[time-nuts] Switching oscillators

Robert LaJeunesse rlajeunesse at sbcglobal.net
Sun Dec 9 22:22:35 UTC 2012

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

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