The heart of this design is based on the µP ATM8.
This one drives a high speed switch to encode left and right channels in concordance with FM rules.

The µP provides also the 19KHz signal to synchronise the fm receiver during the demodulation.

The design includes preemphasis (50µS), and switching filter to limit the audio response to15 KHz.
The left and right channels are 8x sampled.
Finally, a low pass filter ensures spurious rejection to 60 dB and an operationnal amplifier allows this coder to be compatible with all FM transmitter.

 

 

 

 

Specifications

Stereo seperation : >50 db
Pilot frequency: 19 KHz +/- 2 Hz
Pilot THD : <0.1 %
Pilot level : 9%
38 khz rejection : >65 db
IMD and beats : >60 db
Input level : -10 db to +20 db
Output level : +6db
spurious> 100 KHz : < -60 dbc
spurious> 200 KHz : < -80 dbc
Pre emphasis : 50us

 

Stereo coder Schematic (PDF File)