Re: [time-nuts] Switching oscillators

Tom Miller Sun, 09 Dec 2012 18:06:33 -0800

Ok, thanks for the explanation.

Tom

----- Original Message -----From: "Bob Camp" <li...@rtty.us>To: "Discussion of precise time and frequency measurement"<time-nuts@febo.com>

Sent: Sunday, December 09, 2012 8:51 PM
Subject: Re: [time-nuts] Switching oscillators


Hi

That's one interpretation and one method of measurement. The other method isto measure frequency shift from say one hour after power on to 24 hoursafter power on. A lot depends on the requirements of the system the OCXO wasintended to be used in.


If you use the measure / power off / warmup / measure approach, you need to:

1) Define the time on before the first measure
2) Define the power off time
3) Define the warmup time

Change any of those numbers and the retrace number will be different.Generally the temperature(s) involved are also defined. The most common caseis that all of this is done at 20 or 25C.


Bob

On Dec 9, 2012, at 8:47 PM, "Tom Miller" <tmil...@skylinenet.net> wrote:

So if I understand it, you allow the OCXO several days of warm-up to setthe frequency. Then when turned off for a while, then restarted. Aftersome warm-up period, the retrace spec would give an indication of howclose the frequency will be?


Thanks,
Tom

----- Original Message ----- From: "Bob Camp" <li...@rtty.us>

To: "Discussion of precise time and frequency measurement"<time-nuts@febo.com>

Sent: Sunday, December 09, 2012 7:58 PM
Subject: Re: [time-nuts] Switching oscillators


Hi

Retrace assumes that the oscillator has some "normal" frequency that onlymoves according to the aging rate. Retrace occurs after the oscillator hasbeen off power for some period. The rate of change is greater than theaging rate. Warmup and retrace are obviously inter-related. Warmup isgenerally described as a short term (sub 1 hour) process. Retrace is oftenlooked at as a day to multiple day sort of thing.

Since none of this is tightly defined, you will see various specs lookingat the same issues a bit differently. Often those differences roll up tosome sort of system requirement.


Bob


On Dec 9, 2012, at 7:45 PM, "Tom Miller" <tmil...@skylinenet.net> wrote:

Hi Bob,

Can you give a good definition of retrace as it applies here?

Thanks,
Tom


----- Original Message ----- From: "Bob Camp" <li...@rtty.us>

To: "Discussion of precise time and frequency measurement"<time-nuts@febo.com>

Sent: Sunday, December 09, 2012 7:24 PM
Subject: Re: [time-nuts] Switching oscillators


Hi

Your TCXO will have the same sort of retrace issues as your OCXO. Pastsome number of minutes (5,10,15…) you will always be better with a modernOCXO than with a TCXO.


Bob

On Dec 9, 2012, at 7:05 PM, WB6BNQ <wb6...@cox.net> wrote:

Hi Joe,

I think you all are not looking at this correctly.

1.    First, as has been pointed out, a TCXO will vary around till the
environment it is in has returned to its nominal operating temperature.

2.    A typical TCXO is nominally spec'ed around +/- 0.5 x 10-7
neighborhood.  Not a stellar number.

3.    The real spec to look at is the "RETRACE" factor of a good OCXO.

  Many of the modern PCB CAN manufactures do not or are quite hazy on
  this point.  Vectron, for example, on their double oven high
  performance WIDGET (model DX-170) claims a warmup time of 5 minutes
  to +-10ppb of final frequency, however, they also include this
  cryptic statement "(1 hour reading) @ +25DEGC" on the same spec.  I
  am not sure, but it suggests that they are reading the final
  frequency at the one hour point after turn-on.  Taking it at face
  value, it suggest that the oscillator is within +/- 1 x 10-8 at 5
  minutes.  That is a whole decade better than the TCXO under any
  condition.

  Looking at something real like the HP 10811A/B Quartz Crystal
  Oscillator, you will see they spec the retrace as "Warmup 10 min.
  after turn-on within 5 X 10-9 of final value, at 25DEGC and 20 Vdc.
  See Notes 1 & 2.  Notes: 1. For oscillator off-time less than 24
  hours. 2. Final value is defined as frequency 24 hours after
  turn-on."  Here, we are talking about two whole decades better than
  the TCXO ever could be.

4.    So the real question is just what the hell are you doing at 10
minutes or less that would require all this worry ? ?

5.    The real answer to your dilemma is to have some serious battery
backup and if it is truly to be considered a critical necessity, then
maybe you need a power generator that is automatic when the shore power
drops.

My two cents,

Bill....WB6BNQ

Joseph Gray wrote:

  For my use, I don't think the switching glitch will be a concern.
  Most
  of the time, the equipment will be in a standby mode. There is a
  high
  probability that the OCXO will be in use most of the time.

  I guess my original idea of simply waiting a sufficient time for the
  OXCO to warm up and then switching is probably the thing to do.

  Joe

  On Sun, Dec 9, 2012 at 3:16 PM, Bob Camp <li...@rtty.us> wrote:

Both would need some sort of timer to drive them. Both would

  disrupt the instrument when the switch took place. I know of no
  "cheap / easy" ways to take care of the switching glitch. There are
  fancy / expensive ways.

Bob


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