Experimental techniques for the measurement of low-level far-from-carrier noise in cellular telephone power amplifiers

A necessary requirement for a transmit power amplifier (PA) used in analogue cellular telephones is a low noise output level at the receive frequency. Since the system operates in full duplex, excessive noise from the power amplifier will reduce the receiver sensitivity. A large degree of rejection of this noise is provided by the duplexing filter (s) but limitations on size and cost of these filters places a limit on the allowable PA noise. Thus it is important to be able to measure the noise output of a PA accurately before design-in, to ensure that the overall system specifications are met. This paper discusses typical noise specifications for power amplifiers for 900 MHz analogue cellular telephones and describes a test-set which has been developed to make accurate measurements of noise output under large signal conditions