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Old 8th May 2005, 04:58 AM   #1
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Question FM transmitter stereo encoder help

Im not sure if this is a proper forum or not, but here goes:

I dont have a schematic, since I built it myself using different concepts from other schematics, but Ill describe it the best I can.


The problem: NO stereo seperation. none. nill. If you listen very hard, there is very very very little seperation. but mainly the channels are mixed together. It was working perfect one day, and the next day, i just have no seperation. makes no since. All parts test good.

The design: Here is the layout. I have a 555 timer tuned to 76khz, with the output feading to a CD4013 IC. This divides it by two and gives me both a normal and inverted 38khz signal which is fed into the gates of a CD4066. then the /Q is fed into the other side which developes the 19khz pilot. Its then fed into a 741 op-amp with a slick-filtering design I found on anohter schematic to filter the pilot. Its then fed into a transistor along with the 38khz modulated signal from the CD4066, both mixed together with another filter. It sounded very clean when it worked. Then one day, I lost seperation. I even went as far as replacing all the ICs and rebuilding the simple transmitter.

I get a strong pilot. the stereo decoding switches in on all my tuners except for one, which it used to at one point and stopped.

If i switch my tuner from mono to stereo to back, I hear no difference except the low level white noise generated from the decoders. No seperation.

The transmitter is simply a colpits oscillator fed into a buffer stage centered around 106.5mhz, delivering about 5W output. But im not certain of this, since my freq counter cant count that high

Any Ideas? the 4066 is switching as it should, and I hooked a relitivly low freq counter up to the osc stages and outputs, they are all within spec. Hmmmm.

Everything is prototyped onto breadboards with wires everywhere. The encoder runs off of 12V and the transmitter 26V. the only thing on circuit is the osc, since they cant be prototyped easily.
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Old 8th May 2005, 07:52 AM   #2
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From your description and the things you tried, it seems unlikely that there is a problem with a part. What I would do in your place would be to recheck and doublecheck again all the interconnect wiring especially to make sure that you are looking/listening at the right points. It being a proto sort of, there is the possibility of an unseen short/open that happened one time after some mech movement.

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Old 8th May 2005, 10:20 AM   #3
moamps is offline moamps  Croatia
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Default Re: FM transmitter stereo encoder help

Quote:
Originally posted by mbates14
...The problem: NO stereo seperation. ...
... then the /Q is fed into the other side which developes the 19khz pilot. Its then fed into a 741 op-amp with a slick-filtering design I found on anohter schematic to filter the pilot. Its then fed into a transistor along with the 38khz modulated signal from the CD4066, both mixed together with another filter.......
Hi,

It is essential that the pilot (19kHz) is in phase with the subcarier (38kHz) after all filtering and before final mixing. Use the scope to do this. Also, the pilot level should be max. 10% of max. deviation.
Btw, channel separation is also limited with the transmitter modulator's phase characteristic.

Regards,
Milan
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Old 8th May 2005, 12:56 PM   #4
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"I get a strong pilot. the stereo decoding switches in on all my tuners except for one, which it used to at one point and stopped."

Does this tuner use a frequency synthesized front end? If so, your transmitter may be operating slightly off frequency ... too far off for the receiver to lock to it.
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Old 8th May 2005, 07:44 PM   #5
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no, this is an oooold 70s dial tuner. IT used to switch on, but not anymore. It will on other strong stations.


I have another dial tuner the stereo comes on, and my sony digital pll tuner, the stereo light comes on.

if I send the filtered pilot output to a 386 op-amp and boost the output signal, the Stereo will switch on, on that tuner, but i get more noise if I amplify the signal.
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Old 9th May 2005, 09:43 PM   #6
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Quote:
Btw, channel separation is also limited with the transmitter modulator's phase characteristic.

can you elaborate on that a little bit?


I noticed the highs got that "AM" sound, stereo or not. dont matter.

I dunno whats causing that but oh well.
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Old 9th May 2005, 09:54 PM   #7
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I switched the connection from q to /q on the pilot, didnt make a difference. AARRGGH.

one thing I did notice, is when I switched it, the forground voices sound like it went in the background, so I switched it back


The only thing I can think of, is the modulator bandwidth is screwed. because the highs sound like AM. never did before.
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Old 9th May 2005, 10:05 PM   #8
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the only thing I havent done is change the oscillator transistor. Im using a 2n3053 oscillator stage. It works, and it worked up to this point. then seperation went awall.
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Old 9th May 2005, 10:07 PM   #9
moamps is offline moamps  Croatia
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Quote:
Originally posted by mbates14
[B].....because the highs sound like AM. never did before....
Did you apply preemphasis circuit (50 or 75uS)?
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Old 9th May 2005, 10:41 PM   #10
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no, winamp does that automatically, with Sound Solutions DSP.
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