The (Wavetek) Rockland 7530B is an FFT spectrum analyzer plug-in for 7000-series scopes.
Manufacturer was Rockland Scientific Corporation (later acquired by Wavetek).
The 7530A and 7530B Spectrum Analyzer plugins for Tek 7000 scopes are outstanding, and unique, Spectrum Analyzers.
In the mid-1970s, Rockland Instruments was a small company driven by brilliant engineers who thought outside the box to create state of the art instruments. It was no surprise that Wavetek, which had greater resources and name recognition, bought them several years later to get access to their engineering team.
In 1976 they were among the first, if not the first, to create a spectrum analyzer using an A/D to sample the incoming waveform, convert it into a stream of digital values, perform an FFT (Fast Fourier Transform algorithm) on the data, then use a D/A to convert the results back into the display that appears on the CRT of the 7000 series scope.
HP was not far behind Rockland Instruments. By comparison, Tektronix was so deeply committed to swept-IF Spectrum Analyzers that it took over a decade before they realized the advantages of FFT Spectrum Analyzers.
The only limitation the 7530 has is due to the speed and performance of microprocessor chips available in the mid-1970s. The only solution at the time that would be fast enough and precise enough was to create a 12-bit wide CPU using bit-slice microprocessor chips.
The result is a 3-wide 1 Hz to 100 kHz FFT Spectrum Analyzer plugin for the 7000 series scopes that meets or exceeds the specs of the 7L5 in the same frequency range.