Bruce Griffiths wrote: > Hans > > Hans Rosenberg wrote: > >> Hi Time-nuts people, >> >> I'm new to this forum and really enjoyed reading all the design strategies >> on oscillators. >> >> I'm currently designing a low noise oscillator and I wonder if you can help >> me with some questions. I've attached a picture of the oscillator core I'm >> using. I decided to go for a Pierce design some time ago after looking at a >> few topologies. I took a number of measures to give the design a low 1/f >> noise. >> >> >> 1. Low DC gain. There is a transformer in the drain line. >> 2. Ultra low noise supply (not on the schematic) using an ad586 buried >> zener reference with a -3dB lowpass at 0.1Hz. This 5V reference is then >> increased to 12V using a low noise discrete buffer. >> 3. The phase shift across T1 is only a few degrees (<5degrees), low >> phaseshift in the amplifier reduces 1/f noise. >> 4. I chose a fet in order to minimize the load at the 'output' of the >> crystal, the only load I have now is the bias resistor of 100k. This >> resistor does cause low frequency noise (at higher frequencies C1 shorts >> it). This may be the problem in this circuit, Cgd is modulated by this noise >> and a low frequency voltage is applied at the gate, which is however not >> amplified because of the currentsource in the source line which should cause >> nearly infinite feedback for low frequencies making the DC gain even lower. >> 5. The current source produces low frequency noise. I have to have a >> current source though (I could use a resistor but I calculated that produces >> more noise). I could increase the voltage across R5 and make R5 bigger to >> reduce the noise, however, this will reduce Vgd which means modulation of >> Cgd becomes worse. I think I'm ok here by dividing the voltage across the >> currentsource and the active oscillator element in half. Cgd of a J309 is >> around 2.5pF at Vds=10V (it is a little higher in my case since Vgd is lower) >> 6. L2 can be mounted to accommodate overtone crystals. ( I still have to >> calculate a value for L2 and C2 to get the correct impedance at resonance) >> 7. F1 and F2 are ferrite beads with a low impedance at DC (far less then >> an ohm) and rising impedance at higher frequencies to prevent oscillation of >> the RF transistors. >> 8. I've found a really good overtonecrystal from Citizen. CM309S. I >> measured the unloaded Q to be 313000, the loaded Q (using simple estimation) >> should be around 280000 in the circuit. (C0=2.5pF, R=20Ohm, C1=0.75fF, >> L=29.444mH, can be aquired at digikey). The spec for Rs was <130Ohms, I >> guess reality is much better :-) >> 9. The transformeroutputs are going to an isolationchain using 3 cascodes >> and then a discrete limiter not shown on the schematic. >> 10. This whole oscillator core will be placed in an RF shielding can. The >> isolationchain will have a can of it's own to so pulling should be nearly >> eliminated. >> >> Now my questions are: >> >> >> 1. I don't see a pierce design in any of the low noise oscillator circuits >> in the discussion threads about low noise oscillator design. Is there >> something fundamentally wrong about this topology? >> 2. I read a few times that ferrites in inductors (and I assume >> transformers as well) can be a real problem for 1/f noise. I'm using a >> transformer (TC1-1t from minicircuits) which is made with a ferrite bead. Do >> you know of any transformers or inductors that have low 1/f noise ferrite >> material. >> 3. Have I missed something fundamental in the design. The goal is to build >> a very good oscillator. I would like to achieve something like -110dBc/Hz at >> 10Hz distance at 33.8688MHz. >> >> Thanks in advance for having a look and best regards, >> >> Hans Rosenberg >> >> >> > 1) A LED plus a BJT and resistor makes a much lower noise current source > than a zener based reference plus a BJT and resistor. > Some shielding of the LED from ambient light may be required as LEDs (as > all PN junctions) are photosensitive. > A resistor in series with an RF choke can be used to replace the current > source. > > 2) A BJT with a correctly proportioned coupling network will not > significantly load the crystal. > > 3) There is little isolation between the oscillator and the load, a > slight change in topology will improve this. > > 4) Modified Pierce overtone crystal oscillators using JFETs were popular > in the 1960's and 1970's. > > 5) The amplitude limiting mechanism in the oscillator is important as it > affects the phase noise. > > 6) A lower noise audio BJT will make a much lower (flicker) noise > current source than a 9GHz transistor. > > 7) Its usually better to use the crystal to filter the output signal. > > 8) One of the Driscoll BJT oscillators or a variant thereof is a good > stating point. > AGC via a varactor based attenuator can be used. > The output signal can be extracted from the second transistor collector > using a transformer with little interaction with the collector tank tuning. > > > Bruce > > Addendum
9) Having a dc voltage across the crystal isnt a good idea as it can lead to unwanted phase modulation. 10) The current source transistor collector base voltage seems a little low. Bruce _______________________________________________ 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.
