If one has a high end sound card then it could be used to implement the
bandpass filter and replace the zero crossing detector.
It may be necessary to insert a pilot tone to calibrate the sound card
sampling clock frequency.
A noise floor of about 1E-13/Tau should be achievable.
This simplifies the DMTD system by replacing the zero crossing detector
with a low gain linear preamp.
If one analyses the resultant data off line then one can also try out
different techniques such as a Costas receiver rather than a simple
bandpass filter plus zero crossing detector.
However 1000 seconds of data for 2 channels of 24 bit samples at 192KSPS
will result in a file with a size of at least 1.15GB.
Bruce
Bruce Griffiths wrote:
If one were to use a bandpass filter with a Q of 10 to filter the beat
frequency output of the mixer, then if the input frequency is 10MHz
and the filter component tempco is 100ppm/C then the resultant phase
shift tempco is about 16ps/C referred to the mixer input frequency.
This phase shift tempco is certainly low enough not to have
significant impact when measuring the frequency stability of a typical
10811A if the temperature fluctuations are kept small enough during
the run.
The effect of using a bandpass filter with too narrow a bandwidth is
to artificially reduce ADEV for small Tau, so it may be prudent to use
a higher beat frequency that 1Hz or even 10Hz and not calculate ADEV
for Tau less than say 10(??) times the beat frequency period. A trade
off between this and the effect of aliasing is required.
Bruce
Bob Camp wrote:
Hi
With most 10811 range oscillators the impact of a simple bandpass
filter is low enough to not be a major issue. That's for normal lab
temperatures with the circuitry in a conventional die cast box. No
guarantee if you open the window and let the fresh air blow in during
the run.
That's true with a heterodyne. I can see no obvious reason it would
not be true on DMTD.
Bob
On Feb 6, 2010, at 5:12 PM, Bruce Griffiths wrote:
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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