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: "[email protected]" <[email protected]> To: Discussion of precise time and frequency measurement <[email protected]> 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 <[email protected]> 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 _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
