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Tunnel Diode Pulser

April 27, 2013

Here is an interesting device that can be used to check oscilloscope Y rise time and trigger jitter. It came with a bunch of Tektronix type 547 oscilloscope parts.
Tunnel Diode Pulser
Tunnel Diode Pulser Scheme
It will change state extremely rapidly, entering a negative resistance region where as the forward current is increased the current suddenly drops causing a sudden voltage increase at the anode. It can be used to build extremely fast negative transconductance oscillators and in the trigger circuits of fast oscilloscopes. The static forward resistance of the diode is just a few ohms and in this case, this small resistance forms the shunt leg of a potential divider, since the input resistance is 3.48k, there is almost no output until the diode changes state. In this case, the change in state occurs at an input threshold of 32.5V. Here it is driven by a sine wave shown on a type 545A oscilloscope:
Tek 454A Tunnel Pulser driven by Sine
Here it is on the type 545A at 200nS/cm driven by the square wave calibrator set at 50V P-P. The comparative slowness of the leading edge of the square wave to that of the tunnel diode is very clear!
Tek 545A Tunnel Pulser 200nS:cm
The slight forward lean just visible at 200nS/cm on the edge of the tunnel diode state change, maybe 15nS, is mostly due to the rise time of the type 545A Y amplifier which is itself, extremely quick. This is why for many years, tunnel diodes were used to generate the trigger pulse for fast oscilloscopes being at least an order of magnitude faster than a triode or transistor Schmitt trigger. Type 545A pre-dated the use of tunnel diodes for triggering and at high frequencies, the timebase is synchronised, not triggered. The later type 547 used tunnel diode triggering allowing the timebase to be triggered at any speed the Y amp could handle (in the case of type 547, greater than 50MHz). Here is the same set up viewed on a type 547 oscilloscope:
Tek 547 Tunnel Pulser 200nS:cm
Interestingly, the trace on the type 547 is less clear than that of the older type 545A, I think the CRT in this type 547 is getting near the end of its life. Also, disappointingly, I could not get a jitter free trace at a speed higher than 200nS/cm. The type 545A is proving its mettle!
In an attempt to do better, I tried on a type 475 and the result was worse, I could not obtain a jitter free trace above 1µS/cm!
Tek 475 Tunnel Pulser 1uS:cm
This type 475 is a real disappointment being a 200MHz scope. The triggering is weak and the trace definition poor. I cleaned all the switch contacts and that yielded an improvement, the trace went from very fuzzy to fuzzy and the Y stability improved. However, when I want to see something tricky, the 547, 545A or even 535A and 535 types perform better. If you are expert in restoring the performance of a type 475, or know somebody who is, please leave a comment so that I can contact you!

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