067-0681-01

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 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, 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 in theory by

• r_d = 0.025V / 0.006 = 4.2 ohms

Empirical measurements were closer to 7 ohms. This, plus the 43-ohm output resistor, makes for a 50-ohm output resistance when the tunnel diode is in the high-voltage state. To prevent the 43 ohm 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 ohm resistor’s lead. Because of this, the resonant frequency of the TD and its parasitics is around 3GHz and well damped. For any scope with less than about 1.5GHz BW, the ring would likely be invisible.

• Tektronix 067-0681-01 Datasheet (PDF)