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067-0681-01: Difference between revisions
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and produces a 250mVpp rectangular waveform | and produces a 250mVpp rectangular waveform | ||
with risetime less than or equal to 125 picoseconds. | with risetime less than or equal to 125 picoseconds. | ||
The output pulse is produced by a 10mA 2pF [[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 | ||
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R2 is 390 ohms. | R2 is 390 ohms. | ||
* (6.2V - 0.7V) / (100 ohms + 390 ohms) = 11.2mA | * Ie (input waveform high) = (6.2V - 0.7V) / (100 ohms + 390 ohms) = 11.2mA | ||
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 (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. | |||
So the tunnel diode gets 11.2mA - 0.75mA = 10.45mA. This 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, | |||
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 | |||
could ring, resulting in aberrations in the flat top of the output pulse. | |||
The dynamic resistance of the tunnel diode in the high voltage state is low, but still significant. | |||
It is approximated by | |||
* r_d = 0.025V / 0.006 = 4.2 ohms | |||
The ringing current of a parasitic L-C resonator dissipating into the tunnel diode's dynamic resistance | |||
could result in significant time-domain voltage wiggles which would be difficult to distinguish from transient response | |||
problems in the vertical amplifier of the oscilloscope under test. | |||
* [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)] | ||