So if I understand it, you allow the OCXO several days of warm-up to set the
frequency. Then when turned off for a while, then restarted. After some
warm-up period, the retrace spec would give an indication of how close the
frequency will be?
Thanks,
Tom
----- Original Message -----
From: "Bob Camp" <[email protected]>
To: "Discussion of precise time and frequency measurement"
<[email protected]>
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 only
moves according to the aging rate. Retrace occurs after the oscillator has
been off power for some period. The rate of change is greater than the aging
rate. Warmup and retrace are obviously inter-related. Warmup is generally
described as a short term (sub 1 hour) process. Retrace is often looked at
as a day to multiple day sort of thing.
Since none of this is tightly defined, you will see various specs looking at
the same issues a bit differently. Often those differences roll up to some
sort of system requirement.
Bob
On Dec 9, 2012, at 7:45 PM, "Tom Miller" <[email protected]> wrote:
Hi Bob,
Can you give a good definition of retrace as it applies here?
Thanks,
Tom
----- Original Message ----- From: "Bob Camp" <[email protected]>
To: "Discussion of precise time and frequency measurement"
<[email protected]>
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. Past
some number of minutes (5,10,15…) you will always be better with a modern
OCXO than with a TCXO.
Bob
On Dec 9, 2012, at 7:05 PM, WB6BNQ <[email protected]> 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 <[email protected]> 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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