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While comparing schematics of two old analog oscilloscopes (Tektronix 475 and Goldstar 9020,) I was expecting many differences to get better frequency response since the first is a 250MHz and the second is only 20MHz.

My question here is about the way of getting the voltage offset to control the vertical position.

The following figure shows both circuits:

enter image description here

In the Goldstar it is quite straightforward since it creates an offset on the base of one of the emitter-follower driven push-pull.

In the Tektronix a kind of Wheatstone bridge is used to create the offset at the input of the common-base transistors.

Could you please tell the benefits of using the Tektronix solution?

JRE
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Paul Ghobril
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2 Answers2

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The tektronix circuit looks like a bit like a bridge but in reality there are two independent simple voltage dividers between -8V and +8V. One pot controls the top circuit and the other the bottom circuit. There is a third pot to move both circuits against each other simultaneously. Both devices are not that different.

Carsten B.
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    So, what are the benefits (as asked by the OP)? – Andy aka Dec 11 '20 at 12:55
  • Both potentiometers move together but in the opposite direction (logically not physically) to always get the same two opposite resistors of the bridge. there is a reason to add this complexity. You have another difference which is where the offset is applied it must also have a reason. And for the third pot its task is to tune the zero position. – Paul Ghobril Dec 11 '20 at 12:57
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After searching and reasoning about the circuit I think I have something to share concerning the benefits of Tektronix solution and I hope with the help of your comments I can confirm, rectify and complete the answer.

First although it is not a Wheatstone bridge it has the same topology:

enter image description here

Concerning the benefits of not applying the offset at the first stage. Pushing the circuit toward the last stages has the following benefits:

  1. Reducing the interference with the input signal through the adjustment potentiometer.
  2. Reducing the noise since it is mainly imposed by the noise figure of first stages.

Concerning the benefits of the topology (two mechanically constrained potentiometers connected head to tail) creating a balanced offset on both inputs of the common-base transistors:

  1. Improving linearity compared to applying the offset at only one transistor. We must not forget that the push-pull deflection solution is initially preferred over single-ended deflection in order to enhance the linearity. If we apply the offset on only one end we are moving towards a single-ended deflection.
  2. Thermal noise cancelling.
Paul Ghobril
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