# Right value for low-pass filter capacitor ?

Discussion in 'Wireless & RF Design' started by dr.emp, Nov 10, 2018.

1. ### dr.emp Thread Starter New Member

Nov 13, 2017
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I am using this RF receiver microcontroller I had from an old teardown of a rc car. The schematic of the IC is here:https://www.diodes.com/assets/Datasheets/PT8A977B978B978BL.pdf All values for the components are specified on page 9, except for C6.

My question
: Is R1 and C6 some kind of low-pass filter ? If yes, then what value should C6 be ? And can I calculate that value with the formula f= 1/2pi*RC ?

Last edited: Nov 10, 2018
2. ### ericgibbs Moderator

Jan 29, 2010
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hi dr,
The C6 value for tuning the frequency.
E
Woops incorrectly read the capacitor notation.!

MOD: Clipped and posted the circuit, easier to read.E

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3. ### dr.emp Thread Starter New Member

Nov 13, 2017
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Thanks for the answer. Well i asked around in other forums and they told me it was this or that, but they never came to a conclusion. So I applied the values of C3 (10pF) and L1(calculated by measuring the coil, can't remember the value of it) in the formula f= 1/2pi*square(LC), resulting a resonant frequency of about 25 MHZ, which I supposed that can be leveled up with the core of the inductor.

Maybe I should have mentioned that I want it to receive in the range of 27 MHZ ?

4. ### ericgibbs Moderator

Jan 29, 2010
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hi dr,
Using C6 10pF this is LTSpice sim for that circuit, Fout =27MHz.
Very rough sim be it suggests the 10pF is OK.
E

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5. ### dr.emp Thread Starter New Member

Nov 13, 2017
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Thank you very much . It was the last component I had to solder to the PCB. I was thinking about using LTSpice like you did, but I didn't know how to make the antenna. I want to use the circuit to control a Lego Technic car.

6. ### ericgibbs Moderator

Jan 29, 2010
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hi,
Would be interested to hear how the project works out.
E

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Dec 20, 2007
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