|
Hoping this finds you all well with a good start into 2022.
Back at this. I've replaced the dual diode with an HSMS-8202. That solved DC offset, with the no-input trace right in the center of the display with the offset control centered, and the mysterious bi-level "noise" went away. DC offset can shift the trace off top or bottom of the screen. That gets me back to showing an attenuated and somewhat slow representation of the input on that channel. Attenuated to ~1/4 expected amplitude with rise/fall time up closer to 1 ns vs. ~400 ps on the healthier channel. The LO NOISE still amplifies ~2x and shifts the displayed signal, but with less shift. The attenuation far exceed the small range of the front panel gain adjustment. I tried adjusting the loop gain, dc balance, and memory balance on the sampler for lack of a better idea. The procedure includes adjusting DC BALance to minimize trace shift while twiddling LOOP GAIN, then setting the loop gain just below where the displayed response to a square edge starts to overshoot. The DC balance control was able to reduce but not eliminate trace shift while twiddling loop gain. Increasing loop gain *decreased* displayed square wave amplitude (increase attenuation) down to zero -- i.e. reduce wave height above baseline -- then *continued* to depress the wave tops below baseline, inverting the wave shape. Further increasing loop gain produced an inverted signal with greater amplitude than the maximum at zero gain, but still less than input amplitude, before going into overshoot. That sounds like maybe a clue to what's wrong, but I haven't decoded it. For all of the unity-or-less loop gain range, the LO NOISE mode displays the same upright, ~2x amplified trace regardless of loop gain. The MEM BALance control was able to affect but not eliminate trace shift with LO NOISE mode. For slow signals, the square wave tops appeared to follow a sine wave shape. Adjusting L. F. COMP appeared to alter the phase of that sine pattern relative to the square wave without affecting it's amplitude. The amplitude of the sine increased with lower frequencies. Pictures will likely help with describing that... I haven't messed with the AVALANCHE control for lack of a lower lever (~250mV) square edge source which the calibration procedure calls for. Needing an adjustable source voltage or attenuator. My current thought is to solder a calculated SMD resistor across the back of the input from my fast(ish) square wave source -- which is stuck directly on the scope input BNC. The manual doesn't suggest that will influence amplitude, but it's another knob to twist to see what it reveals. The behavior described here follows the sampler board when swapped between channels. My small lot of HSMS-8202s, an obsolete part, came from a marginal source. They are marked as described in a found datasheet: 2R{datecode}. At DC the one I've used functions as two diodes with Schottky-ish forward voltages matched to 3 decimals. Does anyone re-mark or fake SOT23s and package them in tape? Seems that would have to be a *very* efficient operation. I added a photo of the installed SOT23 packaged HSMS-8202 to the album [/g/TekScopes/album?id=270747]. Had to undo/redo the battery receiver to make room for that. Could say more about that; maybe later; maybe in that topic vs here [/g/TekScopes/topic/87551264]. So far it seems the reduction/inversion of sampler output with increasing loop gain is the best clue. Dunno what to make of it yet. Happy to hear any suggestions! |
