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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 100Vpp rectangular waveform  
It was designed by [[John Addis]].
(e.g., from the calibrator output of an oscilloscope)  
 
and produces a 250mVpp rectangular waveform  
It takes in a 60 to 100 V<sub>pp</sub> rectangular waveform (e.g.  
with risetime less than or equal to 125 picoseconds.   
from the calibrator output of an oscilloscope) and produces a 250 mV<sub>pp</sub>
The output pulse is produced by a 10mA 2pF [[tunnel diodes|tunnel diode]], part number [[152-0177-02]].
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.


The circuit starts with a DC restorer. 
==Circuit==
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-ohm potentiometer (assume 100 ohms). 
R2 is 390 ohms.


* Ie (input waveform high) = (6.2V - 0.7V) / (100 ohms + 390 ohms) = 11.2mA
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 Ic = Ie.
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 (70mV)
and Ic of Q1 is 11.2mA.  Ic splits between the tunnel diode and R7, which is 43 ohms).
Assume the output is terminated by a 50-ohm load.  The current in R7 is


* I_R7 (tunnel diode off, input waveform high) = 70mV/(43+50) = 0.75mA.
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).


So the tunnel diode gets 11.2mA - 0.75mA = 10.45mA.   
Assume the output is terminated by a 50 Ω loadThe current in R7 is  
This is enough to 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 4mA) 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,
* I<sub>R7</sub> (tunnel diode off, input waveform high) = 70 mV / (43+50) Ω = 0.75 mA.
there is a chain of three 1k composition resistors in series.   
 
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 = 0.025V /  0.006 = 4.2 ohms
* r_d = 0.025 V /  0.006 A = 4.2 Ω


Empirical measurements were closer to 7 ohms.   
Empirical measurements were closer to 7 Ω.  This, plus the 43 Ω output resistor,  
This, plus the 43-ohm output resistor,  
makes for a 50 Ω output resistance when the tunnel diode is in the high-voltage state.   
makes for a 50-ohm output resistance when the tunnel diode is in
To prevent the 43 Ω output resistor from appearing inductive and spoiling the flat top  
the high-voltage state.  To prevent the 43 ohm output resistor from  
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. The center
conductor of the connector has a small hole to accept the 43 ohm
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 5GHz and well damped, though the  
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 1.5GHz BW, the ring would
likely be invisible. 


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]]

Revision as of 06:16, 15 June 2014

The Tektronix 067-0681-01 is a pulse generator for adjusting the transient response of high frequency oscilloscopes such as the 475, 485, 7A11, 7A24, 7A19/7904, and 7A29/7104. It was designed by John Addis.

It takes in a 60 to 100 Vpp rectangular waveform (e.g. from the calibrator output of an oscilloscope) and produces a 250 mVpp rectangular waveform with rise time less than or equal to 125 picoseconds.

The output pulse is produced by a 10 mA, 2 pF tunnel diode, part number 152-0177-02.

The input and output connectors are BNC. The gender of the pulse output connector is appropriate for connecting directly to a BNC input of an oscilloscope (i.e., with no cable between the pulse generator and the oscilloscope to spoil the risetime).

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 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.

  • IE (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. The 2N5087 has a minimum beta of 250, so we can assume IC = IE.

Assume the tunnel diode is in its low-voltage state (70 mV) and IC of Q1 is 11.2 mA. IC 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

  • IR7 (tunnel diode off, input waveform high) = 70 mV / (43+50) Ω = 0.75 mA.

So the tunnel diode gets 11.2 mA - 0.75 mA = 10.45 mA. This is enough to 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.

Output

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. It is approximated in theory by

  • r_d = 0.025 V / 0.006 A = 4.2 Ω

Empirical measurements were closer to 7 Ω. This, plus the 43 Ω output resistor, makes for a 50 Ω output resistance when the tunnel diode is in the high-voltage state. To prevent the 43 Ω output resistor from appearing inductive and spoiling the flat top of the output waveform, the resistor is actually mounted inside the BNC output connector. The center conductor of the connector has a small hole to accept the 43 Ω resistor’s lead. Because of this, the resonant frequency of the TD and its parasitics is around 5 GHz and well damped, though the exact resonant frequency depends on exact lead length of components which varies somewhat during production.

For any scope with less than about 1.5 GHz BW, the ring would likely be invisible.

Data sheet