(volts) markings on the INPUT SENSITIVITY switch dial. Note that the maximum input levels for the 30 V, 3V 0.3V, etc. ranges are actually 32V, 3.2V and 0.32V,'corresponding to full-scale meter deflection. When the amplitude VERNIER is rotated counterclockwise away from the CAL position, the gain of the Input Circuit decreases, the maximum input level increases and the markings on the INPUT SENSITIVITY switch dial no longer apply. Table 3-3 lists the maximum input levels for
each INPUT SENSITIVITY setting with the amplitude VERNIER in the CAL and fully counterclockwise positions. Observing these maximum input levels will ensure optimum performance on all ranges.
3-40. Sensitivity.
3-41. Maximum Sensitivity. Maximum Sensitivity refers to the smallest signal that can be detected by the analyzer. The maximum sensitivity of the analyzer is limited by its own internally generated noise and is commonly defined as the point where the signal level is equal to the noise level. This is sometimes called "tangential sensitivity".

3-42. Nyquist's Noise Equation* reveals two important  things about noise that apply to the 3581:  a. Noise is proportional to the square root of band- width. .. Noise level decreases and sensitivity increases as the BANDWIDTH setting is narrowed. b. Noise is proportional to the square root of input resistance...The 3581 has a high (1 megohm) input resistance. This means that noise is largely dependent on the source resistance placed at the INPUT terminals. Signal sources having low output resistances will produce a lower noise level than those having high output resistances.
3-43. Noise level is also dependent on the tuned frequency of the instrument. Semiconductors in the input stages of the instrument exhibit surface noise which has a 1/f frequency spectrum. This surface noise is predominate at frequencies below 1 kHz. When the 3581 is tuned below 1 kHz, the noise level increases and sensitivity decreases.
3-44. Figure 3-11 is a family of curves showing the  specified noise levels vs. frequency for the 300 Hz 30 Hz and 3 Hz BANDWIDTH settings. Typically, if the' source resistance is less than 10 kilohms, the noise levels will be below those indicated by the curves. 3-45. Full Scale Sensitivity. Full scale sensitivity defines the input level that will produce full scale deflection on any given range. For absolute measurements, full scale sensitivity ranges from 30V rms to 0.1 uV rms in the Linear mode and from + 30 dBV/dBm to - 140 dBV/dBm in the Log (90 dB) mode. With the amplitude VERNIER control set fully counterclockwise, full scale sensitivity ranges from approximately 100 V rms to 0.3 mV rms in the Linear mode