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FDC2214EVM: How to select a better clock oscillator as an external off board signal?

Guoxing Gu
Intellectual 590 points
Part Number: FDC2214EVM
Other Parts Discussed in Thread: FDC2212, FDC2214

Hi,

I am testing FDC2214EVM, and have a few questions about its clock oscillators. Both the schematics and the physical board show it has two clock oscillators, Y1 and Y2. Y1 is a 24 MHz oscillator in the MSP430 microcontroller section and Y2 is a 40 MHz oscillator in the FDC section.

The user guide (section 2.7.2) has following description

2.7.2 Selecting the Clocking Source: While the device contains an internal oscillator which requires fewer components in a system, it is recommended to use an external oscillator for precision applications. The EVM includes a 40MHz oscillator on-board, but an external off board signal can be used.

I tried both the internal oscillator (43.4 MHz) and the external oscillator (40 MHz), the internal oscillator achieved ± 0.015 pF precision, while the external one achieved ± 0.002 pF, which is a significant improvement.

Here are my questions: 

1. How is the 43.4 MHz derived from Y1 (24 MHz) and Y2 (40 MHz)? The 43.4 MHz shows in the configuration page. My understanding is that a sub-frequency can be derived using a divisor, resulting a sub-frequency lower than its base.

2. If the Y2 (40 MHz) is considered as an external oscillator, the internal one should be Y1 (24 MHz) in terms of the MSP430 MCU. I checked both specifications, Y1 (ABM8-24.000MHz-B2-T) has a frequency stability ± 20 ppm, while Y2 (625L3C040M00000) has a frequency stability ±50 ppm. Y2 is poorer than Y1 in frequency stability, which is not consistent with their actual performance. How to select a better oscillator as an external off board signal? can someone recommend an oscillator that outperforms Y2? Frequency range and stability, which one plays the most important rule?

Y1 (ABM8-24.000MHz-B2-T) link https://abracon.com/Resonators/abm8.pdf

Y2 (625L3C040M00000) link https://www.ctscorp.com/wp-content/uploads/2015/11/008-0329-0.pdf 

3. I am going to use one MSP430 to control multiple FDC2212/2214, can Y1 be replaced with Y2 in the MSP430 section and eliminate all Y2s in the FDC sections?

 

Regards,

Gu

 

  • 0
    TI__Mastermind 23810 points

    Gu,

    The 43.4MHz internal oscillator is inside of the FDC2214 IC. The external oscillator is Y2, the 40MHz oscillator on the EVM. Y1 is only for the MSP430 and is not used by the FDC2214 IC.

    The parameters to look for is frequency stability over temperature and low jitter. The internal oscillator has a decent temperature drift specification, but the jitter is enough to introduce more noise in the measurement.

    The MSP430 has its own specification for oscillator frequency. From a quick glance at the datasheet, it does not look like it supports up to 40MHz.

  • 0
    TI__Genius 17440 points

    Hello Gu,

    Y1 (24MHz) is required for the MSP430 operation. It is not the internal oscillator in the FDC2214. The internal oscillator is built into the silicon. 

    The best external oscillators are 40MHz with low jitter and very good stability over temperature. If you need a better oscillator than the one included on the board (Y2), then select one with better temperature stability and jitter specifications than the CTS oscillator. Definitely select a 40MHz oscillator, as this is the highest external reference frequency that the device can accept and resolution increases with the reference clock frequency.

    Don't swap Y1 and Y2.

    Best Regards,

  • 0 in reply to Kristin Jones93
    Intellectual 590 points

    Hi Kristin,

    Thank you for your quick response. 

    The hidden 43.4 MHz oscillator made me a puzzle. 

    The Y2 (625L3C040M00000) link https://www.ctscorp.com/wp-content/uploads/2015/11/008-0329-0.pdf has 3 stability options, 50, 25 and 20 ppm. Can the one with 20 ppm stability outperform the Y2 currently on the board?

    CTS has 520 series https://www.ctscorp.com/wp-content/uploads/2015/11/008-0371-0.pdf, which has very low stability, +/- 0.5-2.5 ppm, but  they are Clipped Sine Wave Output. Are they suitable for the FDC?

    I also found ECS-TXO-2520, https://www.ecsxtal.com/store/pdf/ECS-TXO-2520.pdf, which has very low stability, +/- 0.2-2.0 ppm

    Could you please let me know your general opinion on those oscillators and recommend me the best one? 

    Regards,

    Gu

  • 0 in reply to Guoxing Gu
    TI__Genius 17440 points

    Hi Gu,

    The CTS HCMOS clock oscillator with higher stability could outperform the oscillator populated on the EVM across temperature. However, to improve the resolution at room temperature, you would need to find an oscillator with better jitter performance.

    I do not see jitter specifications in the other two oscillators you mentioned, so I'm hesitant to recommend them. In general I think a clipped sine oscillator would be fine as long as the minimum voltage high and voltage low levels are met (specified in the datasheet). We have not tested with a clipped sine oscillator, so I'd recommend experimenting with it yourself.

    Best Regards,

  • 0 in reply to Kristin Jones93
    Intellectual 590 points

    Hi Kristin,

    I spent more time on searching for the oscillators and did find a few that have better stability and jitter, e.g. 25 ppm and 5 ps, 50 ppm and 1 ps, 2.5ppm and 3 ps. I also placed an order for them already. 

    Thanks.