How to correctly use the same TXCO in more than one place?

[Navarro]

Hi everyone,

I'm doing a project here and the same TXCO I use both in a microcontroller and in a DDS. After using the DDS I saw the frequency running a little bit (around 10~20Khz). I think It's because the way I'm using the TXCO, connecting it output directly to a microcontroller and the DDS.

Is there any "better correct way" to connect the TXCO to both the DDS and the microcontroller?

Regards, Felipe

[medical-nerd]

Hiya

Can't you use a simple buffer amplifier between the TXCO and the DDS or MCU?  The load may be pulling the frequency off.

e.g.

http://jaunty-electronics.com/blog/wp-content/uploads/2012/08/BufferAmplifier.jpg

Cheers

[Navarro]

I was thinking about exactly this but let me wait and see if someone else arrives with another solution.

The TXCO in use is a FOX924.

[ArthurDent]

Is it possible to have a microcontroller output programmed as a clock buffer? If so feed the output of this buffer to the DDS

[CJay]

Is it possible to have a microcontroller output programmed as a clock buffer? If so feed the output of this buffer to the DDS

I think that'd be rather difficult to do and probably wouldn't work quickly enough, plus it could introduce all sorts of jitter (though the counter timer modules of PICs are rather good)

A buffer, couple of tinylogic gates would do the job if board space is tight

https://www.fairchildsemi.com/collateral/TinyLogic.pdf

[voltsandjolts]

Erm, schematic?

I think you mean TCXO (rather than TXCO) and such modules normally provide a buffered output which would be capable of driving two clock inputs.

Are you using a bare crystal or a TCXO module?

If using a crystal (driven by the microcontroller oscillator) remember to take the clock to the DDS from outputs side of the microcontroller oscillator buffer. Although that will still pull the frequency somewhat.

Why do you think the frequency is 20kHz adrift - what are you using to measure that and what is its accuracy spec?

[Navarro]

Erm, schematic?

I think you mean TCXO (rather than TXCO) and such modules normally provide a buffered output which would be capable of driving two clock inputs.

Are you using a bare crystal or a TCXO module?

If using a crystal (driven by the microcontroller oscillator) remember to take the clock to the DDS from outputs side of the microcontroller oscillator buffer. Although that will still pull the frequency somewhat.

Why do you think the frequency is 20kHz adrift - what are you using to measure that and what is its accuracy spec?

Not a bare crystal, I'm using a FOX924 ( https://www.foxonline.com/pdfs/fox924.pdf )

I think I'm going with the TinyLogic idea. They're really small and won't get too much board space!

[Navarro]

Is it possible to have a microcontroller output programmed as a clock buffer? If so feed the output of this buffer to the DDS

I don't think so, I'm using a STM32F4xx as microcontroller and the PLL in this puppy is pretty crap.

[David Hess]

If using a crystal (driven by the microcontroller oscillator) remember to take the clock to the DDS from outputs side of the microcontroller oscillator buffer. Although that will still pull the frequency somewhat.

If an external oscillator is driving the microcontroller through its 2 pin crystal connection, then the crystal output pin on the microcontroller which would normally be unconnected can be used to drive other circuits.

The load on the TCXO output should not be affecting the frequency unless something is wrong like poor power supply decoupling.