[time-nuts] Switching oscillators

Bob Camp lists at rtty.us
Sun Dec 9 22:28:44 UTC 2012


The gotcha with a comparison is that the TCXO likely is further off than the OCXO's settling characteristics. Put another way, the OCXO goes from 10 ppm to 1 ppm in no time at all. Going from 1 ppm to 0.1 takes longer. It's the 0.1 to 0.01 range that really tells you about settling and warmup. The TCXO is not going to help you there. It's probably seeing a bit of a temperature transient as things turn on ….


On Dec 9, 2012, at 5:22 PM, Robert LaJeunesse <rlajeunesse at sbcglobal.net> wrote:

> It could be done cheaply with a small micro that is clocked by the device's TCXO 
> and uses an internal timer to measure the external clock frequency (or external 
> frequency divided by 10 or 16 if that better suits the timer capabilities). The 
> micro, when satisfied the frequency is close enough for long enough, then drives 
> a SPDT analog switch to select the appropriate oscillator source. After adding 
> the necessary protection components and conditioning the clocks to the micro's 
> level & edge requirements the total cost should come in well under $20.  
> Alternately a micro-free design could be crafted with a some one-shots 
> (monostable multivibrators) to evaluate the external clock for presence and 
> coarse frequency, a timer for some sort of delay to prevent switchover during 
> external OCXO startup, some glue logic, and the same protection components and 
> conditioning and SPDT analog switch as above. 
> Bob LaJeunesse
> ________________________________
> From: "lists at lazygranch.com" <lists at lazygranch.com>
> To: Discussion of precise time and frequency measurement <time-nuts at febo.com>
> Sent: Sun, December 9, 2012 5:05:09 PM
> Subject: Re: [time-nuts] Switching oscillators
> You would probably want to see when the external oscillator frequency is close 
> to the internal oscillator. I suppose than could be done with a mixer and glue 
> circuitry. I don't think this is cheap though. 
> -----Original Message-----
> From: Joseph Gray <jgray at zianet.com>
> Subject: [time-nuts] Switching oscillators
> I have a device that has an internal TCXO. I want to feed it with an
> external OCXO, but I don't want to completely replace the TCXO.
> Here is the scenario. On initial power on, or after a power loss, I
> want the internal TCXO to be used. Once the OCXO is up, I want to
> switch to it. How could this be done easily and cheaply?
> Joe Gray
> W5JG
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