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38khz Oscillator problem

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betwixt

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Re: Need help about 38khz Oscillator

Yes you can, it will work exactly the same although the phase of the pilot tone may be reversed. As the circuit has no way of establishing the initial pilot phase anyway it shouldn't make any difference. You may find the left and right channels are randomly reversed in this design!

The pilot level can be adjusted by varying the value of R14 but fine adjustment of the phase isn't easy unless you make the circuit much more complicated.

Brian.
 

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Re: Need help about 38khz Oscillator

Don't use a lousy old LM358 dual opamp for audio because it has crossover distortion, hiss and poor high audio frequency outputs. Use a dual audio opamp instead (TL072 or equiv).
 

waldopulanco

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Re: Need help about 38khz Oscillator

there is a problem when in i try to simulate the stereo coder by harry! there is no output 38khz only 19khz! the 38khz was cancelled out because the Right output is positive and the Left output is negative, the 38khz in left is negative and the 38khz in right is positive. i double check my connection but its right according to harry,s diagram.

the problem is the 38khz is also invert to negative instead of left audio only was invert to negative!!

my conclusion is the right audio was modulated in 38khz and the left audio was modulated in 38khz after that the modulated Left is invert to negative..

the correct is The left is invert then the left and right is mix then modulate to 38khz..

any idea to correct this circuit?

thanks!!
 

betwixt

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Re: Need help about 38khz Oscillator

That is correct, there is no 38KHz output. The way it works is the left (L) and right (R) signals are alternately selected at 38KHz rate. What comes out of the encoder switching stage is LRLRLRLR but the switching rate is too high to be audible and the ear hears it as a mono mix of the two channels. The 19KHz pilot is phase locked to the 38KHz switch because they come from the same source. 19KHz is low enough in frequency to be transmitted over an FM audio link while 38KHz isn't. What happens in the decoder is it filters the 19KHz, doubles it's frequency back to 38KHz then uses it to operate a switch which passes the audio to one channel then the other. If everything works properly, the switch in the encoder and the switch in the decoder work at exactly the same time so the original L signal is passed to the L output and the R signal to the R output.

Brian.
 

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Re: Need help about 38khz Oscillator

The L minus R is amplitude-modulated with a 38kHz carrier but the carrier is suppressed and is not transmitted. Instead the 38kHz sidebands are transmitted. The 19kHz pilot tone is transmitted so that the decoder can make its own phase-locked 38kHz carrier to decode the sidebands.
 

waldopulanco

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Re: Need help about 38khz Oscillator

thanks!! but how to make it work i think there is something wrong in bilateral switch!! any idea?

thanks!!

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i was searching on google about basic stereo encoder all i saw is the same the the positive 38khz is modulate with right channel and the negative 38khz is modulate with left channel then the 2 channel with positive and negative was mixed!!
 

E-design

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waldopulanco

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Re: Need help about 38khz Oscillator

Thanks!! the 4066 connection is correct and there is an 0utput of 38khz and Positive right and negative left!!
 

E-design

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Re: Need help about 38khz Oscillator

This can be used with an active device to make an oscillator. A filter will give you some phase shift that can be combined with a feedback amplifying device to add enough phase shift together with amplification to overcome the losses in the circuit to sustain oscillations.
 

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Re: Need help about 38khz Oscillator

As I stated before, there should be no output at 38KHz. That is the switching frequency but only the 19KHz should be present at the output. You can see on the circuit posted by E-design that it uses an LC low pass filter to remove any residual 38KHz that leaked through the 4066. Please don't even consider using a 555!

You can use the 456KHz resonator but to make it 38KHz you have to divide it by 12. You would have to do that by dividing by 6 then by 2 so the final output would be a square wave. It isn't difficult but extra circuitry would be needed. An alternative approach would be to phase lock a 38KHz oscillator to 456/12 but that would probably be even more complicated.

Brian.
 

waldopulanco

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Re: Need help about 38khz Oscillator

As you said this is true at almost basic stereo coder that there is no 38khz output? only 19khz output?

- - - Updated - - -

Only 19khz pilot is available in the output here is the spectrum analyzer

Stereo_coder_456khz.JPG

is it true to all basic stereo coder that there is no 38khz output? i tried all basic stereo coder that i search on the internet but all of them there is no 38khz output, im not try to BA1404 because i don't have spice model for that IC to simulate..

how to overcome this problem? or is it Ok even though that there is no 38khz output but good sound quality on the stereo receiver?

thanks!!
 

betwixt

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Re: Need help about 38khz Oscillator

38KHz is only used INSIDE the encoder, it should not appear at the output so the simulation is correct. If you were to transmit 38KHz using an FM transmitter it would use almost twice the width of an FM channel so you wouldn't be able to pick it up on a normal radio. Instead, the frequency is divided by 2 and 19KHz is transmitted instead. In the receiver, the 19KHz is doubled in frequency to recreate the 38KHz needed to operate the decoder switch.

The frequencies were chosen for that very reason, 19KHz is deemed to be just above the average human hearing range so it wouldn't be objectionable to listeners. Most receivers have audio low pass filters that limit the top frequency to around 16KHz anyway. In a radio system, higher frequencies in the modulation require wider bandwidths so going higher than 19KHz would simply waste band space. 19KHz was chosen as a good compromise. The trouble with actually switching the channels at 19KHz is it would make strange noises as it created beat notes with higher audio frequencies, for example a sound at 15KHz would produce a (19-15) 4KHz sound as well. By switching channels at 38KHz it avoids the beat notes and by dividing it by 2 allows it to be transmitted at 19KHz while still allowing the original 38KHz to be re-created in the receiver by doubling it.

Brian.
 

waldopulanco

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Re: Need help about 38khz Oscillator

thank you betwixt!! i have two version of schematic first is using 38khz and the other is using 456khz! i will post the picture tomorrow to analyze and if you or they have any suggestion please let me know to make it good!!

i am going to sleep its been 11pm here!! thank you so much!!
 

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Re: Need help about 38khz Oscillator

here is my 2 stereo coder schematic diagram

using 38khz
Stereo_coder_38khz.JPG

using 456khz
Stereo_coder_456khz.JPG

Please tell me if there is wrong to my schematic diagram!!

thanks!!

- - - Updated - - -

what is the common p-p voltage output on 19khz pilot? 1v p-p is too high?

- - - Updated - - -

the output p-p voltage from 19khz pilot is 200mV to 375mV is it normal?
 

betwixt

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Re: Need help about 38khz Oscillator

I can't read the pins on the 4066 but assuming they are correct it looks good. The 22pF loading capacitors on the 465KHz resonator may be too large but without a specification you will have to experiment.
The pilot tone level should be around 10% of the peak audio level.

Brian.
 

waldopulanco

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Re: Need help about 38khz Oscillator

Which is good Oscillator for 38khz CD4011 or CD4069?

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4066 pin connection
4066.JPG
 

betwixt

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Re: Need help about 38khz Oscillator

Both will work equally well. If you use the CD4011 you should tie both inputs together so it works as a simple inverter. On both devices, connect all the unused inputs to ground.

Brian.
 
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