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Fm Demodulator having clipped output with ripples!!!

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hatim butt

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Could somebody have idea for fm demodulation of 75Mhz signal having 16khz message?? i used sl1454 for this but the problem is my demodulated output is clipped and having ripple how could it be sorted out thanks in advance!!!!
 
Post your existing circuit. Dont post duplicate requests in multiple forums.
 
The datasheet for the SL1454 video decoder says it is obsolete.
You should tune the Quadrature coil so that the S-curve is symmetrical then it will have a bandwidth up to a few MHz, then you signal of only 16kHz will not be clipped. Do you know what an FM S-curve is?
You have ripple at what frequency? Is it from your power supply?
 
frequency vs output voltage plot is s-curve as far as i know.Please tell me what exactly is s-curve of fm along with Q factor? btw for 75Mhz operation(double tuned circuit) the values of r,l and c comes out to be 72ohm ,47nH and 100pf respectively.Can you also elaborate why we should need to vary inductunce or capacitance or tuning. The circuit diagram is attached below and all components are smd thanks for response
 

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fm demodulation of 75Mhz signal having 16khz message
There is some information missing here.

The carrier frequency is 75MHz. That is OK.

The modulating frequency is 16kHz. At least I assume that is what you mean by a "16kHz message".

What is the deviation? We need to know this before we can decide on the value of the components for the quadrature tuned circuit.

The example in the datasheet has a deviation of 21.4MHz (p-p) with a carrier frequency of 140MHz. The deviation is quite large compared with the carrier, which is why they are using a low Q quadrature circuit.

In your circuit, we do not know the deviation and so cannot make any detailed suggestions.
However, if the deviation is small compared with the carrier frequency, then you need a fairly high Q tuned circuit.
My initial wild guess suggestion would be to use a single tuned circuit (like fig4 in the app note), remove the resistor and make the inductor 140nH and the capacitor 33pF.

JimB
 
The deviation is +/-5MHz around center frequency of 75MHz and yes by message i mean modulating signal. Secondly why i would go for single tuned circuit than double and remove loading resistors???
 
The deviation is +/-5MHz around center frequency of 75MHz
OK, so the deviation is a significant fraction of the carrier frequency, so a faily lowQ tuned circuit is probably required.

Secondly why i would go for single tuned circuit than double and remove loading resistors???
Did you read what I wrote in my previous post? ....
if the deviation is small compared with the carrier frequency, then you need a fairly high Q tuned circuit.
My initial wild guess suggestion would be to use a single tuned circuit (like fig4 in the app note), remove the resistor and make the inductor 140nH and the capacitor 33pF.
I was just taking a guess in the absence of information from you.

So, having told us about the signal you are trying to de-modulate, how are you testing your circuit?
How is the circuit built? I hope that you are not using one of those plug-in breadboards, they can give big problems at high frequencies.

JimB
 
So it is cleared that i have to design the demodulator for fairly high Q but how i can estimate its value as datasheet gives S-curve upto Q=6. Again i am asking, you have mentioned to use single tuned circuit i get that point but without "Resistors" why???
 
In my intial testing i have made this circuit on smd testing vero-board and used sma connector (50ohm) for Fm input also i have placed components as close as possible to ic bed. The ciruit is fed by signal generator which generates a fm modulated signal of 75Mhz at specified deviation and signal level,the output is fed to digital scope that it.I have done with pcb design and will testing soon may be ciruit built on pcb results differently.
 
Your deviation is VERY wide at 5MHz so the Q of the quadrature detector must be low. The detector is tuned so that the s-curve is symmetrical (positive and negative swings are the same amount) and the line is as straight as is possible.
 
Thanks audio guru for help so according to design constraints the values of quadrature circuit component at Q=2 comes out to be as follow:
I am using double tuned circuit because of the fact that fairly low q will reduce the video output signal level.

L=47nH
C=100pf
R=47ohm

So now by using low Q and above design can i get rid of clipped output and ripples?
According to datasheet the value of Q have impact on determining the loading Resistance only does it have relation with other elements or not such as L or C?
 
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