155-0076-00: Difference between revisions
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[[File:Tek m94 probe ring decoder identify.png|600px]] | [[File:Tek m94 probe ring decoder identify.png|600px]] | ||
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==RMS Converter== | |||
==Output Logic== | |||
The objective of the output logic is to protect the 50 Ω input attenuator from overheating. | |||
The strategy is to build an electrical model of the 50 Ω attenuator's thermal dynamics, | |||
and to disconnect the input signal from the relatively fragile 50 Ω attenuator | |||
if further electrical input would put it at risk of overheating. | |||
The collector current of Q27 is quasi-proportional to | |||
the instantaneous power dissipated in the 50 Ω input attenuator. | |||
This current is applied to a leaky integrator. | |||
The electrical time constant of the leaky integrator matches the thermal time constant | |||
of the most vulnerable parts of the 50 Ω input attenuator. | |||
The voltage at pin 1 (base of Q33) is inversely related to the modeled temperature of the attenuator. | |||
A 3.0 volt reference is established at the base of Q34. | |||
The voltage on pin 1 is compared with the 3.0 volt reference. | |||
When the input is not overloaded, the 0.1 mA tail current of Q33/Q34 is steered to the Q33's side and Q32 is cut off. | |||
When the input is overloaded, the voltage at the base of Q33 dips below 3.0 volts, and the the tail current is steered to Q34's side. | |||
This turns on Q32. | |||
Thus, the collector current of Q32 is essentially a digital signal: on means overload, off means no overload. | |||
==Used in== | ==Used in== | ||