According to the IRIG-106 telemetry standard, FQPSK-JR and SOQPSK-TG are two interchangeable signals, which have almost the same power efficiency and bandwidth efficiency. The two signals can be demodulated by the universal receiver of OQPSK. The optimal performance degradation of the symbol by symbol demodulation based on the OQPSK structure can reach 2dB while its optimal demodulation may ignore the two signals' characteristics of memory. The 2dB power loss is crucial in the telemetry link for the limit of equipment size and power. The optimum receivers based on their modulation schemes can improve the power efficiency, but the different system of signal generating will lead to different structure of corresponding optimal receiver. In this research, the FQPSK-JR signal in IRIG-106 standard was approximated by CPM modulation, the SOQPSK-TG signal was truncated for an approximate analysis. The result of simulation showed that the signals demodulated by the CPM-based demodulator and by the OQPSK-based demodulator display the same function of little with incremental loss, and demand no need to do signals recognition. In addition, the implementation complexity was also taken into account.