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Design >> Analog Design >> DTCXO architecture options
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Message started by lunren on Sep 14^th, 2012, 8:42am

Title: DTCXO architecture options
Post by lunren on Sep 14^th, 2012, 8:42am

Hi All,

I am working on a Digital Compensated Crystal Oscillator design for real time 32.768kHz clock (part of the whole project). Now there are two options in terms of architecture:
1) temperature sensor+EEPROM+analog option(change tunable xtal load capacitor bank) to change oscillation frequency
2) temperature sensor+EEPROM+digital option( keep the xtal load capacitor unchanged, but digitally change the time counter to add or remove pulse to speed up or slow down the clock counter)
The problem in 1) is we need big area to put the tunable load capacitor on-chip, however we are area limited (if we have enough area, the frequency compensation precision can be high enough);
The problem in 2) is we are not sure if it works well, especially let's say the counter precision up to +/-3ppm.
I am hoping there are some DTCXO/TCXO expert at here can give some comments.

Thanks,

Lunren

Title: Re: DTCXO architecture options
Post by ywguo on Sep 15^th, 2012, 1:55am

Hi Lunren,

What is your target? +/- 3ppm? Do you need a precise 32.768 kHz clock or 1 Hz signal for real time clock ?

Best Regards,
Yawei

Title: Re: DTCXO architecture options
Post by lunren on Sep 15^th, 2012, 7:55am

Hi Yawei,

Target is +/-3ppm. I need a 1Hz for real time clock gen. Any suggestion or experience on this?

Thanks,

Lunren

Title: Re: DTCXO architecture options
Post by loose-electron on Sep 16^th, 2012, 8:18pm

What are you using as your tuning reference?

Also an uncompensated as is 32K RTC is not accurate enough?

If that does not get it done, consider a digital count adjustment method instead.

Doing all kinds of fancy analog tweaking might be more simply
done by using digital number adjustment.

Title: Re: DTCXO architecture options
Post by microe on Sep 17^th, 2012, 2:27pm

Not expert.

Just two cents:

there are some existing products out there in the market. Many companies give lots of information in their datasheet. that tells you which approach is more popular, although may not fit into your application exactly.

Title: Re: DTCXO architecture options
Post by lunren on Sep 18^th, 2012, 11:36pm

Hi loose-electron,

The tuning reference is at room temperature. In our application, uncompensated 32K RTC is not enough. I am just afraid there is some limitation on digital number adjustment since it seems some big companies using analog tuning method instead. I don't know if without any compensation, how good the RTC frequency stability is, let's say +/-100ppm across -25C to 85C?

Thanks,

Lunren