enter Thu Feb 18 14:06:53 2021
- finished routing AC power cables for PSOMA table
Commercial current driver noise
- LD Datasheet
- ITC 502 manual
- Measurement configuration
- ITC 502 LD out connected to DB9 breakout board
- Pins 1 and 5 connected with a 50 Ohm dummy load, to lift the driver interlock
- Pins 3 and 7 connected across a 2V diode and 20 Ohm resistor.
- SR560 channel A measuring voltage across the resistor, AC coupled and high-passed (6dB rolloff at 1 Hz) with G=100. The preamp is running on battery power in 'low noise' mode.
While searching for a functioning SR560, I found 2 with bad batteries (can run on AC but not on batt power), one that overloads with no gain or load attached, and one with a 32 MHz oscillation when running on battery (outside the 1MHz amplifier bandwidth, but still odd). All are now labelled as such.
In the theme for the week, I caught my leg on a BNC cable and dropped the Moku from benchtop height  . Tried to convince myself nothing broke by measuring white noise from an SRS function generator, and comparing the LD driver current noise spectrum to that measured by HP 8560E. There's a spurious peak (floating inputs) on ch 1 at 356 MHz (attachment 1), though this is not the first time I've noticed something similar. Still, it's troubling; when I send the current noise from SR560 to either channel of Moku, the peak remains in the signal-carrying channel, even though the signal is well above the noise floor.
Not to mention, the current noise spectrum looks wack. The signal was barely in range of the 8560, but I ended up with a qualitatively similar spectrum after boosting SR560 gain to 1000 (attachment 4). The spectrum was the same with both functioning SR560 I tried.
Note y axis is in dBm, so -100 to -80 dBm/rtHz is 1-10 nA/rtHz out of the driver. The ITC 502 manual quotes < 5 uA rms from 10 Hz to 10 MHz, so 1nA/rtHz is in the right ballpark.
exit Thu Feb 18 21:00:47 2021 |