Fairchild RC2211A: How to use the formula for FM detection

[richard1991]

Hi. Can someone please help by verifying I understand how to use a formula?  Thanks.

The formula is in this data sheet:

http://www.littlediode.com/datasheets/pdf/Datasheets-RC2/RC2211A.PDF

On page 9 under "Linear FM detection".

V_OUT = R₁ V_R / 100  R₀  Volts/% deviation.

Let's say that the FM carrier signal was at 1 Mhz and the maximum frequency deviation for the system was set at 5Khz (i.e. f_c 1Mhz and the deviation ⁺/_- 5Khz (10Khz frequency swing). Say also the max modulating frequency for the system was set as 5Khz.  That would mean if we, in practice, used a modulating signal of 5Khz, the modulation index would 5Khz/5khz = 1.  Also, we would have 100% modulation.

Mostly, I'm not sure about the last part of the formula (Volts/% deviation). Rich

P.S. Is the acual formula this?: V_OUT = R₁ V_R / 100  R₀  And the last bit telling us something about the answer?

[richard1991]

I think the text is saying that the result is volts per unit of deviation. So, if the deviation were to be 100% you would multiply the anwer by 100.

[floobydust]

Look at the Exar XR2211A datasheet, same equation but with proper formatting, pg. 18

[richard1991]

Thanks for the heads up on the much better document produced by Exar.

The formula then is this: V_OUT     =    R₁         V_REF
                                                          ____________
                                                             100        R₀       

The ouput being output voltage change per unit of FM deviation.