Re: [time-nuts] xtal oscillator phase noise
Henk
Two things will dominate if you want such a low phase noise spec.:- the
loaded Q of the oscillator circuit, and the flicker corner frequency
of the sustaining amplifier transistor. To get a high loaded Q you need
a crystal with a high unloaded Q - maybe 100 000 or more - this isn't
difficult to achieve from a good crystal manufacturer, but you can't
expect any old crystal to work. And the rest of the oscillator circuit
should not load the crystal too much, otherwise the loaded Q, and thus
phase noise, will suffer. Good crystal manufacturers will provide the
necessary measurements of series resistance, motional inductance (or
capacitance, or unloaded Q - doesn't matter which) and static
capacitance. Lesser crystal manufacturers - don't.
Also the flicker corner frequency of the transistor needs to be as low
as possible. Generally speaking, at offsets below the flicker corner
frequency you will get 30dB/decade, above the flicker corner frequency
you should get 20dB/decade, or flat, depending on the level of the phase
noise floor. If you can find a transistor with a lower corner
frequency, the flicker noise will be reduced. In fact, this is one of
the dominant parameters when choosing a transistor as an oscillator -
any old transistor can be made to oscillate, but to do so with a low
flicker corner frequency is not so easy, and the corner frequency is
usually a function of bias current.
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.
Then you have the added problem of the FSUP. It's a superb instrument,
but it has it's limitations. The FSUP data sheet states a phase noise
spec. of -130dBc at 10Hz offset for a 10MHz signal, which gives a
resulting sensitivity of -127dBc - 3dB worse than what you are trying to
achieve. You would need option B60 (cross correlation) to significantly
reduce the effect of the internal source by (say) 15db or so.
regards
Grant
Henk wrote :-
Hello,
Some questions on xtal oscillator phase noise. Attached the
measurement result of my series resonant xtal oscillator.
It is a class A, ibias 5 mA, Ixtal 1 mArms. Transistor selected for
low Rbb' 20 Ohm, Ft 100MHz. Reference voltage 5V from an ADR445,
filtered with 10uF folie cap. Phase noise target -130dBc at 10Hz.
-
Is series resonant better or easier to engineer than parallel
resonant? -
Where should I have 20 dB/decade and where 30 B/decade?
-
Some suggestions for the next 25dB?
-
Is there more to learn from the attached picture?
regards,
Henk
AFAIK, in a crystal oscillator specifically (not oscillators
in general), the oscillator transistor has almost nothing to
do with either close in or far out phase noise. The close in
phase noise is typically limited by the crystal's intrinsic
noise and the far out phase noise is limited by the buffer
amplifier, assuming you take the output through the crystal
as is done in the 10811. Certainly in the case of the 10811,
the oscillator transistor (a selected 2N5179) is not a player
in terms of noise. BTW, the selection is based on startup
issues and has nothing to do with noise. The buffer amplifier
transistor contributes shot noise, however this is determined
by physics, not choice of transistor. So again, choice of
transistor is irrelevent for noise purposes. You do have to
worry about distortion in the buffer amplifier that can
upconvert 1/f noise. This can possibly require transistors
with constant beta vs collector current, depending on the
circuit design.
Rick Karlquist N6RK
Grant Hodgson wrote:
Henk
Two things will dominate if you want such a low phase noise spec.:- the
loaded Q of the oscillator circuit, and the flicker corner frequency
of the sustaining amplifier transistor. To get a high loaded Q you need
a crystal with a high unloaded Q - maybe 100 000 or more - this isn't
difficult to achieve from a good crystal manufacturer, but you can't
expect any old crystal to work. And the rest of the oscillator circuit
should not load the crystal too much, otherwise the loaded Q, and thus
phase noise, will suffer. Good crystal manufacturers will provide the
necessary measurements of series resistance, motional inductance (or
capacitance, or unloaded Q - doesn't matter which) and static
capacitance. Lesser crystal manufacturers - don't.
Also the flicker corner frequency of the transistor needs to be as low
as possible. Generally speaking, at offsets below the flicker corner
frequency you will get 30dB/decade, above the flicker corner frequency
you should get 20dB/decade, or flat, depending on the level of the phase
noise floor. If you can find a transistor with a lower corner
frequency, the flicker noise will be reduced. In fact, this is one of
the dominant parameters when choosing a transistor as an oscillator -
any old transistor can be made to oscillate, but to do so with a low
flicker corner frequency is not so easy, and the corner frequency is
usually a function of bias current.
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.
Then you have the added problem of the FSUP. It's a superb instrument,
but it has it's limitations. The FSUP data sheet states a phase noise
spec. of -130dBc at 10Hz offset for a 10MHz signal, which gives a
resulting sensitivity of -127dBc - 3dB worse than what you are trying to
achieve. You would need option B60 (cross correlation) to significantly
reduce the effect of the internal source by (say) 15db or so.
regards
Grant
Henk wrote :-
Hello,
Some questions on xtal oscillator phase noise. Attached the
measurement result of my series resonant xtal oscillator.
It is a class A, ibias 5 mA, Ixtal 1 mArms. Transistor selected for
low Rbb' 20 Ohm, Ft 100MHz. Reference voltage 5V from an ADR445,
filtered with 10uF folie cap. Phase noise target -130dBc at 10Hz.
-
Is series resonant better or easier to engineer than parallel
resonant? -
Where should I have 20 dB/decade and where 30 B/decade?
-
Some suggestions for the next 25dB?
-
Is there more to learn from the attached picture?
regards,
Henk
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Grant Hodgson wrote:
Henk
Two things will dominate if you want such a low phase noise spec.:- the
loaded Q of the oscillator circuit, and the flicker corner frequency
of the sustaining amplifier transistor. To get a high loaded Q you need
a crystal with a high unloaded Q - maybe 100 000 or more - this isn't
difficult to achieve from a good crystal manufacturer, but you can't
expect any old crystal to work. And the rest of the oscillator circuit
should not load the crystal too much, otherwise the loaded Q, and thus
phase noise, will suffer. Good crystal manufacturers will provide the
necessary measurements of series resistance, motional inductance (or
capacitance, or unloaded Q - doesn't matter which) and static
capacitance. Lesser crystal manufacturers - don't.
A Q of 100,000 at 10MHz produces a Leeson effect phase noise corner of
100Hz
(below which the phase noise slope is 20dB/decade).
This makes it much harder to achieve the desired phase noise at 10Hz
than a crystal with a Q of 1,000,000.
Also the flicker corner frequency of the transistor needs to be as low
as possible. Generally speaking, at offsets below the flicker corner
frequency you will get 30dB/decade, above the flicker corner frequency
you should get 20dB/decade, or flat, depending on the level of the phase
noise floor. If you can find a transistor with a lower corner
frequency, the flicker noise will be reduced. In fact, this is one of
the dominant parameters when choosing a transistor as an oscillator -
any old transistor can be made to oscillate, but to do so with a low
flicker corner frequency is not so easy, and the corner frequency is
usually a function of bias current.
Its not quite that simple, the flicker phase modulation depends on the
voltage and/or
current dependence of the various transistor capacitances etc and the
consequent phase
modulation produced by low frequency noise currents flowing through the
transistor
or low frequency noise voltages developed across such capacitances.
Simply choosing
a low flicker noise transistor without taking its collector base
capacitance, emitter base
capacitance, etc into account is unlikely to reduce the flicker phase noise.
Reducing the dc gain from the base to the collector and ensuring that
the emitter current
low frequency noise is low is likely to be more effective.
Increasing the collector base voltage will reduce the collector base
capacitance and its voltage dependence.
The oscillator power supply noise can also modulate the transistor
collector current and thus increase the phase noise.
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.
Then you have the added problem of the FSUP. It's a superb instrument,
but it has it's limitations. The FSUP data sheet states a phase noise
spec. of -130dBc at 10Hz offset for a 10MHz signal, which gives a
resulting sensitivity of -127dBc - 3dB worse than what you are trying to
achieve. You would need option B60 (cross correlation) to significantly
reduce the effect of the internal source by (say) 15db or so.
regards
Grant
The measured oscillator phase noise floor seems a little high for a
modern low phase noise design.
If the oscillator uses varicap diodes to adjust the oscillation
frequency, these can contribute
significantly to the phase noise especially if their tuning range is large.
Testing the oscillator without varicaps (if possible) and with lower
noise supplies may also be useful.
Failing removal of such varicaps reconfiguring the EFC so that the
varicap has a much smaller tuning range
(supplemented by a manual trimmer) may be useful in reducing the phase
noise contribution of the EFC circuit.
Use a capacitive attenuator to reduce the EFC range, not a resistive
attenuator on the EFC control voltage.
Bruce
On Dec 30, 2007, at 19:03, Grant Hodgson wrote:
Henk
Two things will dominate if you want such a low phase noise spec.:-
the
loaded Q of the oscillator circuit, and the flicker corner frequency
of the sustaining amplifier transistor. To get a high loaded Q you
need
a crystal with a high unloaded Q - maybe 100 000 or more - this isn't
difficult to achieve from a good crystal manufacturer, but you can't
expect any old crystal to work. And the rest of the oscillator
circuit
should not load the crystal too much, otherwise the loaded Q, and thus
phase noise, will suffer. Good crystal manufacturers will provide the
necessary measurements of series resistance, motional inductance (or
capacitance, or unloaded Q - doesn't matter which) and static
capacitance. Lesser crystal manufacturers - don't.
It showed to be very difficult to come lower than what I have now. If
can be the crystal. How can I decide?
Also the flicker corner frequency of the transistor needs to be as low
as possible. Generally speaking, at offsets below the flicker corner
frequency you will get 30dB/decade, above the flicker corner frequency
you should get 20dB/decade, or flat, depending on the level of the
phase
noise floor. If you can find a transistor with a lower corner
frequency, the flicker noise will be reduced. In fact, this is one of
the dominant parameters when choosing a transistor as an oscillator -
any old transistor can be made to oscillate, but to do so with a low
flicker corner frequency is not so easy, and the corner frequency is
usually a function of bias current.
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?
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?
Then you have the added problem of the FSUP. It's a superb
instrument,
but it has it's limitations. The FSUP data sheet states a phase noise
spec. of -130dBc at 10Hz offset for a 10MHz signal, which gives a
resulting sensitivity of -127dBc - 3dB worse than what you are
trying to
achieve. You would need option B60 (cross correlation) to
significantly
reduce the effect of the internal source by (say) 15db or so.
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
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
Not really, it's the circuit topology determines whether a parallel or
series resonant crystal is used. Your circuit appears to be a variant
Just a comment about series vs parallel. The 10811 has a parallel
resonant circuit and the E1938A has a series resonant circuit.
The crystals used are identical except for the package. The performance
is basically identical. There is nothing significant about
series vs parallel in terms of phase noise.
Rick Karlquist N6RK
I think the main difference between parallel and series resonance is that in parallel resonance mode, the capacitance of the crystal holder and wiring to the crystal is part of the frequency determining parameters, and in series mode it is not. So it may have a slight effect on temperature stability (series would be better?) but it should have no other effect.
If the parasitics are well controlled, it should have the same stability.
The crystal is operating the same way, at a slightly different frequency, so if you had it specified for a series mode, you would be on the wrong frequency if you put it into a parallel resonant mode oscillator..
Didier KO4BB
I forgot to mention that the crystals for the E1938A had to
be calibrated for series resonance at 10 MHz.
Rick Karlquist N6RK
Didier Juges wrote:
I think the main difference between parallel and series resonance is that in parallel resonance mode, the capacitance of the crystal holder and wiring to the crystal is part of the frequency determining parameters, and in series mode it is not. So it may have a slight effect on temperature stability (series would be better?) but it should have no other effect.
If the parasitics are well controlled, it should have the same stability.
The crystal is operating the same way, at a slightly different frequency, so if you had it specified for a series mode, you would be on the wrong frequency if you put it into a parallel resonant mode oscillator..
Didier KO4BB
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Grant Hodgson wrote:
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.
The circuit is so simple that one can easily estimate the crystal loaded
Q by hand (if one knows the crystal parameters).
Similarly the phase noise floor can easily be estimated without
requiring a simulator.
Unless the simulators include physically correct models for the flicker
phase noise generation mechanisms they will be of little help.
If they persist in using the Leeson model (which has been shown to be a
gross approximation particularly for flicker noise by Hajimiri and Lee
as well as Demir), then the results are questionable.
With the correct flicker phase noise generation mechanism model its not
too difficult to estimate the flicker phase noise if one has sufficient
data on the oscillator transistor characteristics
Since real crystals exhibit flicker noise such estimates will need to be
supplemented by actual measurements.
If the crystal is a fundamental crystal similar to those offered by
cemac in an HC49 holder and is not a strip crystal then the ESR will be
less than 35 ohms (typically 20ohms??) so with a 5mA oscillator
transistor emitter current the loaded Q will be ~25% less than the
unloaded Q with if the crystal ESR is 20 ohms.
--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' -
rbb adds Johnson noise not shot noise.
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.
Not necessarily true, RF phase noise may have either a higher or lower
flicker noise corner frequency than the transistors low frequency
flicker noise corner.
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.
Increasing the junction area of the transistor reduces the low frequency
flicker noise however this increases the junction capacitances which
increases the Rf flicker phase noise.
It has recently been shown that flicker noise has a lower limit which is
quantum mechanical in nature.
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.
Using transistors with low capacitance can be more important than using
a lower noise transistor see:
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.
The circuit can easily accommodate a crystal specified for parallel
resonance by using an appropriate series capacitor.
If one wishes to trim the oscillation frequency of the circuit such a
series capacitor is necessary.
--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.
Speed of measurement is usually only an issue in production testing.
regards
Grant
Bruce