To a first approximation injection locking alters the loop parameters so
its important to measure the actual PLL characteristics with the loop
closed and not just use the PLL parameters inferred from the OCXO EFC
transfer function etc.
The noise of the OCXO used as a VCXO will limit the noise floor.
An ADEV noise floor of 1E-13 isnt likely when using an HP10811A as the
VCXO for example.
Bruce
Bob Camp wrote:
Hi
It's possible / likely to injection lock with the tight loop approach
and get data that's much better than reality. A lot depends on the
specific oscillators under test and the buffers (if any) between the
oscillators and mixer.
If your OCVCXO has a tuning slope of 0.1 ppm / volt then a part in
10^14 is going to be at the 100 of nanovolts level. Certainly not
impossible, but it does present it's own set of issues. Lab gear to do
it is available, but not all that common. DC offsets and their
temperature coefficients along with thermocouple effects could make
things exciting.
There is no perfect way to do any of this, only a lot of compromises
here or there. Each approach has stuff you need to watch out for.
Bob
--------------------------------------------------
From: "WarrenS" <[email protected]>
Sent: Saturday, February 06, 2010 2:19 PM
To: "Discussion of precise time and frequency measurement"
<[email protected]>
Subject: Re: [time-nuts] ADEV vs MDEV
Peat said:
I would appreciate any comments or observations on the topic of
apparatus with demonstrated stability measurements.
My motivation is to discover the SIMPLEST scheme for making
stability measurements at the 1E-13 in 1s performance level.
If you accept that the measurement is going to limited by the
Reference Osc,
for Low COST and SIMPLE, with the ability to measure ADEVs at that
level,
Can't beat a simple analog version of NIST's "Tight Phase-Lock Loop
Method of measuring Freq stability".
http://tf.nist.gov/phase/Properties/one.htm#oneone Fig 1.7
By replacing the "Voltage to freq converter, Freq counter & Printer
with a Radio shack type PC data logging DVM,
It can be up and running from scratch in under an Hr, with no high
end test equipment needed.
If you want performance that exceeds the best of most DMTD at low Tau
it takes a little more work
and a higher speed oversampling ADC data logger and a good offset
voltage.
I must add this is not a popular solution (Or a general Purpose one) but
IF you know analog and have a GOOD osc with EFC to use for the
reference,
as far as I've been able to determine it is the BEST SIMPLE answer
that allows High performance.
Limited by My HP10811 Ref OSC, I'm getting better than 1e-12 in 0.1
sec (at 30 Hz Bandwidth)
Basic modified NIST Block Diag attached:
The NIST paper sums it up quite nicely:
'It is not difficult to achieve a sensitivity of a part in e14 per Hz
resolution
so one has excellent precision capabilities with this system.'
This does not address your other question of ADEV vs MDEV,
What I've described is just a simple way to get the Low cost, GOOD
Raw data.
What you then do with that Data is a different subject.
You can run the raw data thru one of the many ADEV programs out
there, 'Plotter' being my choice.
Have fun
ws
*************
[time-nuts] ADEV vs MDEV
Pete Rawson peterawson at earthlink.net
Sat Feb 6 03:59:18 UTC 2010
Efforts are underway to develop a low cost DMTD apparatus with
demonstrated stability measurements of 1E-13 in 1s. It seems that
existing TI counters can reach this goal in 10s. (using MDEV estimate
or 100+s. using ADEV estimate). The question is; does the MDEV tool
provide an appropriate measure of stability in this time range, or is
the ADEV estimate a more correct answer?
The TI performance I'm referring to is the 20-25 ps, single shot TI,
typical for theHP5370A/B, the SR620 or the CNT81/91. I have data
from my CNT81showing MDEV < 1E-13 in 10s. and I believe the
other counters behave similarly.
I would appreciate any comments or observations on this topic.
My motivation is to discover the simplest scheme for making
stability measurements at this performance level; this is NOT
even close to the state-of-the-art, but can still be useful.
Pete Rawson
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