TDA8722 / RF modulator IC / changing the sound freq

[loneant2002]

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.

 

[AL0117]

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