The Nelson-Ross PSA-011 is a spectrum analyzer plug-in for 500-series scopes. The center frequency range is 10 Hz to 20 kHz. It can display amplitude in linear or log scale.
The input is high-impedance and is AC-coupled. The PSA-011 is a single-conversion architecture with a pair of 6BH6 tubes serving as a balanced mixer. After IF filtering and amplification, the signal is detected using a 1N34A germanium diode. The detected voltage from the diode directly drives pin 3 (vertical -) of the scope plug-in interface connector. Pin 1 (vertical +) of the scope plug-in interface connector is fed a DC vertical position voltage by the PSA-011.
The voltage-controlled local oscillator is tuned using a variable-reactance tube, V6, a 6201.
There were at least two major versions produced, without changing the model number. Compare the picture from the manual below with the other front panel photographs.
There was also a tracking generator, called the Nelson Ross 101 SYNCHRO-SWEEP Generator. These are very rare.