Fault on PPM input. Sbus its ok

Hello everybody:
My name it’s Oier and I have a problem. I connect my 9x8cv2 turnigy receiver to an Rmilec pwm-ppm-sbus converter. If I connect to pixhawk on rc input trough ppm conversor output, on radio calibration page, the radio channel values disappears and rarely appears very quickly and disappears again. If I connect trough sbus output and rc input, the radio channel appears and works fine.
I check the ppm output with an oscilloscope and signal seems to be OK. On sbus an ppm, the signal voltage its near 3volts.
Any idea why works on sbus and no on PPM?
Thanks in advance