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Hi All,

I've just uploaded a PDF I created that I hope helps describe the S-52 Pulse Generator output.

The S-52 generates a fast 25ps output edge for use in the 7S12 (and other) Time Domain Reflectometers. I was introduced to these fairly recently, and was quite confused by the Service Manual description. My S-52's behavior was suspect, and I couldn't be sure it was working or not. After help from members of this group I've come to understand its functionality a bit better and am on my way to resolving its issues.

By the way, to avoid confusion, the pictures used in the uploaded document are NOT of my S-52s output. Mine continues to not produce the pulse at the center of the waveform. Why is yet to be determined.

I don't claim to know these devices well, and I'm no great technical writer. But I hope this description clarifies this devices operation.

Dave

Link: /g/TekScopes/files/S52%20Functional%20Description.pdf

Hi Dave,

You produced a very nice document! I can't compete with that with my poor pencil notations for personal use.

I think the description of what happens in step 2) of the sequence of operations is not fully correct. There might be some leakage which causes the voltage at C90 to drop during the next counts. But that's not intended. The trick in my opinion is that while charging C90 the voltage at C90 lags behind compared with the voltage at the base of Q90. The charging current (mainly depending on R74) also has to pass through R90. This causes a small voltage across R90. Hence at the moment of firing the voltage at C90 is slightly below the voltage at the base of Q90. That base voltage drops to the C90 voltage after charging has stopped. The setting of R90 determines the amount of voltage drop.

Albert

Thanks for the insights Albert.

Makes a lot more sense than just a design that relies on just leakage alone.

In the "T.D. Bias Current" portion of the SM Circuit Description it describes: "Q74 reverse biases CR90 and the charge on C90 is held (except for a very small Q90 base current through R90) to control the arming bias current in period 7." That was the basis of my understanding that it was "leakage" that brought the bias level to just below the TD trip point.

However, the description follows that sentence with "Trig Level control R90 sets the arming bias current to a value below the peak current for the Tunnel Diode.", which I took to mean that it's variability allows setting of the arm bias level. But it's also saying what you're saying. One of those sentences that can be read more than one way?

Given your description, and my growing understanding, it seems that the actual operation is a combination of what you've described, including some leakage (and voltage drop from R90 in C90s storage mode) all contribute to the small reduction of the armed bias level to something below the trip point. I think your description is probably the more significant contributor to the reduced level of the bias setting.

Thanks again for the additional insight Albert.
Dave

Hi Dave,

As far as I can see the way how R90 functions is not described at all in the manual. The (I would say disturbing) leakage by Q90 (and possibly also by some reverse biased diode) ill have been the reason to choose a Darlington there.
I share your feelings about the manual and remember that the circuit descriptions were far from clear to me at first reading.
BTW the effect of R90 on Q90 current can be quite large, but most of the TD current is already supplied via R97.

Albert