The HP 8447F OPT H64 is a low noise, high gain pre-amplifier used to increase the sensitivity of measurements. It contains two preamplifiers. One provides 26dB minimum gain to signals from 9 kHz to 50MHz. The other provides 25dB minimum gain to signals from 100 kHz to 1300MHz.  

 Where would this be used? The P 8447F OPT H64 is predominantly used within Radiated Emissions testing to amplify signals whilst keeping the noise floor constant.