P
Paul G.
Guest
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.
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.