[time-nuts] Switching oscillators

Bob Camp lists at rtty.us
Mon Dec 10 00:24:15 UTC 2012


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


On Dec 9, 2012, at 7:05 PM, WB6BNQ <wb6bnq at 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 <lists at 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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