The 9503 series is a family of Precision Lock-in Amplifiers which incorporate circuitry and system innovations which advance the state of the art and enable low level ac signals to be accurately measured - even in extremely noisy environments. Six models are available. These differ in the response modes available and the digital output facility. There are three basic versions; the standard 9503 with the normal (squarewave) response mode, the 9503C, which adds the analogue correlation mode and the 9503SC, which in addition to these two modes includes Sinetrac (fundamental only) mode and digital correlation mode. The SC version also has the signal gating facility. Each of these three versions is also available with a digital display and BCD output -these models carrying the suffix ’D’. Both the Sinetrac and analogue correlation modes are based on new pulse carrier modulation techniques*. These facilities either match the instrument response to the signal energy frequencies, even for complex waveforms, or suppress all harmonics to give a ’’fundamental only" response. When harmonic suppression is wanted, or when operating at low frequencies (<lkHz), these options will greatly extend conventional lock-in performance with convenient operation. Further details on all of these response modes are given in the detailed operating instructions in section 5.4 and in greater detail in various Technical Notes available on request. All the 9503 versions have excellent dynamic reserve and output stability - optimised by mode switching pushbuttons - so that it is very suitable for applications when the signal recovery power of a system has to be stretched to the very limit as well as for those applications where the measurement demands extreme precision. When the frequency range of operation has to be extended, the extra bandwidth options make such measurements possible -wide band and without range or circuit card changes. All the necessary interface requirements are incorporated to build up the system capability by means of the range of slot-in modules. These can be fitted as specific applications requirements develop.