Re: 7A26 attenuator compensation problem

Fabio Trevisan
 

Apologies to everyone, I sent the message before finishing... Resuming now:

On Mon, Jan 28, 2019 at 06:16 PM, Nenad Filipovic wrote:

Thank you all for your hints, but still no go.
Yes, 10mV and 20mV are pass-through...
Hello Nenad,
Arrived a little bit later to this, but I see there might be a confusion or misunderstanding that can ruin all the process down the road.
I do own a 7A26 myself and 10 and 20mV ARE NOT the ranges where all the high impedance attenuators are BYPASSED.
They're 5mV and 10mV / div, being that 10mV/div is the attenuation that shares the same preamplifier GAIN with all the other (higher) attenuation factors, while the 5mV/div setting stands alone on its own, as it employs no input attenuation (just as the 10mV does) and doubles the gain of the pre-amplifier.
If you're performing the preamplifier's LF and HF response adjustments at 10mV/div you're fine but, if you're performing them using the 20mV range, believing this range is a BYPASS range, then you will be compensating on the preamplifier's AC response adjustments, whatever deviation your X2 attenuator block might have, and this is not right.

So - risking to digress from Tektronix a little bit - basically what you would want is:
1. Go through the checks of the DC attenuation factors, to rule-out that there might be some bad contacts on the attenuator blocks and/or the contact leaves.
2. Using a low impedance source (e.g. going through a 50 Ohm past-through terminator), and using the 10mV range (and not the 5mV range in my opinion), perform all the preamp AC adjustments (LF and HF step response).
3. Changing over from the 50Ohm terminator to the 1M Input normalizer, adjust the pre-amlipfier's input capacitance (C130 alone or C130 and C134, depending on the serial number of your 7A26).
4. Then perform each attenuator block's Frequency compensations and Input capacitances in the following order:
4a. Change from 1M Input normalizer to 50Ohm terminator, Set range to 20mV, Set generator's amplitude accordinglly, adjust C106 (this is x2 att.'s freq. compensation).
4b, Change from 50Ohm term. to 1M Input normalizer, leave everything else untouched, adjust C107 (this is x2 att's input capacitance).
4c. Change from 1M Input normalizer to 50Ohm terminator, Set range to 50mV, Set generator's amplitude accordinglly, adjust C110 (this is x5 att.'s freq. compensation).
4d, Change from 50Ohm term. to 1M Input normalizer, leave everything else untouched, adjust C111 (this is x5 att's input capacitance).
4e. Change from 1M Input normalizer to 50Ohm terminator, Set range to 100mV, Set generator's amplitude accordinglly, adjust C114 (this is 1st x10 att.'s freq. compensation).
4f, Change from 50Ohm term. to 1M Input normalizer, leave everything else untouched, adjust C115 (this is 1st x10 att's input capacitance).
4g. Change from 1M Input normalizer to 50Ohm terminator, Set range to 1V, Set generator's amplitude accordinglly, adjust C118 (this is 2nd x10 att.'s freq. compensation).
4h, Change from 50Ohm term. to 1M Input normalizer, leave everything else untouched, adjust C119 (this is 2nd x10 att's input capacitance).

Some might disagree from me, about keeping changing over between 50Ohm terminator and 1M Input normalizer, but the net result is the same, and by doing the frequency compensation adjustments always from a low impedance source, helps preventing that the input capacitance from the block being adjusted (which is not yet calibrated up to each step) will interfere in the compensation adjustment itself, even if it's grossly off.

Of course that this method implies that your generator is capable of sourcing at least 10Vpp on 1M load, and 5Vpp on 50Ohm (for an optimal 5 div display at 1V/div).

Doing this way has always yielded predictable results for me.

Rgrds,

Fabio

Join TekScopes@groups.io to automatically receive all group messages.