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Cause for TX frequency difference

kellogs

Jan 7, 2014
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i have an ISM band transmitter device based on the MAX41460 chip. It expects a 16Mhz, 12 pF crystal.

Switched from set A of parts:
- crystal: https://www.tme.eu/ro/en/details/16.00m-smdxt224/smd-quartz-crystals/yic/
- PCB antenna feed inductor: https://www.tme.eu/ro/details/nl03jtcr15/inductoare-smd-0603/viking/

To set B of parts:
- crystal: https://eu.mouser.com/ProductDetail/ECS/ECS-160-12-33-AGN-TR3
- PCB antenna feed inductor: https://www.tme.eu/ro/details/lqw18anr15g00d/inductoare-smd-0603/murata/

With same everything else, i am seeing a frequency difference of about 6 kHz at 433.92 MHz, OOK, room temperature. Difference seen via SDR.

What are the causes for this difference, and what do I need to be more careful about when swapping these two parts ?

Thank you
 

bertus

Moderator
Nov 8, 2019
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hello,

there is a difference in tollerance of the crystals.
set a has a tollerance of 10 ppm
set b has a tollerance of 30 ppm

bertus
 

kellogs

Jan 7, 2014
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Oh yes... do you think this amount, 6 kHz, would be enough for a black box 433 MHz receiver to always process the signal vs never process the signal over a 50 deg C range ?
 

kellys_eye

Jun 25, 2010
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The crystal is probably associated with two capacitors and you can - if necessary - replace one of the capacitors with a variable type and trim the oscillator frequency using it.
 

kellogs

Jan 7, 2014
88
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I have adjusted the frequency from software, problem persists. What about the two feed inductors used, how much of an effect can they have ?

A: SRF = 1050 MHz DCR = 0.15 ohm Q = 15
B: SRF = 1400 MHz DCR = 1.5 ohm Q = 32

Thank you
 
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