, LED 102, R116) sufficiently > stabilize the power inside the crystal, and thus the output power? No > 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. Except that Hfe is pretty stable over time > > Can the noise induced by Q103 be further decreased by increasing C109? Not really > Or is there a reason why C109 is just 10nF? Look at the loop stability of the current source and the R/C between the 10K and C109 > Stability maybe? > If stability is the problem, how about using an RC low pass filter? > > 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 In series with C105, with a blocking cap to the crystal and buffer sides. > The bias voltage would be then applied directly at the crystal/C105/varicap > node. Is this correct or is there a better way? Block the DC > > > What are the criteria to choose the transistors? Low noise, more specifically the combination of low 1/F noise and low noise figure at RF and a low mixing coefficient. > There are also a number of “tweaks” (added components) typically put into that circuit for best performance. Bob > Thanks in advance > > Attila Kinali > > -- > Reading can seriously damage your ignorance. > -- unknown > _______________________________________________ > 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.