My MX919B product works fine at 9600bps, but when I increase the data rate to 19.2kbps, the BER degrades badly. What is going on?
A. The problem is most likely the IF filter bandwidth. When the 9600bps radio is receiving the preamble, the symbol sync pattern ( +3+3-3-3 ) effectively has a 2400bps fundamental frequency component. When the preamble moves into the other data packets (frame sync, data), the higher frequency content of that data is still within the IF filter skirts and no phase distortion occurs: BER is acceptable. When the 19.2kbps radio trains, its symbol sync has a 4800bps fundamental frequency component, and this likely passes through the IF filter with no problem. However, if the 19.2kbps and 9600bps radios have the same IF filter bandwidth, the higher frequency content in the 19.2kbps data stream is likely bleeding over into the IF filter skirts. This can cause phase distortion and will lead to incorrect decisions about incoming signal zero crossings. This will then lead to poorer BER.
