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.
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.
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.
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?
Regardless, care must be taken to ensure high enough bandwidth compared
to the tau for the measurements not to be affected.
Cheers,
Magnus
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