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 phase between the 19KHz & 38 Khz signals is digitally controled by the µP. The adjustment is performed by acting on the microswicthes.

The design includes 2 XLR connectors, 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.

This new coder stereo integrates input RDS and a 19K reference ouput to synchronize the RDS coder.

 

 

 

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 : -3 to +9 dbm
Output level : -0.7dbm
spurious> 100 KHz : < -60 dbc
spurious> 200 KHz : < -80 dbc
Pre emphasis : 50us

Stereo coder Schematic (Fichier PDF)

PCB & BOM (PDF File)