[time-nuts] xtal oscillator phase noise

[Grant Hodgson]

Henk ten Pierick wrote:
--snip--
> 
> It showed to be very difficult to come lower than what I have now. If 
> can be the crystal. How can I decide?

As an absolute minimum, you need to know the crystal dynamic (or 
motional) parameters - the crystal supplier should be able to provide 
these.  If not, you can measure them on a network analyser whilst you 
look for another crystal supplier.

Then you should be able to simulate the phase noise of the oscillator 
using a harmonic balance or similar method as used in Microwave Office, 
Genesys, ADS, Ansoft Desginer, QUCS etc.  Failing that, a small-signal 
(linear) open-loop analysis would at least give an estimate of the 
loaded Q, which can be used to predict phase noise - but ignoring 
flicker noise.  Some SPICE-based simulators might be able to help.

--snip--
> 
> I used the BC375 for the low Rbb' and assume that the noise corner must 
> be low as a result of that. Is this not true?

There are many different types of noise - the base bulk resistance of a 
transistor contributes to shot noise, which is close to being 'white' - 
i.e. equal magnitude /Hz at all frequencies.  This does not have a 
significant effect on phase noise at offsets close to the carrier, and 
at 30Hz offset the flicker noise dominates.  Flicker noise is not 
'white' noise - flicker noise increases at a rate of 1/f, or 10dB/decade 
as the offset frequency is reduced, and simply choosing a transistor 
with low Rbb' is not sufficient - the noise mechanisms are different.

For an 11MHz oscillator I would use 2N5179s as advocated by Rick for 
both the sustaining amplifier and the limiter - this is a very popular 
transistor for oscillators in this frequency range.  I'd be surprised if 
the BC375 generated less noise than the 2N5179.  This would mean 
changing the circuit topology to use an NPN transistor instead of the 
BF450 which is PNP.


>> At 11MHz, most crystal oscillators use parallel resonant crystals,
>> although some are series resonant, such as the excellent Driscoll
>> oscillator which is capable of the performance you desire with a
>> suitable crystal.
> 
> I was aware that most lower frequency circuits are parallel resonant. I 
> used series in class A because I thought is was better, it is easier to 
> use the current though the xtal. Is there a fundamental difference 
> between parallel ore series w.r.t performance?

Not really, it's the circuit topology determines whether a parallel or 
series resonant crystal is used.  Your circuit appears to be a variant 
of the Driscoll oscillator, which usually uses a series resonant crystal 
and is capable of exceptionally high performance, however there are a 
number of differences in your circuit, which I've never seen before, 
although I can't claim to be an expert oscillator designer.  Circuit 
simulation is a good (no - essential) starting point, and would give you 
a good idea of the relative merits of the features of your circuit.

--snip--
> 
> I do normally not have access to a FSUP but borrowed the instrument for 
> two weeks. To my luck it has the B60 option and I used this of coarse. 
> There must be a reason for my employer to buy this fantastic tool.
> 
> Henk
> 

OK, but given that the noise level is currently way above the noise 
floor of the FSUP, using cross-correlation doesn't add anything - it 
just slows down the measurement.  Cross-correlation would only be of 
benefit to reduce the noise floor of the instrument if/when the phase 
noise of the oscillator has been reduced enough to justify it - it can 
seriously slow down the measurement.

regards

Grant
>