An adapter/converter circuit converts transmissions by a mobile telephone built for one standard (e.g., PCS) from the telephone transmit/base station receive frequency of that one standard to the telephone transmit/base station receive frequency of another standard (e.g., cellular), and converts transmissions by a base station or similar device (e.g., a base station simulator) from the telephone receive/base station transmit frequency of the one standard to the telephone receive/base station transmit frequency of the other standard. The two frequency conversions occur simultaneously on two parallel legs under the control of a PLL dual frequency synthesizer, which is in turn controlled by user input (in the case where the adapter is used to allow, e.g., PCS telephones to be tested on cellular test equipment) or by the base station (in the case where the adapter is used to allow, e.g., PCS telephones to be used with a cellular base station).