Magnus posted:
The main point I am trying to make is that it may not be useful to read the
number of the data-sheet,
One of my short comings, I never even considered one would do that.
I guess it would be possible if one only needed say 50% accuracy for small
differences.
Right it needs to be calibrated, It's analog.
Hand-trimming the coarse offset should be done if far away.
Yes, the TPLL BB has provisions for
course manual offset, med H/W offset and Software offset
Can put that one under the disadvantage list also.
It is not like Digital, One has to understand the differences and when to
use which offset.
Yes, but since we derive our measurements from that EFC our
sampled data will change so it will creep into the sample-series.
What I find effective is to recheck the TPLL reference by exchanging the DUT
with a known freq every day or so (with my reference), or can be done as
much as one needs ( 100 sec, hr etc) as long as it is properly taken into
account.
It takes only a second or so to place a full accuracy calibration marker on
the plot or the data log,
that takes into account all drifts including the TPLL ref osc, based on a
separate fixed reference freq that can be CS or anything.
You may get very low on some of these, but what I was aiming
for was a more average price for the average builder.
The point I was trying to make that the total cost was
more around 200 USD including ref oscillator and ADC.
I just think 200 USD is a more realistic cost (value).
No disagreement,
My point which is different than yours, is it all depends on how much effort
& design time one wants to put into it.
Total parts cost can be kept under $10 in small production runs (even
without eBay) including all but the Reference OSC.
For the reference OSC there is all kinds of lower and higher cost devices
that would be suitable for some.
We seem to agree on all the major points so far.
Hard to have much of a discussion when two agree.
So time to bring up something that we may NOT agree on.
Bandwidth and the bandwidth filter freq used with the TPLL has little effect
as long as its freq is > than about 2x tau0 freq,
Unlike most other Phase methods, where the optimal Bandwidth is tau0 or 1/2
trau0.
Before some go ballistic over the comment, consider what the effect of the
Tau0 integration is on the oversampled TPLL Frequency data.
Example: because most are confused by my mixed use of freq, time constant,
tau0 and BW
Tau0 = 0.1 sec, 100 ms, 10 sample per second (all the same)
Phase methods need an optimal Bandwidth of 5Hz, or 10 Hz or 100ms or ...
depending on which paper you read.
The optimal wsTPLL method BW for tau0 => 20 Hz
(greater BW does not change the ADEV results, like it does with Phase).
and the TPLL optimal BW for tau0 of 1 sec is => than 2 Hz
and the optimal BW for TPLL of tau0 of 10ms is => than 200 Hz
Note that BIG difference is, unlike Phase methods, a single filter freq can
be selected that works for all TPLL tau0
It does not have to be changed for each tau0
(in the above example the H/W BW filter should be => than 200 Hz for any
tau0 <=10ms)
AND I'm suggesting that goes at the top of the advantages list, right above
simple.
Now I expect that will restart the name calling from some before they even
think.
ws
**********************
[time-nuts] Advantages & Disadvantages of the TPLL Method
Magnus Danielson magnus at rubidium.dyndns.org
Mon Jun 14 22:25:11 UTC 2010
On 06/14/2010 06:13 PM, WarrenS wrote:
Long explanations, cause I try to explain, the best I can, when I say
something is "WRONG or misleading"
Magnus Posted:
EFC linearity will remain an issue for analog oscillators.
The oscillator gain will differ depending on offset voltage and
temperature.
TRUE it is an issue, but somewhat misleading because it need NOT be a
problem or limitation (mostly)
EFC Linearity can be an issue because the TPLL is limited by the
"performance" of the reference oscillator in lots of ways.
BUT
Oscillator EFC gain or linearity are not likely to be of much concern or
a limitation for high end performance.
The gain nonlinearity I've measured can vary two to one over the full
range of a good Oscillator but it is more like 10% over the normally
used range, if one stays well away from the end points.
NOT so good but livable if you are not making something real accurate.
BUT
For all my accurate stuff, when using a HP 10811, I limit the full-scale
change to 1e-9 or 1e-8 at most.
This uses such a small part of the total EFC range, that the
nonlinearity effects are generally below the noise level and of little
concern at all.
The fact that Oscillator gain does differ with the EFC voltage (offset
voltage), means if you want to get max accuracy out of the TPLL, it will
need to be calibrated at the EFC offset voltage it is being used at. One
simple solution, if the OSC also has a independent manual Freq
adjustment like the single oven 10811, is to use it always set the EFC
voltage to be near zero volts.
BTW calibration need not be much of a problem, because it can be a
static calibration. What I use for a finial calibration & check is the
2G turn over, which I measure very accurately by other means before hand
and then use that as a known freq offset to check operation and
calibration. Of course there are any number of other ways.
The main point I am trying to make is that it may not be useful to read
the number of the data-sheet, but calibration methods should be
performed. Should not be particularly hard to do, but if you do not make
provisions for it, it will become a scaling error issue.
Hand-trimming the coarse offset should be done if far away.
As far as temperature having ANY effect on EFC gain, that is a total NON
issue.
If temperature had any effect on EFC Gain then Temperature would also
effect Osc Frequency at a fixed EFC voltage,
which would then effect the OSC freq drift and stability,
that would then effect anything that the Osc was used for, NOT just the
TPLL.
The TPLL actually has a slight advantage over other methods,
because the PLL will adjust the freq to be correct, even if the EFC
effect should change.
Yes, but since we derive our measurements from that EFC our sampled data
will change so it will creep into the sample-series. Hand-calibrating
towards zero EFC and let it stabilize should work well enough.
Identifying potential problems is the first step to finding ways to
avoid or compensate for them.
I think it is reasonable to assume that a TPLL weighs in at about
200 USD with all support mixers, amplifiers, ADCs etc. if you don't
have the parts
It is still a fairly cheap solution.
Yes I think that is ONE reasonable number to use and a fair conclusion.
BUT there are others.
The EBAY cost of the TPLL can be easy under $10, not including the
reference Osc and the ADC.
The point I was trying to make that the total cost was more around 200
USD including ref oscillator and ADC.
Do note, NONE of items above are plural, Only one is needed per system
unlike some other methods.
Because the cost of the Ref Osc is so variable and depends so much on
what one is doing, I have noticed that its cost is generally not
included in the base price. I think even on the $20K+ TSC 5120A that the
reference Osc is an extra cost option.
The reference oscillator plays a different role in that system.
The ADC is another BIG variable, depending on your needs and skill level
and junk box, almost no limit in cost at the high end,
and can be as low as $0.00 dollars if you are a student doing a science
project.
It can also be as low as $1.00 if one is good at programming PICS or
other micros with built in ADC's.
Indeed. Cheap audio-boards could be hacked up.
The only other major part in the TPLL with any cost over $1 is the Phase
detector.
The one I use most is a micro-circuits $15 single price device, but I've
used all sorts of dual balanced mixers,
and if one is real cheap and good at design, I have found that a PD
based on a 50 cent XOR gate works fine.
You may get very low on some of these, but what I was aiming for was a
more average price for the average builder. You are usually not all that
lucky when you want to. Also, recall that packaging and transport adds
on top of that, including import taxes and VAT... which I did not included.
Your milage may vary a lot. I just 200 USD is a more realistic value. It
doesn't make it less valuable as a tool, I am not trying to say that at all.
Cheers,
Magnus
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