Help me understood this stereo decoder schematic

[oasis4355]

Stereo Decoder [Help]

Hi,

http://circuitos.cl.tripod.com/schem/r45.gif

I come across this stereo decoder, i understand that 19khz pilot is a signal for the decoder to dectect that it's a MPX signal that it's receiving.

I'm wondering the usage of Pin 15 of this IC, LM1800, what's the oscillator for?
And the "pilot monitor" on pin11, what does that actually means?

Please help

Thanks in advance
Charis

 

[Borber]

Re: Stereo Decoder [Help]

First look at datasheet:
**broken link removed**
This decoder is PLL type. VCO is RC type and on pin 15 must be connected appropriate RC for frequency of oscillation which is 76kHz. Pin 11 is monitor pin where you can measure frequency of VCO divided by 4. It must be near 19kHz when PLL is not locked, no MPX signal present. With MPX present you adjust R5 (VCO frequency) untill it locks and stereo lamp on pin 7 lights up.

 

[oasis4355]

Re: Stereo Decoder [Help]

Hi,

I still have some questions...

Why the VCO must be 4 times of the pilot frequency?
Can i replace the lamp with some LEDs?
Since i can adjust VCO untill the lamp is on, what use is pin 11?
Does pin 11 connected to some other circuit?
Can i just leave pin 11 open?

By the way, do you know where i can find a not PLL type of decoder? I tried google it, can't find much of it.

Thanks in advance
Charis

 

[Borber]

Re: Stereo Decoder [Help]

Frequency of VCO was choosen as low as possible for simplicity reasons. Needed 38kHz demultiplexer signal, which must have 50:50 duty cycle can be acheived with simple divider by 2. Nobody said it must be 76kHz.
Lamp can be replaced with LED and appropriate series resistor.
Pin 11 is used for monitoring if needed. Normaly it is left open.
First stereo decoders build with discretes like tubes or transistors were using 19kHz pilot signal and frequency doubler to produce 38kHz.
All stereo decoder chips from that generation are using PLL technique for generation 38kHz demultiplexing carrier.