Noise measurement and units conversion

Andrew,
Thank you for your response. I’m sure that this could work assuming, as you say 50 ohms and 1Hz. However, the lock-in measurement is at an unknown impedance and it is a 600Hz chopped signal. On a spectrum analyzer I think I’ll expect to see a signal spike at 600Hz and noise below the skirts where the whole trace would have a 50% duty cycle in time domain. The UUT is a biased PbS flame sensor with a blackbox radiator producing the stimulating signal. There is a fan between the BB radiator and the UUT that alternately passes and block the radiated signal at a rate of 600Hz. I was hoping that I could make the measurement in volts. Of course, this would require a high input impedance at the SA input. I think that if I could, then I could set the noise markers at some distance in the noise, take the square root of the difference in frequencies and divide the voltage measurement by the square root of the difference in frequencies to get my nV/SQRT(Hz) result. I don’t know if this is even reasonable, correct or within the capabilities of the SA44. What do you think?