Re: [time-nuts] Advantages & Disadvantages of the TPLL Method
Thanks for the positive contrbution, A good example of one of the TPLL's
obvious disadvantages.
The simple cheap analog version of the TPLL is limited by it's need to have
a dedicated Ref OSC.
One way I have got around that problem, which would not apply to all, is to
put the DUT unit as the controlled OSC, and use a special Tbolt as the
reference Oscillator.
The other way around the problem is the Digital version of the TPLL that
uses DSS.
BTW that limitation is not nearly as big as one would think. This is because
the long term accuracy is already limited by the reference osc, so one would
not generally use this kind of system out past 1000 sec or so anyway. So If
doing long term multichannel Osc, One would likely be MUCH better off with a
more basic undersampled Phase system for long term testing and just go thru
and cheek each Osc one at time for a short time with a low tau tester such
as this type.
Keep the advantages and disadvantages coming in, so the Time Nuts can
compare which methods work best for their application.
Now if we just had some place to log the responses.
Summery: If you have multi oscillators to test simultaneously that do not
have EFC input, and that you want to do continuous sampling on, and do not
have multiple TSC boxes, the TPLL is not the right tool for the job.
Be better off with one simple lower resolution multiplexed time stamped TI
phase system and a single TPLL.
Bruce posted:
The poor cost scaling of the tight PLL system is another reason
why it has fallen out of favour for those who have more than
2 frequency standards to compare simultaneously.
Thanks for that opinion, but I don't think we should list the above as a
unique disadvantage.
Maybe need a new column heading for that one, Any name suggestions?
Does not sound all that valid or unique of a reason to me.
It seems the same can be said about a TSC or any new high cost system.
I would think a more important reason is that the simple TPLL is not a
universal do all system.
Because the simple analog version is "Limited by it's reference Osc" in many
ways,
This does give it some possible major disadvantages like not working so good
with a CS or Rb standard.
If one has more time than money, there are ways around that.
ws
Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Jun 13 01:25:13 UTC 2010
Another disadvantage of the Tight PLL system that only applies to
multichannel systems is that a dedicated reference oscillator is
required for each channel.
i.e. for an N channel system N reference oscillators are required.
If correlation techniques were to be employed then an N channel system
requires 2N reference oscillators.
N channel versions of Dual Mixer systems by contrast only need a single
offset oscillator and a single reference oscillator.
Similarly an N channel heterodyne system only requires a single offset
oscillator.
An N channel direct RF phase sampling system (like that employed by the
2 channel TSC5120A) only requires a single samplign clock source.
An N channel time interval counter that periodically (eg at a 1Hz rate)
measures phase differences between 2 RF signals only requires a single
reference source.
The above system can be regarded as an undersampled version of the
direct RF phase sampling system.
The poor cost scaling of the tight PLL system is another reason why it
has fallen out of favour for those who have more than 2 frequency
standards to compare simultaneously.
Bruce
WarrenS wrote:
Great start
Now if we just had a list that someone would add the advantages and
disadvantages to, so that any non relevant stuff could be easily seen
and removed or moved to a third list, It would all become much clearer.
ws
Magnus Danielson wrote:
On 06/12/2010 11:29 PM, Bruce Griffiths wrote:
WarrenS wrote:
subject: Advantages& Disadvantages of the TPLL Method.
Here is a new and unique Idea that may be useful for many.
Rather than focusing on what some members may or may not already know,
or how good or bad one specific working BB configuration is.
How about focusing on what the TPLL method can and can not do well.
If someone will make a place to post and compile a couple of list,
I can start it off with what I've learned so far:
DISADVANTAGES of the TPLL method:
#1) The TPLL method is limited by it's reference OSC.
This isn't necessarily correct, one could use a pair of tight PLL loops
and use correlation techniques to reduce the contribution of the
reference oscillator noise.
True. The same technique is being used for LPLL phase noise
measurements. The reference oscillator will still be a limit, but
wither you can go below the reference oscillator noise or not is what
makes the difference. Such a setup costs about twice of a
single-channel TPLL. Usually there is two ADC channels available.
Yes the cost of the reference oscillator dominates the system cost, the
additional $10 (omitting the cost of the phase detector) to implement
the tight PLL is relatively insignificant.
The cross-correlation processing isn't too hard to achieve and is
efficiently performed using FFTs and a little support-processing. FFTW
is a good tool to toss the FFT processing to. The remaining wrapping
is in a few ten lines of codes or so. Going down the FFT path will
give the frequency plot for free, getting it back into the time-domain
cost extra.
If one is calculating the FFT then it is possible to calculate ADEV
directly from the FFT (of the frequency samples) with little additional
effort, for the relevant formulae see:
http://hal.archives-ouvertes.fr/docs/00/37/63/05/PDF/alaa_p1_v4a.pdf
Note such processing doesn't increase the cost of the system as one
needs a PC to calculate frequency stability measures, unless one
wants/needs to do it in real time.
One disadvantage of a tight PLL system is that finite EFC range and EFC
non linearity may preclude its application to noisier sources.
Linearising the EFC transfer function will help but the reference
oscillator EFC range will ultimately provide an upper limit to the
measurable noise.
The ref osc (or the DUT) needs to have an Analog&/or Digital EFC
control input with a bandwidth that is wider than the desired Tau0
#2) It basically measures Freq and not Phase differences, and few
understand how and why it works so well or it's many advantages.
This is not true, there is no inherent SNR advantage in measuring
frequency changes as opposed to measuring phase differences.
When the phase measurement system and the frequency measurement systems
being compared have the same noise bandwidth then the measurement
floors
are comparable.
For example, the TSC5120A is a narrow band system based on measuring
phase differences with a comparable or lower noise floor than your
implementation of the tight PLL.
The common technique of using a time interval counter to measure the
phase difference between 2 RF signals once ever second or so is a
wideband technique with severe undersampling, consequently the system
noise floor is much higher than for narrow bandwidth techniques. If the
phase difference between the 2 signals were measured more frequently
and
digitally low pass filtered the noise will be much lower.
Using time-stamping counters at high rate would be possible if being
able to cope with the rate of samples. You want a frontend to do that
if you want to run continously.
As for digital filtering. When doing measurements in the 0,1 - 1000 s
range for the G.813 measurements, a 10 Hz low-pass filter is being
required.
Since one has to calculate average frequency from the frequency samples
by integration/averaging this is mathematically equivalent to
reconstructing the phase change between the start and end of the
averaging time (Tau0).
Depends on the details. Some counters (SR620 for instance) can have
biases for frequency data which their time-difference measures do not
have. A TPLL does not suffer from that particular problem, as it
cranks out its frequency estimation by a different method.
Yes, but I thought that we were calculating the required averages from
the frequency (EFC) samples by approximating the required integrals.
One effect of undersampling is to convert (in the sampled data) a
proportion of any flicker phase noise (and other non white phase noise
components) to white phase noise.
The effect of this is to change the ADEV vs Tau plots from their true
shape.
Care to hand a reference or two for this statement?
References for the whitening effect of undersampling:
http://www.obs-besancon.fr/tf/publis/metrologia98a.pdf
http://www.obs-besancon.fr/tf/publis/metrologia98b.pdf
The change in shape of the ADEV vs Tau plot is a consequence of the
whitening of the phase noise.
Regardless, care must be taken to ensure high enough bandwidth
compared to the tau for the measurements not to be affected.
Cheers,
Magnus
Bruce
WarrenS wrote:
Thanks for the positive contrbution, A good example of one of the
TPLL's obvious disadvantages.
The simple cheap analog version of the TPLL is limited by it's need to
have a dedicated Ref OSC.
One way I have got around that problem, which would not apply to all,
is to put the DUT unit as the controlled OSC, and use a special Tbolt
as the reference Oscillator.
The other way around the problem is the Digital version of the TPLL
that uses DSS.
BTW that limitation is not nearly as big as one would think. This is
because the long term accuracy is already limited by the reference
osc, so one would not generally use this kind of system out past 1000
sec or so anyway. So If doing long term multichannel Osc, One would
likely be MUCH better off with a more basic undersampled Phase system
for long term testing and just go thru and cheek each Osc one at time
for a short time with a low tau tester such as this type.
Keep the advantages and disadvantages coming in, so the Time Nuts can
compare which methods work best for their application.
Now if we just had some place to log the responses.
Summery: If you have multi oscillators to test simultaneously that do
not have EFC input, and that you want to do continuous sampling on,
and do not have multiple TSC boxes, the TPLL is not the right tool
for the job.
Be better off with one simple lower resolution multiplexed time
stamped TI phase system and a single TPLL.
If one has a production requirement to test/compare several hundred
oscillators simultaneously, the TSC5120A and its variants, being 2
channel instruments, aren't really that useful even if one could afford
several hundred of them. Such a requirement may be difficult to meet
within a modest budget whilst still achieving the performance
requirements (eg 1E-13/tau system noise).
Even with a much smaller number of oscillators (eg 8 -16) devising an
affordable measurement system may be challenging.
Bruce
Bruce posted:
The poor cost scaling of the tight PLL system is another reason
why it has fallen out of favour for those who have more than
2 frequency standards to compare simultaneously.
Thanks for that opinion, but I don't think we should list the above as
a unique disadvantage.
Maybe need a new column heading for that one, Any name suggestions?
Does not sound all that valid or unique of a reason to me.
It seems the same can be said about a TSC or any new high cost system.
I would think a more important reason is that the simple TPLL is not a
universal do all system.
Because the simple analog version is "Limited by it's reference Osc"
in many ways,
This does give it some possible major disadvantages like not working
so good with a CS or Rb standard.
If one has more time than money, there are ways around that.
ws
Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Jun 13 01:25:13 UTC 2010
Another disadvantage of the Tight PLL system that only applies to
multichannel systems is that a dedicated reference oscillator is
required for each channel.
i.e. for an N channel system N reference oscillators are required.
If correlation techniques were to be employed then an N channel system
requires 2N reference oscillators.
N channel versions of Dual Mixer systems by contrast only need a single
offset oscillator and a single reference oscillator.
Similarly an N channel heterodyne system only requires a single offset
oscillator.
An N channel direct RF phase sampling system (like that employed by the
2 channel TSC5120A) only requires a single samplign clock source.
An N channel time interval counter that periodically (eg at a 1Hz rate)
measures phase differences between 2 RF signals only requires a single
reference source.
The above system can be regarded as an undersampled version of the
direct RF phase sampling system.
The poor cost scaling of the tight PLL system is another reason why it
has fallen out of favour for those who have more than 2 frequency
standards to compare simultaneously.
Bruce
WarrenS wrote:
Great start
Now if we just had a list that someone would add the advantages and
disadvantages to, so that any non relevant stuff could be easily seen
and removed or moved to a third list, It would all become much clearer.
ws
Magnus Danielson wrote:
On 06/12/2010 11:29 PM, Bruce Griffiths wrote:
WarrenS wrote:
subject: Advantages& Disadvantages of the TPLL Method.
Here is a new and unique Idea that may be useful for many.
Rather than focusing on what some members may or may not already
know,
or how good or bad one specific working BB configuration is.
How about focusing on what the TPLL method can and can not do well.
If someone will make a place to post and compile a couple of list,
I can start it off with what I've learned so far:
DISADVANTAGES of the TPLL method:
#1) The TPLL method is limited by it's reference OSC.
This isn't necessarily correct, one could use a pair of tight PLL
loops
and use correlation techniques to reduce the contribution of the
reference oscillator noise.
True. The same technique is being used for LPLL phase noise
measurements. The reference oscillator will still be a limit, but
wither you can go below the reference oscillator noise or not is what
makes the difference. Such a setup costs about twice of a
single-channel TPLL. Usually there is two ADC channels available.
Yes the cost of the reference oscillator dominates the system cost, the
additional $10 (omitting the cost of the phase detector) to implement
the tight PLL is relatively insignificant.
The cross-correlation processing isn't too hard to achieve and is
efficiently performed using FFTs and a little support-processing. FFTW
is a good tool to toss the FFT processing to. The remaining wrapping
is in a few ten lines of codes or so. Going down the FFT path will
give the frequency plot for free, getting it back into the time-domain
cost extra.
If one is calculating the FFT then it is possible to calculate ADEV
directly from the FFT (of the frequency samples) with little additional
effort, for the relevant formulae see:
http://hal.archives-ouvertes.fr/docs/00/37/63/05/PDF/alaa_p1_v4a.pdf
Note such processing doesn't increase the cost of the system as one
needs a PC to calculate frequency stability measures, unless one
wants/needs to do it in real time.
One disadvantage of a tight PLL system is that finite EFC range and EFC
non linearity may preclude its application to noisier sources.
Linearising the EFC transfer function will help but the reference
oscillator EFC range will ultimately provide an upper limit to the
measurable noise.
The ref osc (or the DUT) needs to have an Analog&/or Digital EFC
control input with a bandwidth that is wider than the desired Tau0
#2) It basically measures Freq and not Phase differences, and few
understand how and why it works so well or it's many advantages.
This is not true, there is no inherent SNR advantage in measuring
frequency changes as opposed to measuring phase differences.
When the phase measurement system and the frequency measurement
systems
being compared have the same noise bandwidth then the measurement
floors
are comparable.
For example, the TSC5120A is a narrow band system based on measuring
phase differences with a comparable or lower noise floor than your
implementation of the tight PLL.
The common technique of using a time interval counter to measure the
phase difference between 2 RF signals once ever second or so is a
wideband technique with severe undersampling, consequently the system
noise floor is much higher than for narrow bandwidth techniques. If
the
phase difference between the 2 signals were measured more frequently
and
digitally low pass filtered the noise will be much lower.
Using time-stamping counters at high rate would be possible if being
able to cope with the rate of samples. You want a frontend to do that
if you want to run continously.
As for digital filtering. When doing measurements in the 0,1 - 1000 s
range for the G.813 measurements, a 10 Hz low-pass filter is being
required.
Since one has to calculate average frequency from the frequency
samples
by integration/averaging this is mathematically equivalent to
reconstructing the phase change between the start and end of the
averaging time (Tau0).
Depends on the details. Some counters (SR620 for instance) can have
biases for frequency data which their time-difference measures do not
have. A TPLL does not suffer from that particular problem, as it
cranks out its frequency estimation by a different method.
Yes, but I thought that we were calculating the required averages from
the frequency (EFC) samples by approximating the required integrals.
One effect of undersampling is to convert (in the sampled data) a
proportion of any flicker phase noise (and other non white phase noise
components) to white phase noise.
The effect of this is to change the ADEV vs Tau plots from their true
shape.
Care to hand a reference or two for this statement?
References for the whitening effect of undersampling:
http://www.obs-besancon.fr/tf/publis/metrologia98a.pdf
http://www.obs-besancon.fr/tf/publis/metrologia98b.pdf
The change in shape of the ADEV vs Tau plot is a consequence of the
whitening of the phase noise.
Regardless, care must be taken to ensure high enough bandwidth
compared to the tau for the measurements not to be affected.
Cheers,
Magnus
Bruce
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Warren,
On 06/13/2010 05:45 AM, WarrenS wrote:
Thanks for the positive contrbution, A good example of one of the TPLL's
obvious disadvantages.
The simple cheap analog version of the TPLL is limited by it's need to
have a dedicated Ref OSC.
One way I have got around that problem, which would not apply to all, is
to put the DUT unit as the controlled OSC, and use a special Tbolt as
the reference Oscillator.
The other way around the problem is the Digital version of the TPLL that
uses DSS.
I think one should best break the TPLL into three different cases, as
they have different characteristics.
BTW that limitation is not nearly as big as one would think. This is
because the long term accuracy is already limited by the reference osc,
so one would not generally use this kind of system out past 1000 sec or
so anyway. So If doing long term multichannel Osc, One would likely be
MUCH better off with a more basic undersampled Phase system for long
term testing and just go thru and cheek each Osc one at time for a short
time with a low tau tester such as this type.
The TPLL is a mid-tau stability test, since it's sweet-spot is in the
0,1 - 1000 s range. Short-term is better handled in LPLL phase-noise
measurements.
Keep the advantages and disadvantages coming in, so the Time Nuts can
compare which methods work best for their application.
Now if we just had some place to log the responses.
Could be arranged. Best way would naturally be to whip up a draft
article providing a survey. It may be useful to break up detailed
analysis in separate articles. There is many details in this.
Summery: If you have multi oscillators to test simultaneously that do
not have EFC input, and that you want to do continuous sampling on, and
do not have multiple TSC boxes, the TPLL is not the right tool for the job.
Agreed... for large N.
Be better off with one simple lower resolution multiplexed time stamped
TI phase system and a single TPLL.
Actually that might not be what you want, the articles Bruce referred to
points out some interesting problems which also needs to be understood
as one progresses down that path.
For me, finding those articles was a good side-consequence of this
discussion.
Bruce posted:
The poor cost scaling of the tight PLL system is another reason
why it has fallen out of favour for those who have more than
2 frequency standards to compare simultaneously.
Thanks for that opinion, but I don't think we should list the above as a
unique disadvantage.
Maybe need a new column heading for that one, Any name suggestions?
It's a conditioned disadvantage, but still a disadvantage. Doesn't apply
for 1 or 2 channel systems.
Does not sound all that valid or unique of a reason to me.
It seems the same can be said about a TSC or any new high cost system.
Indeed. This is the N channel condition. The DTMT system proved much
cheaper for many-channel setups. Infact, the many-channel setup
situation has very few solutions even today when looking at commercial
boxes.
I would think a more important reason is that the simple TPLL is not a
universal do all system.
Because the simple analog version is "Limited by it's reference Osc" in
many ways,
This does give it some possible major disadvantages like not working so
good with a CS or Rb standard.
If one has more time than money, there are ways around that.
Sure is. The TPLL method does not fit all needs. It's fine, we just need
to quantify properly what needs it fits. The N-channel case is a
limitation which may or may not apply to a particular case.
For most hobbyists and many commercial usages, the N-channel case is not
a serious limit.
It could also be argued that for small N (say 8) the cost would still
not be prohibiting. Personally I lack mixers and such to setup an
8-channel system, as I already have more than 8 10811 and an 8-channel
ADC system suitable for the task. Oh, I still count myself as a hobbyist.
Cheers,
Magnus