Can this crystal be used in a Colpits oscillator

[Aussie Susan]

I have a PIC12LF1840T39A that I want to use to generate anOOK signal in the 433.94MHz ISM band. The application schematic in the datasheet is:

This shows a crystal from the XTAL pin (pin 10) to ground in what I take is a Colpitts oscillator configuration (but I could well be wrong). The datas heet for the MCU also says that the load capacitance of the crystal should be 15pF.

The local supplier lists the following as the only SMD 24MHz 'crystal' that has 15pF load capacitance: https://www.farnell.com/datasheets/4129071.pdf. To my mind this looks more like an external oscillator (with the 'Vcc' 'Output' and 'OE' pins). [Ignore the frequency of the crystal in the schematic - that is for the 868/915MHz bands]

Can I use this "crystal" (as per the PDF) or must I look further for an actual crystal?

Susan

 

[KlausST]

Hi,

I don´t think this is what you need.

The PIC datasheets suggests a bare XTAL ... while the shown device is a complete "CMOS XTAL oscillator". Sadly the "multicomp" datasheet (farnell generated) misses to say so. As you correctly wrote .. you can see this, because it has an VCC and GND pin and a supply current.

Better use farnell item #4228673 or #2854166.

Klaus

 

[FvM]

PIC12LF1840 datasheet doesn't specify RF transmitter operation with a crystal oscillator instead of a crystal. One obvious problem is that you'd need an additional pin to control oscillator enable. It can be that disabled oscillator current is still a high multiple of PIC sleep current.

--- Updated Monday at 6:59 AM ---

In addition, consider that RF transmitter might need tight frequency tolerances, depending on receiver bandwidth, e.g. 5 to 10 ppm.

 

[Aussie Susan]

Thanks both - you have confirmed what I thought.
My reason for asking is that I'm very familiar with the other Microchip oscillator circuits where you can use either a crystal (across the 2 'OSCn' pins which are themselves at either end of an inverter) or apply an external oscillator to the OSC1 pin (that feeds the input of the aforementioned inverter).
I'm not so familiar with the (assumed) Colpitts analog oscillator and was wondering if you could also feed an external oscillator into the pin for the crystal.
I'm using one of the larger (and cheaper!!!) SMD crystals now.
Susan

 

[FvM]

The datasheet doesn't specify much about the oscillator. It's obviously designed to connect a crystal. It may be possible to connect an external oscillator but you don't know the acceptable signal level.