There is a substantial difference between a potentiometer used as voltage divider and a potentiometer used as rheostat. The former (which is likely what Tektronix uses) does not present many difficulties because things can be arranged such that near-zero current flows through the wiper. The later however *is* troublesome as the (ill-defined, noisy and nonlinear) contact resistance is directly in series with the wanted resistance, and the use of expensive parts does only partly aleviate the troubles.
Samuel
Just read about the lightspeed, very curious. The diode effect is eliminated using wire wound pots because the wiper and the wire are the same material so of no more consequence than a gold to gold RCA as example.
The main problem with pots for AC signals is the coupling capacitance of the parts inside to each other. Pots for RF work are all from 25-100 ohms. For telecom the pots are 300, 500, or 600 ohms. Only in audio are high resistance pots used instead of stepped attenuators on the AC part of the signal. That "variable" gain pot in the center of the stepped attenuator on your scope... likely a 500 ohm or less in the gain feedback circuit and not used as a passive device. Personally gave up using pots more the 500 ohms about 20 years ago. Lots of testing showed the pot had no real sound character of its own at 500 ohms. Even the fanciest most expensive pots of the 10K ohm variety sound pretty strange when compared to a 500 ohm wire wound. The wire wound and a wire could not be distinguished from each other. Something that was very easy to do with the 10k ohm pot no matter the type or cost. The same is true for the stepped attenuator made of a bunch of resistors in series at 10K ohms. The "proper" way to make a stepped attenuator is to have many 2 resistor attenuators and select which attenuator is used.. Bad pot is bad pot and includes any value over 500 ohm for AC signals.
The main problem with pots for AC signals is the coupling capacitance of the parts inside to each other. Pots for RF work are all from 25-100 ohms. For telecom the pots are 300, 500, or 600 ohms. Only in audio are high resistance pots used instead of stepped attenuators on the AC part of the signal. That "variable" gain pot in the center of the stepped attenuator on your scope... likely a 500 ohm or less in the gain feedback circuit and not used as a passive device. Personally gave up using pots more the 500 ohms about 20 years ago. Lots of testing showed the pot had no real sound character of its own at 500 ohms. Even the fanciest most expensive pots of the 10K ohm variety sound pretty strange when compared to a 500 ohm wire wound. The wire wound and a wire could not be distinguished from each other. Something that was very easy to do with the 10k ohm pot no matter the type or cost. The same is true for the stepped attenuator made of a bunch of resistors in series at 10K ohms. The "proper" way to make a stepped attenuator is to have many 2 resistor attenuators and select which attenuator is used.. Bad pot is bad pot and includes any value over 500 ohm for AC signals.
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