FE-5650A option 58 tuning word for 10 MHz output
Hello guys
I'm new to this list. I got myself a FE-5650A Rubidium Standard off of
ebay.
It's the "option 58" 1 pps output variant, hence I have to modify the
tuning
word used in the DDS phase accumulator to get 10 MHz out. I found a
vast amount
of awesome descriptions on how to do that on the web and in particular
on this
list. One write-up that stood out was this one by Mark Sims:
http://www.mail-archive.com/time-nuts@febo.com/msg13486.html
I think I can pull this off since everything is documented so nicely.
However,
I'm having trouble calculating the right tuning word and this is why:
Mark notes
that the reference frequency reported by the unit is the one with the
C-field
pot at the lowest frequency position. He gives a number of suggestions
on how to
deal with that. Since I didn't get that hydrogen maser for Christmas,
the best
approach seems to be "to calculate the true reference frequency from
the saved
(minimum C-field) R=reference frequency and F=divisor word and use that
value to
calculate divisor words." I don't understand how the saved minimum
C-field
reference ties into this calculation.
My approach would have been to calculate the true reference frequency
from the
saved divisor alone, ignoring the minimum C-field calculation. I don't
see how
the minimum C-field reference frequency would help me since the C-field
pot is
not in the min position anymore due to factory tweaking. To be
specific, this is
what I would do:
The unit returns the following string upon 'S':
OK50255055.760840Hz F=2ABB5046B34A2E00
Now based on this, the tuning word should be coded in the first 8
characters, of
F, i.e. '2ABB5046'. I'm a bit confused about the remaining characters
being
non-zero. Any documentation I came across has a number that ends in 8
zeroes...
In any case, 0x2ABB5046 is 716918854 in decimal and the resolution
would
therefore be
2^23 / 716918854 = approx. 0.0117 Hz which makes sense.
The physics package would then output a frequency of
f_ref = (2^23 / 716918854) * 2^32 = approx. 50255055.809934 Hz
This is higher than the reference given in the 'S' output, which is in
line with
what Mark wrote. However, scaling this with the average correction
factor he
gave yields
f_ref * 1.000000002150 = approx. 50255055.917982 Hz
Which is higher than what I would expect. Then again I'm not entirely
sure what
I would expect because various errors add up in the above calculation.
I'd be
interested in what people with more experience think about those
results.
I would then use
M = 10000000/(2^23/716918854) = approx. 854633872.509003
to find the 10 MHz tuning word, which I would then round up
(unfortunately it's
smack in the middle between two integers...) and convert to hex,
yielding
0x32F0AD91. This does in fact result in a 10.000 MHz output waveform
but I have
no means to check its accuracy (yet?). I'd appreciate any hints about
where
things could have potentially gone wrong, especially with respect to
the minimum
C-field reference frequency that I ended up not using.
On a slightly related note, I have cooked up a small PCB with a local 5
V
regulator and status LEDs that mates with the amphenol connector used
on this
standard. I have to complete the write-up on it and will probably put
up a
video about the mod on my youtube channel; once this is done I'll be
sitting on
9 spare boards since I got 10 boards done. If there is interest, I
could send
off the spares without profit, i.e. for about 5 bucks or so. I imagine
this
could be of use to those who have the same standard. The board doesn't
do
anything funky, it is just neat. In any case I'd like to ask if it
would be OK
to formally place this offer on the list once I got everything ready.
Thanks a lot and best regards!
Matt
Hoi Mathias!
Wie goht's wie stoht's? :-)
On Sat, 31 Dec 2016 15:33:53 +0100
Mathias Weyland mathias@weyland.ch wrote:
I'm new to this list. I got myself a FE-5650A Rubidium Standard off of
ebay.
It's the "option 58" 1 pps output variant, hence I have to modify the
tuning
word used in the DDS phase accumulator to get 10 MHz out.
May I ask what you want to achieve? Resp. what you need a 10MHz reference for?
In general I would agree with Bill Houlne's comment that it's probably
easier to use a different reference that already has a 10MHz output.
Also keep in mind, that an output that has been designed as an PPS output
might not work as well as an output for 10MHz.
On a slightly related note, I have cooked up a small PCB with a local 5V
regulator and status LEDs that mates with the amphenol connector used on this
standard. I have to complete the write-up on it and will probably put up a
video about the mod on my youtube channel; once this is done I'll be sitting on
9 spare boards since I got 10 boards done. If there is interest, I could send
off the spares without profit, i.e. for about 5 bucks or so. I imagine this
could be of use to those who have the same standard. The board doesn't do
anything funky, it is just neat. In any case I'd like to ask if it would be OK
to formally place this offer on the list once I got everything ready.
I am not master of desaster on this list, but it's usually ok to offer
this kind of stuff, as long as it is time-nut related.
Gruess und guets neus!
Attila Kinali
--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neil Stephenson
On 2017-01-02 12:18, Attila Kinali wrote:
Attila!
Nice to see you around here ond au der es guez nois!
May I ask what you want to achieve? Resp. what you need a 10MHz
reference for?
I've always been wondering about those devices, I guess out of pure
curiosity. Once I learned that those were available for cheap I knew
I had to get one. But there is a secondary goal, which is to have
a standard around to check my other gear against every now and then.
Lately, I've been building and repairing 1.3 GHz radios and lost
some time because I did not realize how much off both of my service
monitors where. I could get away with a properly calibrated OCXO
but the Rb was so much more sexy. So for the most part I need the
10 MHz for my own entertainment. :-)
In general I would agree with Bill Houlne's comment that it's
probably
easier to use a different reference that already has a 10MHz output.
Also keep in mind, that an output that has been designed as an PPS
output
might not work as well as an output for 10MHz.
Yes I agree with that, but I'd also say that based on the reading I
have done, it would be foolish to assume to get something that is
guaranteed to work anyway. With all the things that could go wrong in
general (dead unit, worn-out discharge lamp, drifted sweeping
boundaries
of the oscillator etc.), the 1 pps to 10 MHz conversion for this
particular
model(!) seems fairly straight forward. In fact the only issue I've
been
facing so far is the one that I'm trying to resolve in this thread. And
even if this cannot be resolved, the converted unit is still suitable
for
my application.
On the output issue you are rising: This particular model has a DDS
that
takes a 50ish MHz reference and synthesizes another frequency which is
a power of 2. A coax cable is taking that to a divider board where it's
brought down to 1 Hz. This is all happening outside of the physics
package. The plan is to reconfigure the DDS to synthesize 10 MHz
instead
and bring out that coax. I would have to open a hole on the face plate
to do that, but I'm fairly confident that this outer enclosure does not
contribute to the actual magnetic shielding of the chamber... All
modifications are strictly outside of the physics package (i.e. no
water jet cutting...)
I am not master of desaster on this list, but it's usually ok to
offer
this kind of stuff, as long as it is time-nut related.
OK thank you and hope to meet you soon!
Matt
Hoi Mathias!
On Tue, 03 Jan 2017 17:55:32 +0100
Mathias Weyland mathias@weyland.ch wrote:
On 2017-01-02 12:18, Attila Kinali wrote:
May I ask what you want to achieve? Resp. what you need a 10MHz
reference for?
I've always been wondering about those devices, I guess out of pure
curiosity. Once I learned that those were available for cheap I knew
I had to get one. But there is a secondary goal, which is to have
a standard around to check my other gear against every now and then.
Lately, I've been building and repairing 1.3 GHz radios and lost
some time because I did not realize how much off both of my service
monitors where. I could get away with a properly calibrated OCXO
but the Rb was so much more sexy. So for the most part I need the
10 MHz for my own entertainment. :-)
So it's the usual curiosity and the need to have a reference for
your 23cm radios? For the latter I would recommend using a GPSDO
like the Trimble Thunderbolt or the Jacksonlabs Fury. There are also
a few Trimble OEM GPSDOs on ebay, but I have very little knowledge
about those. Advantage of an GPSDO over an Rb is that you know it's
on frequency, while with an Rb it can be off without you knowing it,
if the electronics or the physics package is defect. Of course, the
disadvantages are that most GPSDOs are made for fixed positions and
need a couple of hours of operation to achieve their nominal accuracy.
The short/mid-term stability (between 10s and ~1000s) is also slightly
worse than compared to an Rb.
To get the best of both worlds, there are a couple of GPSD-Rb around,
but commercial ones are usually quite expensive and modifying regular
GPSDOs to Rb is not always possible (at the very least you need to change
the control loop amplification).
BTW: there is a whole new way of looking at statistics of time series
in the field of time and frequency control, which you might find interesting.
The NIST Technote 1337[1] and NIST special publication 1065[2] give a nice
overview of the methods applied.
In general I would agree with Bill Houlne's comment that it's
probably
easier to use a different reference that already has a 10MHz output.
Also keep in mind, that an output that has been designed as an PPS
output
might not work as well as an output for 10MHz.
Yes I agree with that, but I'd also say that based on the reading I
have done, it would be foolish to assume to get something that is
guaranteed to work anyway. With all the things that could go wrong in
general (dead unit, worn-out discharge lamp, drifted sweeping
boundaries
of the oscillator etc.), the 1 pps to 10 MHz conversion for this
particular
model(!) seems fairly straight forward.
Most of the units sold on ebay work fairly well. And even a worn out
discharge lamp can often be recovered using a hot air gun.
On the output issue you are rising: This particular model has a DDS
that
takes a 50ish MHz reference and synthesizes another frequency which is
a power of 2. A coax cable is taking that to a divider board where it's
brought down to 1 Hz. This is all happening outside of the physics
package. The plan is to reconfigure the DDS to synthesize 10 MHz
instead and bring out that coax.
Ah..ok. That should work then.
I would have to open a hole on the face plate
to do that, but I'm fairly confident that this outer enclosure does not
contribute to the actual magnetic shielding of the chamber... All
modifications are strictly outside of the physics package (i.e. no
water jet cutting...)
Unfortunately, it's not that easy :-)
I don't know the exact construction of the FE-5650, but usually the physics
package is just a normal aluminium microwave cavity, without shielding.
The mu-metal shield is usually the housing of the whole device itself.
Why this kind of construction is usually employed I do not know, but it
makes any modification of the case cumbersome. Mu-metal itself is quite
sensitive to shock and vibration and will lose (part of) its permeability
when being machined. You can of course try to drill as slowly as possible
to keep the heat and vibration at a minimum, but I have no idea how well
this works and I do not have any hard numbers on the sensitivity of
mu-metal either.
Attila Kinali
[1] "Characterization of Clocks and Oscillators", NIST Technote 1337,
by Sullivan, Allan, Howe, Walls, 1990
http://tf.nist.gov/general/pdf/868.pdf
[2] "Handbook of Frequency Stability Analysis" NIST Special Pub 1065,
by W.J. Riley, 2008
http://tf.nist.gov/timefreq/general/pdf/2220.pdf
--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neil Stephenson
On 2017-01-04 11:08, Attila Kinali wrote:
Hoi Attila
So it's the usual curiosity and the need to have a reference for
your 23cm radios?
Yes exactly.
Advantage of an GPSDO over an Rb is that you know it's on frequency,
while with an Rb it can be off without you knowing it, if the
electronics or the physics package is defect.
This is in line with what I've read. Maybe the decision to go for the
Rb was not the best one but I don't think that it's that bad either.
I was offered the opportunity to calibrate my modified Rb standard
against a fellow ham's two 10 MHz standards and his GPSDO.
BTW: there is a whole new way of looking at statistics of time series
in the field of time and frequency control, which you might find
interesting.
The NIST Technote 1337[1] and NIST special publication 1065[2] give a
nice
overview of the methods applied.
This is very interesting thank you for sharing those!
Unfortunately, it's not that easy :-)
I don't know the exact construction of the FE-5650, but usually the
physics
package is just a normal aluminium microwave cavity, without
shielding.
The mu-metal shield is usually the housing of the whole device
itself.
Hm I thought I had done my homework on this one but I never have
issues with being proven wrong. For what it's worth, here are two
pictures
of the unit in question:
http://2.bp.blogspot.com/_8uoOi3ZInLQ/SdZfVfpcBaI/AAAAAAAABB8/YvZ2-l0GhMI/s400/DSCN4592.JPG
http://www.dty.sakura.ne.jp/keisoku/rubidium/FE5650-2.jpg
The shiny face plate is where I would be drilling. For what it's worth,
it is very light, nonmagnetic, does not look like the mu-metal I've
seen in disassembled hard disks and very much looks like machined
aluminum... Would a compromised mu-metal shield show during calibration
if the standard was physically rotated?
Best regards and thank you
Matt