The only major issue with DMTD systems is that they undersample the
phase fluctuations and hence are subject to aliasing effects.
The low pass filter has to have a bandwidth of the same order as the
beat frequency or the beat frequency signal will be significantly
attenuated.
Since the phase is only sampled once per beat frequency period the phase
fluctuations are undersampled.
Various attempts to use both zero crossings have not been successful.
In principle if one can overcome the increased phase shift tempco
associated with a bandpass filter, using a bandpass filter can in
principle ensure that the phase fluctuations are oversampled.
Bruce
Bob Camp wrote:
Hi
A straight heterodyne system will get you to the floor of most 10811's with a
very simple (2 stage) limiter. As with the DMTD, the counter requirements
aren't really all that severe.
Bob
On Feb 6, 2010, at 4:24 PM, WarrenS wrote:
"It's possible / likely for injection lock ... to be a problem ..."
Something I certainly worried about and tested for.
What I found (for MY case) is that injection lock is NOT a problem.
The reason being is that unlike most other ways, where the two OSC have to be
completely independent,
The tight loop approach forces the Two Osc to "Lock with something like 60 + db
gain,
so a little stray -80db injection lock coupling that would very much limit
other systems has
no measurable effect at e-13. Just one of the neat little side effects that
make the tight loop approach so simple.
"then a part in 10^14 is going to be at the 100 of nanovolts level."
For that example, just need to put a simple discrete 100 to 1 resistor divider
in-between the control voltage and the EFC and now you have a nice workable
10uv.
BUT the bigger point is, probable not needed, cause you are NOT going to do any
better than the stability of the OSC with a grounded shorted EFC input.
as you said and I agree is so true:
"There is no perfect way to do any of this, only a lot of compromises ... you need
to watch out for".
But you did not offer any easier way to do it, which is what the original
request was for and my answer addressed.
This is the cheapest easiest way BY FAR to get high performance, at low tau,
ADEV numbers that I've seen.
ws
***************
----- Original Message ----- From: "Bob Camp"<li...@cq.nu>
To: "Discussion of precise time and frequency measurement"<time-nuts@febo.com>
Sent: Saturday, February 06, 2010 12:09 PM
Subject: Re: [time-nuts] ADEV vs MDEV
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"<warrensjmail-...@yahoo.com>
Sent: Saturday, February 06, 2010 2:19 PM
To: "Discussion of precise time and frequency measurement"<time-nuts@febo.com>
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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