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Old 23rd May 2005, 06:26 AM   #1
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Default My Stereo Encoder Design not working -- PLS HELP..!

Hello Friends..

i m currently desigining a stereo encoder circuit so that i can convert my useless 1W mono xmitter to a complete stereo one..

i have made the design using the DSBSC theory.. I m tryin it out on a bread board.. i m working on it since last 3 WEEKS..!!! But my fate is too rude on me..!! The entire stuff looks useless as it gives NO SIGN OF STEREO SEPERATION.. might be bats maybe hearin it.. but not me..!!

okay.. let me explain what's my circuit :

using LF351 i generated L+R and L-R signals..

i used a 555 oscillator to generate accurate 76Khz signal with the help of my multimeter... by feeding it to a 4013C i got a 38KHz square wave signal and a 19Khz Square signal...

using a active 20Khz lowpass filter i converted 19khz square signal to a sinusoidal one.. Everywhere i used LF351 op-amp..

then i feed the 38khz signal to C4066 chopper to modulated L-R signal.. Then the output of 4066 is connected to a 38khz active bandpass filter to generate DSBSC signal.. (I DOUBT IS THIS THE WAY DSBSC CAN BE GENERATED ???????)

by mixing 19khz pilot, DSBSC and L+R to a op-amp Adder i fed it to my mono transmitter and played a song which didnt seperate in my stereo FM...

SO AFTER THIS STORY i wanna know; is my design alright..?? I even used isolation buffer after L+R and L-R , to prevent impedance changes at the input affect the output..

When i start the transmitter, the FM Stereo LIGHT of my receiver glows... but no sepration occurs..?? only a background stereophoic sense distortion is bein felt after i start the pilot..

I feel that my 38 Khz subcarrier has some critical error.. SO PLS GUIDE SO AS TO WHAT CHANGES MAY I TRY OUT..!!

Waitin for an earliest response..
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Old 23rd May 2005, 07:57 AM   #2
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Your 38 kHz subcarrier must have a flat bandpass from about 23 kHz to about 53 kHz.
The phase of the 19 kHz pilot must be correct also.
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Old 23rd May 2005, 08:43 AM   #3
moamps is offline moamps  Croatia
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Quote:
Originally posted by Frank Berry
Your 38 kHz subcarrier must have a flat bandpass from about 23 kHz to about 53 kHz.
The phase of the 19 kHz pilot must be correct also.
And a modulator in Tx must have a linear frequency and phase characteristic up to 57kHz.
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Old 23rd May 2005, 09:31 AM   #4
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Also,don't forget to disable the pre-emphasis,if your transmitter has it.
(It should be moved to the inputs of the stereo encoder.)
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Old 23rd May 2005, 10:05 AM   #5
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Also make sure your 38 kHz subcarrier has a 50% duty cycle.
This is critical.
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Old 23rd May 2005, 11:54 AM   #6
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Hi..!

Thanks for your reply folks..!

I have used a 2 stage MFB filter with a band width of 30khz.. So the pass band is exactly about 23khz to 53khz.. and have also put 50ms preemphasis across the encoder input...

BUT, i dont know if there already exists a pre-emphasis in my MONO FM xmitter ..?? Because i havent located such parallel RC combination neighter at the input...I HAVE ATTACHED MY MONO XMITTER CIRCUIT DEGISN WITH THIS POST.... So pls look at it and help in locating its pre-emphsis stage, if its really present..!!

And finally, i have purchased the MONO Transmitters from a electronics kits shop quiet some time back when i got the taste of wireless electronics.. The transmitter is actually for transmitting VOICE Conversations (by connecting its input to a condenser mic) and not for directly connecting any audio..

is that the problem because in such a case the modulator of the xmitter may not have linaer freq and phase charactersic upto 57KHz and might create some big problems...??

Pls reply..
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Old 24th May 2005, 03:11 PM   #7
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Quote:
Originally posted by moamps


And a modulator in Tx must have a linear frequency and phase characteristic up to 57kHz.


How can i assure that my Tx has linear freq. phase characteristic upto 57 khz..??

And one thing, the mono fm Tx that i have is actually for transmitting VOICE CONVERSATIONS....

so i doubt that the characteritics might be linear till 57khz..!!! Pls elborate on that...

-- Prasanna (xitronics)
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Old 24th May 2005, 07:03 PM   #8
moamps is offline moamps  Croatia
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Hi,

this Tx isn't intended for stereo FM. Q1 is oscillator in grounded-base connection with FM-AM modulation in base. Frequency band that can be modulated is limited with R3C2 LP, which is around 30kHz. Also, modulation linearitiy is very poor with high level parasitic AM.
Usually, varicap-controlled/modulated oscillator is used in stereoFM transmitters.
A good book on small transmitters is:
R.F. Graf, W. Sheets "Build Your Own Low-Power Transmitters"
ISBN 0-7506-7244-7

Regards,
Milan
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Old 27th May 2005, 01:46 PM   #9
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Default Regarding my STEREO ENCODER -->> A small doubt.. pls respond..

Quote:
Originally posted by moamps
Hi,

this Tx isn't intended for stereo FM. Q1 is oscillator in grounded-base connection with FM-AM modulation in base. Frequency band that can be modulated is limited with R3C2 LP, which is around 30kHz. Also, modulation linearitiy is very poor with high level parasitic AM.
Usually, varicap-controlled/modulated oscillator is used in stereoFM transmitters.
A good book on small transmitters is:
R.F. Graf, W. Sheets "Build Your Own Low-Power Transmitters"
ISBN 0-7506-7244-7

Regards,
Milan

Hi..!

Thnks for ur response...

I had another small doubt.. In my encoder, i chopped L-R and R-L signals alternatively at 38khz and passed the summed output to the 23-53 khz bandpass filter.. My drawing some waveforms and math, i feel, generating DSBSC by this way might be alright..??

I have a doubt about this, because i do not have a oscilloscope and so i cannot determine the nature of the signal produced.. i only assume it to be right that way..

Pls. reply..

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Old 27th May 2005, 08:53 PM   #10
moamps is offline moamps  Croatia
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Default Re: Regarding my STEREO ENCODER -->> A small doubt.. pls respond..

Quote:
Originally posted by xitronics
I had another small doubt.. In my encoder, i chopped L-R and R-L signals alternatively at 38khz and passed the summed output to the 23-53 khz bandpass filter..
http://home.tiscali.be/vrijeradio/radio/stereo.html
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