Fm Radio Question..?
there is attached below the schematic of a Traditional fm receiver circuit by the use of tda7000 ic fo use in the band 88 - 108 MHZ
Now is there anybody helping me in finding the values of L1 and L2 inductors of related filters with its calculation details
i have used BB809 varactor with the datasheet also attached , instead of bb329
will you solve my problem
thanks
Input filter C1, C2, L1:
it is simple parallel circuit, total parallel capacitance is Cpi=C1*C2/(C1+C2), resonant frequency has to be band midfrequency 98MHz
L1=1/[(2*pi*f)2*Cpi]
it is about 80nH.
Oscillator circuit L2, DV, 1.8nF:
calculate L2 for max oscillator frequency which is 70kHz higher than input frequency, but the difference is small and we can say the frequencies are equal. For good band coverage we will chose 110MHz frequency first, it should be when varicap voltage is 5V. Diode capacitance at 5V is about 22pF. Series capacitor 1.8nF does not change total capacitance to be considered. The input capacitance at pin 6 is about 8.5pF. So total capacitance at varicap voltage 5V is Cpo=Cdv+C6
Inductance L2 should be L2=1/[(2*pi*fmax)2*Cpo] and is about 70nH.
Next thing to do is to verify L2 calculations on the band low frequency limit. Varicap voltage in this case is only 1.4V. Take into account new Cdv capacitance and calculate resonant frequency it should be about 85MHz. If not reduce R3 to allow lower varicap voltage.
Actual values are dependent to circuit parasitics which I ignored. Anyway you will have to tune the receiver.
Look allso APP Notes AN192 and AN193 from Philips.
well first one is the input of the reciever so it should be (filter) with not so high Q and the other is the tuning one