Re: 7a26 rise time


On Thu, May 17, 2018 at 10:16 pm, lop pol wrote:
Messing around today after i THOUGHT i had things straighted out, I was
looking at the rise time again and 50mV is good 10mV is fine but now 20 mV
only has horrible rise time. Do you guys think that means another cap is open
If it's only happening on the 20mV/div setting, it's probably something to do with the attenuator compensation capacitors. These are also prone to problems, though I've found that it's not so much the capacitors that cause trouble as the solder joints to the pins of the attenuator module. They need a careful hand and a fine iron to tidy up.

You won't be able to set the attenuator compensation correctly without an RC normalizer on the input. I made my own with a little box containing a 1Mohm resistor and a trimmer cap (set the trimmer cap like you would a scope probe on a known-good input). Without the normalizer, the attenuator will appear to set up correctly though some adjustments have very little effect, but in fact the result is badly wrong. The adjustments on the attenuator modules interact because they're cascaded, so it's important to follow the procedure in the manual. I found that the 100mV/div setting (the first 10x attenuator) was hard to get right so that the square wave looked right both with and without the normalizer, so I had to make gentle tweaks adding and removing the normalizer to get the best setting.

The point of all this is to get the frequency compensation and input capacitance the same at all attenuator settings.

The procedure I use if I can't be bothered to find the manual is this:
- set attenuator to 10mV/div (straight-through, no attenuation)
- connect fast rise square wave generator to input with in-line 50R terminator (to make sure there are no aberrations due to the cable), *no* RC normalizer
- set up LF/MF/HF trimmers on amplifer board for optimum settings
- insert RC normalizer between square wave gen and input. You can remove the terminator if it makes it easier for your generator to produce the right output, because we no longer need such fast edges for the rest of the adjustments
- adjust normalizer trimmer for optimum square wave
- set attenuator to 20mV/div. Adjust 2x attenuator trimmers for optimum corner and flat top.
- set attenuator to 50mV/div. Adjust 5x attenuator trimmers for optimum corner and flat top.
- set attenuator to 100mV/div. Adjust first 10x attenuator trimmers for optimum corner and flat top.
- check 200mV/div and 500mV/div settings on the way past
- set attenuator to 1V/div. Adjust second 10x attenuator trimmers for optimum corner and flat top.
- check 2V/div and 5V/div settings
- go back and check all attenuator settings again
- remove normalizer
- check all attenuator settings again
- repeat until it's good enough

Don't do this without the screening cover over the attenuators because it makes a significant difference to the adjustments. I wasted enough time with that!


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