CC1200: Crystal accuracy sizing

MDO

Prodigy 110 points

Part Number: CC1200

Hi,

I need to size crystal accuracy for 433.92MHz receiver application with a CC1200.

For this, I'd like to take into account the frequency drift of emitter and thus its crystal accuracy.

I tried to do this with the following formula that I found in the application note SWRA122:

BW channel > BWsignal + (4 × XTALppm × fRF)

However, this formula seems to be available in case of same ppm of the crystals for the emitter and the receiver, which is not my case.

Here are my features:

Emitter:

Xtal ppm: 55ppm
Carrier frequency: 433.92MHz
Frequency shift: +-25kHz (deviation 50kHz)

Receiver:

Carrier frequency: 433.92MHz
RX filter: 300kHz

How deduce Rx crystal ppm with this features?

Best regards

Martial

over 2 years ago

0 RGW over 2 years ago

TI__Mastermind 29560 points

Hi,

The crystal tuning is documented in Chapter 6 in the following app note: https://www.ti.com/lit/swra640

If you would like to offset the frequency offset, then this can be done by using the internal register or by using external load caps on the crystal.

Regards,

Richard

0 TER over 2 years ago

TI__Guru**** 317180 points

In the equation BW channel > BWsignal + (4 × XTALppm × fRF):

2*XTALppm will be for the TX side and 2*XTALppm for the RX side.

Meaning you have this equation:

BW channel > BWsignal+2*55ppm*433.92M+2*X*433.92M.

Here, BW channel is 300 kHz is you keep the RX filter setting. To get an accurate measurement of BWsignal, measure the 95% OBW using a spectrum. Then you can find X.

0 MDO over 2 years ago

Prodigy 110 points

Hi guys,

Thanks for your feedback.

I already deduced the mentioned formula but I had some doubt.

BW channel > BWsignal+2*55ppm*433.92M+2*X*433.92M

About OBW, is there a formula to deduce it?

Can we use Carlson’s rule? Datarate + 2 * fm_deviation

Best regards

Martial

+1 TER over 2 years ago in reply to MDO

TI__Guru**** 317180 points

If you use GFSK, carsons rule give a lot higher OBW than you actually have. You can use it to get a very conservative number or you can measure OBW using a spectrum analyzer.

0 MDO over 2 years ago in reply to TER

Prodigy 110 points

Thanks!

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CC1200: Crystal accuracy sizing