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NJM567 V LM567 - Tone Decoder Formula Help

stuart sutherland

Jul 6, 2015
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Jul 6, 2015
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All,

I am looking at two devices:

1. NJM567 (Made by JRC then Phillips)
2. LM567 (Made by TI)

These look fundamentally the same. The data sheet for each looks identical (same equivalent schematic). However, the formula quoted for calculating the central tone frequency differs in both data sheets (I have attached both).

1. Fo = 1 / (1.07 x R x C)
2. Fo = 1.1 / (R x C)

Before I start designing I want to understand why these are different?
I cant understand why they would be from the data sheets.

Does anyone have any experience in using either device and can fill in the blanks?
I thought equation 2 would be one I used, however I got confused when I seen the formula in data sheet 1.

Many thanks,
Stuart
 

Harald Kapp

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Nov 17, 2011
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No experience with these chips, but: Look-alike components do not necessarily have to be identical. Small variations in parameters may lead to small differences in behaviour. Not uncommon. Use the formula that matches the specific chip you are using.
Then again: The equations differ by ~17 %. If you use standard capacitors (20 %) and resistors (10 %) the change in center frequency implied by component tolerances is higher than that. As a one-size-fits all solution you could just as well use f0 = 1/(R*C) for designing the circuit.
 
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