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
_______________________________________________
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
_______________________________________________
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
