The EPM-E441A and EPM-442A instruments were the first EPM power meters released by HP, along with the meters HP also released 2 new sensors the ECP-E18A (18 GHz) and ECP-E26A (26 GHz) both diode sensors which featured an EEPROM to store the calibration factors and linearity calibration data. The range is -70 dBm to + 20 dBm. The EPM-442A features two channels. A short time later Agilent changed the model to E4418A (single channel) and the sensors to E4412 and E4413A. Later on the A model was replaced by the B model which added additional interface options. Around the same time the E9300 series of sensors was introduced, along with a retrofit kit to add compatibility to the legacy A meters The E4419A/B is the dual channel version of the E4418A/B. Max speed is 200 readings per second. These meters are compatible with the HP848x sensors. The EPM-441A/E4418A/B is code compatible with the HP437B RF power meter. The EPM-442A/E4419A/B with the HP438A power meter. Additionally they comply to the SCPI standard. It is advisable to perform a firmware upgrade, if applicable. Refer to the Keysight Technical Support page. A support program must be used: dloadwin32.exe, that must be run in XP compatibility mode. Follow the instructions on screen. For example: after the upgrade procedure a EPM441A behaves like a E4418A. Obviously you need an interface: the HP82357 USB-GPIB will work fine. A weakness of the E44x power meter is the power supply (specifically at 220-240 Vac, 24/7). Once plugged the SMPS (OEM product) is always “on”. Internally an electrolytic capacitor (4.7 uF, 60 V) adjacent to a hot 100 kOhm 2 W resistor can get cooked. Best to replace this cap preventive and bent away from the resistor. Also the keypad needs replacement at times. The E441x series have different smps.