The HP3581A/C is a fairly popular instrument, it is used for measuring distortion, noise, and audio signals. It is very sensitive, and you can directly measure resistor noise levels (values > 10Kohms). I was measuring very low level signals and noise with a HP 3581A, and I noticed a signal at approximately 520Hz (plus harmonics), about 1.5 uV, with the input connector shorted. The noise originates from the LED display driver circuit. Does your 3581A/C do the same? The noise depends on the LED brightness, turn down them down, and the noise is reduced. I was able to further reduce the noise by increasing C7 (on assembly A8, the display board) from the normal 200 uF to 1000uF. The circuit uses a daft arrangement that runs CMOS and TTL circuits with gnd, -3.16v, -10v. Bypassing the other supplies with larger caps has no effect. The 520 Hz is derived from the 3KHz display strobe, which is divided amongst the 5 LEDs + out-of-range indicator. I was able to get the offending tone down to about 130nv, but the instruments background noise is about 50nv. Naturally I'd like to completely eliminate the 520Hz disturbance. Otherwise there no significant signals present, other than noise. The powerline interference is barely noticeable.... that's really good when you are measuring down the these low voltages. I can't tell if this is unique to my 3581A, or its an HP design flaw. Anyone ever seen this problem?
Paul G.