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TDA8722 / RF modulator IC / changing the sound freq

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loneant2002

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tda8722

Hi ,
i would like to ask if any of you have experience using this RF modulator IC . I am trying to figure out how to change the sound carrier freq, as the datasheet only has 5.5MHz example without explaining how it was built , or why these components were used. It is a LCR tank circuit with adjustable inductance, so that 4.5, 5 , 5.5 or 6 MHz can be buit using these external components. Thanks in advance to anybody who can enlighten me, or give me some clues. Thanks all.
 

tda8722 datasheet

Hi,
You can use the same inductor and change the capacitor value based on the "Thompson" formula:
f=1/(2*PI*sqrt(L*C))

for known frequency (ex 5.5 MHz)and L, C values also you can calculate easy the new capacitor for the new frequency

C2=C1*(f1/f2)exp2

for 6.5 MHz this means approximatelly 39 pF capacitor.

regards, Al
 

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