[time-nuts] Ultra low noise Pierce oscillator???

Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed Jan 14 16:09:16 EST 2009


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

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.


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