about the modulation fail due to the external regulator

here is a problem about the spectrum of GSM .
A GSM Module , which power from external DC-DC regulator. as the osciallator of this DC-DC regualtor works around 400khz , when test GSM spectrum due to modulation , it fail on 400Khz . as its fixed working frequence of regulator , how to resolve it, thanks

The decoupling capacitors and the layout of the DC converter is very important minimizing the generated spurious. Use low ESR decoupling capacitors, for Vcc and Bias lines of the GSM module. Use proper ground for these capacitors. Ferrite beads with high resistance at 400kHz will help also.
The ground of charging capacitor from the output of the DC converter shall not be crossed by the GSM module ground.

thanks