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The Tektronix 067-0681-01 is a pulse generator | The '''Tektronix 067-0681-01''' is a pulse generator for adjusting the | ||
for adjusting the transient response of high frequency oscilloscopes | transient response of high frequency oscilloscopes such as the [[475]], | ||
such as the [[475]], [[485]], [[7A11]], [[7A24]], [[7A19]]/[[7904]], and [[7A29]]/[[7104]]. It was designed by John Addis. | [[485]], [[7A11]], [[7A24]], [[7A19]]/[[7904]], and [[7A29]]/[[7104]]. | ||
It takes in a 60 to | It was designed by [[John Addis]]. | ||
(e.g. | |||
and produces a | It takes in a 60 to 100 V<sub>pp</sub> rectangular waveform (e.g. | ||
with | from the calibrator output of an oscilloscope) and produces a 250 mV<sub>pp</sub> | ||
The output pulse is produced by a | rectangular waveform with rise time less than or equal to 125 picoseconds. | ||
The output pulse is produced by a 10 mA, 2 pF [[tunnel diodes|tunnel diode]], | |||
part number [[152-0177-02]]. | |||
The input and output connectors are [[Connectors#BNC|BNC]]. | The input and output connectors are [[Connectors#BNC|BNC]]. | ||
The gender of the pulse output connector is appropriate for | The gender of the pulse output connector is appropriate for connecting directly to | ||
connecting directly to a BNC input of an oscilloscope | a BNC input of an oscilloscope (i.e., with no cable between the pulse generator and | ||
(i.e., with no cable between the pulse generator and the oscilloscope | the oscilloscope to spoil the risetime). | ||
to spoil the risetime). | |||
A regular BNC cable is typically used for connecting the input that drives | A regular BNC cable is typically used for connecting the input that drives | ||
the 067-0681-01. The risetime of the input to to the 067-0681-01 is not | the 067-0681-01. The risetime of the input to to the 067-0681-01 is not | ||
important. | important. | ||
==Circuit== | |||
* | The circuit starts with a DC restorer. The output of the DC restorer turns a current | ||
source on and off. The current produced by the current source (at the collector of PNP Q1) | |||
is set by controlling the emitter current using 6.3V zener diode CR3, R1, R2, and Vbe of Q3. | |||
R1 is a 200 Ω potentiometer (assume 100 Ω). R2 is 390 Ω. | |||
* I<sub>E</sub> (input waveform high) = (6.2 V - 0.7 V) / (100 Ω + 390 Ω) = 11.2 mA | |||
Q1 is Tektronix part number [[151-0410-00]] which is a 2N5087. | Q1 is Tektronix part number [[151-0410-00]] which is a 2N5087. | ||
The 2N5087 has a minimum beta of 250, so we can assume | The 2N5087 has a minimum beta of 250, so we can assume I<sub>C</sub> = I<sub>E</sub>. | ||
Assume the tunnel diode is in its low-voltage state (70 mV) and I<sub>C</sub> of Q1 is 11.2 mA. | |||
I<sub>C</sub> splits between the tunnel diode and R7, which is 43 Ω). | |||
Assume the output is terminated by a 50 Ω load. The current in R7 is | |||
Between the collector of the switched current source transistor Q1 and the output tunnel diode, | * I<sub>R7</sub> (tunnel diode off, input waveform high) = 70 mV / (43+50) Ω = 0.75 mA. | ||
there is a chain of three | |||
The purpose of this unusual design is almost certainly to reduce parasitic reactances that | So the tunnel diode gets 11.2 mA - 0.75 mA = 10.45 mA. This is enough to | ||
could slow the output pulse by coupling the tunnel diode to the transistor's capacitance, and could | switch the tunnel diode, even near the high end of its peak current tolerance range. | ||
After the tunnel diode switches to the high voltage state, more current (about 4 mA) | |||
is diverted into the load instead of the tunnel diode. | |||
Between the collector of the switched current source transistor Q1 and the output tunnel | |||
diode, there is a chain of three 1 kΩ composition resistors in series. The purpose of this | |||
unusual design is almost certainly to reduce parasitic reactances that could slow the | |||
output pulse by coupling the tunnel diode to the transistor's capacitance, and could | |||
cause the output to ring, resulting in flat-top aberrations. | cause the output to ring, resulting in flat-top aberrations. | ||
===Output=== | |||
The dynamic resistance of the tunnel diode in the high voltage state is low, but not | The dynamic resistance of the tunnel diode in the high voltage state is low, but not | ||
low enough for the diode to be modeled as a rock-solid voltage source. | low enough for the diode to be modeled as a rock-solid voltage source. | ||
It is approximated in theory by | It is approximated in theory by | ||
* r_d = 0. | * r_d = 0.025 V / 0.006 A = 4.2 Ω | ||
Empirical measurements were closer to | Empirical measurements were closer to 7 Ω. This, plus the 43 Ω output resistor, | ||
This, plus the | makes for a 50 Ω output resistance when the tunnel diode is in the high-voltage state. | ||
makes for a | To prevent the 43 Ω output resistor from appearing inductive and spoiling the flat top | ||
the high-voltage state. To prevent the | of the output waveform, the resistor is actually mounted inside the BNC output connector. | ||
appearing inductive and spoiling the flat top of the output waveform, | The center conductor of the connector has a small hole to accept the 43 Ω | ||
the resistor is actually | |||
mounted inside the BNC output connector. | |||
conductor of the connector has a small hole to accept the | |||
resistor’s lead. Because of this, the resonant frequency of the | resistor’s lead. Because of this, the resonant frequency of the | ||
TD and its parasitics is around | TD and its parasitics is around 5 GHz and well damped, though the | ||
exact resonant frequency depends on exact lead length of components | exact resonant frequency depends on exact lead length of components | ||
which varies somewhat during production. | which varies somewhat during production. | ||
For any scope with less than about 1.5 GHz BW, the ring would likely be invisible. | |||
==Data sheet== | |||
* [http://w140.com/tek_067-0681-01.pdf Tektronix 067-0681-01 Datasheet (PDF)] | * [http://w140.com/tek_067-0681-01.pdf Tektronix 067-0681-01 Datasheet (PDF)] | ||
[[Category:Pulse generators]] |