# foster-seeley detector?

Discussion in 'General Electronics Chat' started by Mathematics!, Jan 17, 2010.

1. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
I am curious from this schematic of the foster-seeley detector

half way down the page

I understand how it basically works from the description on the page and from googleing around.

My questions are if I where going to build this FM detector
What values should I make the C's in this schematic?

I am assuming the ones after the diodes should be the same to balance the circuit and give zero voltage at the output when unmodulated (i.e just the carrier exists at the input).

For the diodes I am using either switch or germinum diodes which ever I have this should be to big of an issue since I can always amplify the output however I want...etc I know though germinum forward voltage drop is the lowest and would be a better pick , but what ever....

I am unsure about the resistors R's I would think they should be the same
for balancing reasons...etc but what value I don't know and I don't know if the value matters that much (other then using the same value for both)???

And what value should the RF choke be , and the capacitor that bridges the 2 tank circuits ?

Also
I am assuming the RC circuit has to do with some kind of filter for this detector or something.... maybe this is where the R and C values get defined but I don't even know what the filter is trying to achive...

If I did I would use the formula 1/2*pi*LC in someway to achive a desired cut off frequency....but duno

Thanks for any help on this egar to build/understand this

Apr 5, 2008
18,695
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3. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
yes , but it doesn't give you any indication on what values to use for some of these components???

Thanks for any help

4. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
does anybody know what these values should be?
I know the 2 tank circuits should have the same L and C values
defined by 1/2*pi*sqrt(LC) which defines whatever carrier frequency your going to use.

But when it comes to the other capacitors, resistors I have no clue on what values would be reasonable for these components....

Like I don't know if this is supposed to be a high or low pass filter of some kind of given cut off frequency. If I knew this then I could determine a reasonable value for the R , C ,...etc

Thanks for any help
Seems like not alot of people know what the values should be.

5. ### beenthere Retired Moderator

Apr 20, 2004
15,815
293
Perhaps you are forgetting that an FM radio IF frequency is 10.7 MHz. That tends to define circuit values.

6. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
ya, so that defines my 2 LC circuits which must be set to have a resonanting frequency of 10.7MHz, So 10.7Mhz = 1/2pi*sqrt(LC)

But what defines the 2 RC circuits and the value for the RF choke?

I am using germinum diodes but the only difference in using other types of diodes in it's place would be the forward voltage drop... which would mean you would have to amplify more ...etc because of more voltage drop....

I am more curious about what the RC circuits values should be , the RF choke , and the capacitor that connects the 2 tank circuits in this schematic should be????????????????????????????????????

Thanks for any help

7. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
I found this picture.
Can anybody explain why they are using these values for the RC
if I calculate using 1/2*pi*sqrt(RC) = 3.3862753849339433142315694334609 which is about 3.4khz and I know the human voice is around 20hz to 3.4khz. And the human ear can hear 20hz to 20khz. Maybe this has significants for those values????

I don't full understand why they are using a 1.0uH for the RF choke why this value?
Also curious about why they choose this 0.047F for the C3 capacitor why that value must be a reason?

Thanks

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