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The '''Tektronix 155-0076-00''' (M94) is a Tek-made input protection and probe logic | [[File:Tek 155-0076-00.jpg|thumb|300px|right|155-0076-00]]The '''Tektronix 155-0076-00''' (M94) is a Tek-made input protection and probe logic | ||
integrated circuit in a 16-pin [[minipac]] package, designed by [[John Addis]]. | integrated circuit in a 16-pin [[minipac]] package, designed by [[John Addis]] in 1971. | ||
The M94 has four subcircuits: | The M94 has four subcircuits: | ||
* DC current sources for biasing | * DC current sources for biasing | ||
* Decoder for the probe attenuation pin signal | * Decoder for the probe attenuation pin signal | ||
* RMS detector | * RMS detector | ||
* | * Output logic and drivers for coil of relay that sets input impedance of scope | ||
The M94 uses an IC fabrication process with NPN and PNP transistors. | |||
The NPN devices in this fabrication process have | |||
good performance and a beta around 100. | |||
In contrast, the PNP transistors are poor-performing lateral devices with beta around 8. | |||
==Biasing== | ==Biasing== | ||
| Line 73: | Line 77: | ||
This current is applied to a leaky integrator. | This current is applied to a leaky integrator. | ||
The electrical time constant of the leaky integrator matches the thermal time constant | The electrical time constant of the leaky integrator matches the thermal time constant | ||
of the most vulnerable parts of the 50 Ω input attenuator. | of the most vulnerable parts of the 50 Ω input attenuator. | ||
In the case of the 11A52, this is a simple RC circuit with a time constant of 294 milliseconds. | |||
The voltage at pin 1 (base of Q33) is inversely related to the modeled temperature of the attenuator. | The voltage at pin 1 (base of Q33) is inversely related to the modeled temperature of the attenuator. | ||
| Line 79: | Line 84: | ||
The voltage on pin 1 is compared with the 3.0 volt reference. | 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 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 | When the input is overloaded, the voltage at the base of Q33 dips below 3.0 volts, and the tail current is steered to Q34's side. | ||
This turns on Q32. | This turns on Q32. | ||
Thus, the collector current of Q32 is essentially a digital signal: on means overload, off means no overload. | Thus, the collector current of Q32 is essentially a digital signal: on means overload, off means no overload. | ||
[[File:Tek m94 integration detection.png|600px]] | |||
---- | |||
Q36, Q37, Q38, and Q39 form an RS flip-flop. | |||
When input overload is detected, Q32 turns on, supplying current to the base of Q36. | |||
The collector voltage of Q36 and Q37 goes to around 0 V. | |||
The pin 13 "MODE INPUT" pin is normally grounded (e.g., in the [[485]]. | |||
In this state, Q38 and Q39 are cut off and their collectors are pulled up by R38. | |||
That causes a current to flow through R53, into the base of Q37, | |||
which keeps the collectors of Q36 and Q37 close to ground potential. | |||
So, once an overload condition is detected, the flip flop stays in the | |||
left-side-active state even after the input is removed. | |||
[[File:Tek m94 output logic.png|600px]] | |||
When the excessive input signal has been removed and the operator wants to re-enable the 50 Ω input, | |||
pin 13 of the M94 is momentarily allowed to float high. | |||
In the 485, this is done by the operator pressing the impedance selector button on the front panel, S125B. | |||
Letting pin 13 float allows current to flow through R45, into the base of Q39. | |||
The resulting collector current in Q39 pulls the collector of Q38/Q39 low, | |||
which stops the current in R53, thus turning off Q37. | |||
Q36 is also off because the excessive input has been removed. | |||
So the collector of Q36/Q37 is pulled high by R36, which also turns on R38. | |||
This leaves the flip-flop in the stable right-side-active state. | |||
When pin 13 is allowed to float high, current flows through R45 into the base of Q44. | |||
The collector of Q44 shunts the R43 current away from the base of Q42, | |||
thus cutting off the current in pin 12. | |||
So, in a 485, while the input impedance selector button is pressed, the | |||
input is disconnected from the 50 Ω attenuator. | |||
There are two cases where the input relay should be de-energized, i.e., | |||
where the input should be connected to the 1 MΩ attenuator instead of the 50 Ω attenuator: | |||
# The operator has selected 50 Ω input impedance, but input overload has been detected. | |||
# The operator has selected 1 MΩ input impedance. | |||
The first case is described above. | |||
It corresponds to the flip-flop being in the stable left-side-active state. | |||
In the second case, where the operator has selected 1 MΩ input impedance, | |||
pin 13 is allowed to float high. | |||
This results in Q44 turning on and shunting current away from Q42, | |||
thus preventing the relay from being energized, leaving the input connected to | |||
the 1 MΩ attenuator. | |||
Note that if pin 13 is allowed to float high, | |||
the state of the flip-flop is of no consequence as far as the MODE RELAY output is concerned. | |||
---- | |||
==Used in== | ==Used in== | ||
| Line 97: | Line 150: | ||
==Links== | ==Links== | ||
* | * https://w140.com/tek_made_sm.pdf | ||
==Photos== | |||
<gallery> | <gallery> | ||
Tek m94 in 485.png|M94 as U80 in [[485]] | Tek m94 in 485.png|M94 as U80 in [[485]] | ||
1971-05-19 M94 Probe Coding early schematic.jpg|Early Pre-Production Schematic of Probe Coding Circuit. It was for the "455" which became the [[485]]. | |||
</gallery> | </gallery> | ||
[[Category:Tektronix-made monolithic integrated circuits]] | [[Category:Tektronix-made monolithic integrated circuits]] | ||
[[Category:Minipac]] | [[Category:Minipac]] | ||