How to correctly use the same TXCO in more than one place? - Page 1
Forum section: electronics / projects

author: Navarro
author_rank: Contributor
timestamp: July 13, 2018, 11:14:25 am
content: 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

author: medical-nerd
author_rank: Regular Contributor
timestamp: July 13, 2018, 11:46:31 am
content: HiyaCan'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.jpgCheers

author: Navarro
author_rank: Contributor
timestamp: July 13, 2018, 12:14:59 pm
content: 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.

author: ArthurDent
author_rank: Super Contributor
timestamp: July 13, 2018, 12:35:40 pm
content: Is it possible to have a microcontroller output programmed as a clock buffer? If so feed the output of this buffer to the DDS

author: CJay
author_rank: Super Contributor
timestamp: July 13, 2018, 01:12:40 pm
content: Quote from: ArthurDent on July 13, 2018, 12:35:40 pmI 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 tighthttps://www.fairchildsemi.com/collateral/TinyLogic.pdf

author: voltsandjolts
author_rank: Unknown
timestamp: July 13, 2018, 02:00:25 pm
content: 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?

author: Navarro
author_rank: Contributor
timestamp: July 13, 2018, 03:24:11 pm
content: Quote from: voltsandjolts on July 13, 2018, 02:00:25 pmNot 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!

author: Navarro
author_rank: Contributor
timestamp: July 13, 2018, 03:24:51 pm
content: Quote from: ArthurDent on July 13, 2018, 12:35:40 pmI don't think so, I'm using a STM32F4xx as microcontroller and the PLL in this puppy is pretty crap.

author: David Hess
author_rank: Super Contributor
timestamp: July 14, 2018, 12:43:15 am
content: Quote from: voltsandjolts on July 13, 2018, 02:00:25 pmIf 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.