067-0681-01: Difference between revisions
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there is a chain of three 1k composition resistors in series. | 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 | The purpose of this unusual design is almost certainly to reduce parasitic reactances that | ||
could ring, resulting in | could slow the output pulse by coupling the tunnel diode to the transistor's capacitance, and could | ||
The dynamic resistance of the tunnel diode in the high voltage state is low, but | cause the output to ring, resulting in flat-top aberrations. | ||
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 by | It is approximated by | ||
Revision as of 20:07, 30 July 2012
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 takes in a 60 to 100Vpp rectangular waveform (e.g., from the calibrator output of an oscilloscope) and produces a 250mVpp rectangular waveform with risetime less than or equal to 125 picoseconds. The output pulse is produced by a 10mA 2pF 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.
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-ohm potentiometer (assume 100 ohms). R2 is 390 ohms.
- 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 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. 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 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.