Hi
My main concern with the low frequency pole in the sound card is
the quality of the R/C used. You can certainly model what ever
you have. If they used an aluminum electrolytic for the "C" it
may not be the same next time you check it ....
On a 10 Hz system, a 1 Hz pole is probably not an issue. It might
get in the way with a 1 Hz beat note.
Another thing I have only seen in passing: "Sigma Delta's have
poor low frequency noise characteristics". I haven't dug into it
to see if that's really true or not. If you buy your own ADC's,
you certainly would not be restricted to a Sigma Delta.
Even with a cheap pre-built FPGA board, you could look into
higher sample rates than a conventional sound card. You would
drop back to 16 bits, but it might be worth it.
Bob
On Feb 6, 2010, at 6:46 PM, Bruce Griffiths wrote:
Even better is to toss out the mixers and sample the RF signals
directly.
However suitable ADCs cost $US100 or more each.
To which one has to add an FPGA and an interface to a PC with
sufficient throughput to handle the down converted I + Q samples.
Bob Camp wrote:
Hi
You probably could put a couple of cheap DAC's
(ADCs are preferable as it avoids having to implement the
conversion logic plus comparator required when using a DAC.)
on a board with a FPGA and reduce the data on the fly. I'd
guess that would be be in the same $100 range as a half way
decent sound card. Clock the DAC's off of a 10 MHz reference and
eliminate the cal issue.
If you are down around 10 Hz or worse yet 1 Hz, the AC
coupling of the sound card will get in the way, even with a
bandpass approach. You really don't know what they may have in
there at the low end. Build it yourself and that stuff's not an issue.
Bob
My sound card has a 1Hz cutoff RC high pass input filter plus
an internal high pass digital filter.
Its not too difficult to measure the sound card frequency
response using a white noise source for example.
Bruce
On Feb 6, 2010, at 6:12 PM, Bruce Griffiths wrote:
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"<[email protected]>
To: "Discussion of precise time and frequency
measurement"<[email protected]>
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"<[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
