[time-nuts] Adjusting HP 5065A frequency
Bob Camp
lists at rtty.us
Mon Oct 22 16:48:37 UTC 2012
Hi
One "simple / dirty" approach, assuming you are starting from 5 or 10 MHz:
Lock up a VCXO at 100 MHz from the source via a wide band PLL. Divide it
down to 1 pps by the usual techniques. Slip the counter (add / subtract one
count) to get things within 10 ns.
Now all you need is a +/- 5 ns adjustment.
Inject a DC offset into the PLL with an DAC to steer it off of it's mid
point. A cheap 16 bit DAC would get you to below 200 fs.
With a wide band (say 1 KHz) loop, the ADEV of the VCXO compared to the
reference should be identical at anything past 100 ms. A well filtered DAC
voltage should not mess that up.
Yes you need to do a little calibration. If you use something like an XOR as
a phase detector, and run the DAC off the same supply as the XOR, cal should
not be too hard.
No, NIST will not be drooling at your gizmo any time soon. You will have
your PPS steered far closer to UTC than any of us is likely to be able to
measure.
Cost wise there's not a lot there. The main thing to work out is the "pop"
when you step the counter (roll over a 10 ns boundary). The gizmo is cheap
enough that you run two of them and ping-pong when you do a roll over (keep
one running while the other settles).
The "big bucks" approach probably is to run a good RF ADC on the input and
then do all the offset stuff as DSP math. The VCXO just sits at it's magic
frequency and never moves. More money / no pops.
Bob
-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of EWKehren at aol.com
Sent: Monday, October 22, 2012 7:05 AM
To: time-nuts at febo.com
Subject: Re: [time-nuts] Adjusting HP 5065A frequency
Tom
I have two questions what should the range, resolution and stability of
the delay generator be and how much do you think a digital loop driven by a
Tbolt would degrade short and medium precision. What is your definition of
short and medium?
Bert Kehren
In a message dated 10/22/2012 12:25:08 A.M. Eastern Daylight Time,
tvb at LeapSecond.com writes:
Three companies come to mind for phase microsteppers. A popular one
decades ago was made by Austron (model 2055A). I got mine on eBay but they
are
not as common now as ten years ago.
The current models by Symmetricom and Spectra Dynamics are extremely
high-end (expensive) and overqualified for use with a vintage rubidium
oscillator. If you visit NIST or USNO you will see these impressive units.
It would be a very fun project to make your own. I suspect other group
members could either help you or would eagerly employ your design for their
own use.
But -- before you decide on a hardware solution see if you can do it in
software.
An analogy is what we do with GPS 1PPS sawtooth errors. There are two ways
to deal with this. One is to capture the correction message over RS232,
measure the DUT vs. GPS 1PPS with a TIC, and then numerically apply the
sawtooth correction with one line of code. Several of the popular GPS
monitor
programs do this automatically for you (TBoltmon and TAC32, for example).
The
software solution is perfect to the granularity of the sawtooth message,
typically 1 ns.
The hardware implementation usually involves a PIC and a programmable
delay generator. The PIC listens for the correction message over RS232 and
then
has plenty of time (up to one second) to program the delay chip. When the
hardware 1PPS arrives it is delayed to compensate for the aforementioned
sawtooth error. The result is a hardware 1PPS that's quite close to the
ideal
1PPS, limited again by the granularity of the message, as well as offset
or linearity errors in the delay chip.
So that's the analogy. To apply this to your rubidium, ask yourself which
instruments or measurements or users are downstream of your 5065A 10 MHz
output. Can they deal with daily software corrections to a stable but
slightly imprecise frequency, or do they really need the frequency to be as
accurate as possible at all times.
There's a third alternative as well. You might consider using your 5065A
as the LO in a GPSDO. This will sacrifice some short- and mid-term
precision
due to additive noise, but it will guarantee the best possible long-term
accuracy.
/tvb
----- Original Message -----
From: Edgardo Molina
To: Tom Van Baak ; Discussion of precise time and frequency measurement
Sent: Sunday, October 21, 2012 8:49 PM
Subject: Re: [time-nuts] Adjusting HP 5065A frequency
Dear Tom,
Good evening. In relation to your last comments on this and other subjects,
I am sharing some thoughts and experience about it. I took the liberty to
separate the topics as to ease the interested parties to follow up
accordingly. TNX.
a. Information you kindly provided and the index for newbies:
Thank you! You just provided me with lots of new ideas and information on
the subject. You have very valuable information in your web site. As Hal
was saying, an index should be done anywhere so it could be easier for the
rest of us to locate the information. I am planning soon to build a web
page
for my lab. In english of course for everybody to share my experiences. I
could work on an index to point out to the various sources of information
and topics that are difficult to find. That I think could expedite things a
little bit.
b. Phase Micro steppers:
I saw the phase micro steppers working at CENAM time scale. I was
wondering that the technique could be translated to my 5065As and not
trying to
touch them so often. If I am assuming correctly and the technique could be
used with the HP Rb standards. Are those phase micro steppers easy to find?
I
mean, affordable in the second market? If there is one of course. I saw the
ones used at CENAM are produced by SpectraDynamics in Colorado. According
to Mike Lombardi it is a small highly specialized company with a small
market to serve. I could translate it as "expensive and exotic" : ) Am I
correct?
c. Thunderbolt and my will to share initial experiences:
I am gathering a lot of information on the Thunderbolts as I am using them
in my thesis work. I bought a couple of them. If my information or novice
experience with these receivers is good for anybody, I would be more than
glad to share it.
Thank you.
Kind regards,
Edgardo Molina
Dirección IPTEL
www.iptel.net.mx
T : 55 55 55202444
M : 04455 20501854
Piensa en Bits SA de CV
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On Oct 21, 2012, at 7:29 PM, "Tom Van Baak" <tvb at LeapSecond.com> wrote:
Hi Edgardo,
What you'll find is that many labs do not periodically adjust the C-field
of their 5061A or 5065A at all.
Instead, any phase or frequency adjustment is done with phase
microsteppers or simply done in software with time and rate adjustments to
the raw
data. These methods avoid all possible physical side-effects of changing
voltages, currents, and fields. It also makes it possible to gather
long-term
data to show how the standard is operating (if you make mechanical rate
adjustments it complicates data that you have already collected).
The other point is that when making stability measurements, there is no
requirement that the reference (e.g., 5065A) be perfectly on-frequency. So
this removes motivation for physically touching and possibly perturbing the
operation of the reference.
Please also take the time to read these pages.
"HP 5065A Rubidium C-Field Resolution"
http://leapsecond.com/pages/hp-5065a-cfield/
"Rubidium Oscillator Stability"
http://leapsecond.com/images/4rb.gif
"Stability and Noise Performance of Various Rubidium Standards"
http://www.ke5fx.com/rb.htm
"Performance of Low-Cost Rubidium Standards"
http://febo.com/pages/oscillators/rubes/
"A close look at a drifting HP 5065A Rubidium Frequency Standard"
http://leapsecond.com/pages/doug-rb/
/tvb
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