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Usefulness of different TDR designs?
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David Hess:
It is irrelevant; at this point I am not convinced that you understand how sequential and random sampling work and what equivalent time sampling means.  Your LeCroy supports random equivalent time sampling (RIS in LeCroy speak) of up to 25 GS/s using real time sample rates of 2, 4, or 8 GS/s depending on the number of active channels.  This does not conflict with its 1.5 GHz of bandwidth at all except of course that it cannot capture 1.5 GHz of bandwidth with all 4 channels operating at the same time in a single shot without aliasing.  That is what RIS is for.

I am still puzzling over chapter 7 of the manual which discusses 5 picosecond timing resolution in connection with 40 picosecond sampling resolution.  I wonder if that is based on 5 picosecond resolution in the time delay counter which measures the difference between the trigger and sample clock.  Usually that is irrelevant because samples are only saved with up to 40 picosecond resolution in this case in the acquisition record and any extra resolution from the time delay counter is lost.  Maybe LeCroy is retaining the 5 picosecond timing resolution somehow outside of the acquisition record.

With the same design, Tektronix discards any extra resolution from the time delay counter.  (1) But timing resolution greater than the acquisition record resolution is still possible when measurements take the vertical signal levels into account.  That is how digital time delay counters produce timing resolution much greater than their sampling rate.  Tektronix uses something like interval arithmetic to calculate and display measurement results with the proper number of significant digits and I wish other DSO manufacturers did the same thing.

(1) Going by memory, the Tektronix 2440 TDC has a timing resolution of 0.5 picoseconds (or maybe 2.0 picoseconds) but this is obviously meaningless.  The extra resolution supports gain, offset, and perhaps linearity calibration of the TDC and I suspect LeCroy does the same thing.
rhb:
If you want to discuss pulse generation circuits, I'm fine with that.  But I have no desire to rehash 3 semesters of grad school mathematics.  Proving the orthogonality of the Fourier basis once was quite enough for me.

If EEs want to have their own special meaning for ETS, they may do so.  But I am not interested in such abuses of language and mathematics.  Wiener, Shannon and Nyquist got it right and stated it properly.
ocw:
My prior post showed the comparison evaluations of short transmission line jumpers as measured by my Tektronix 1503B TDR and Agilent E7495A FDR/Station Test Set.  I am more commonly evaluating lines from 100 to 2,000 feet long.  That first attached picture shows the inner and outer conductors of some slightly overheated 3 1/8" rigid line that I replaced.  There wasn't much left of the Teflon insulators near those points.

With my being more concerned about longer lines I thought that I would include views of about a 175 foot long line to a receive antenna.  It is mostly LMR400 cable although there are some shorter pieces of other cable also included.  You can see those connections much better on the E7495A FDR.  With saved measurements, you can see deterioration of connections or in the line itself.  You can also make measurements on the specific frequency or range of frequencies primarily used.  For comparison, I also included a Riser Bond 1205CX measurement as well.

rhb:
Why is there a reflection at around 140 ft on the Tek and RB displays and not on the Agilent?

Also what is the frequency step on the Agilent?  Does the Agilent correct for transmission loss?  It certainly looks as if it is and it would be much easier to do in the frequency domain than in the time domain.

Thanks for posting these.
ocw:

--- Quote ---Why is there a reflection at around 140 ft on the Tek and RB displays and not on the Agilent?
--- End quote ---

You are missing the obvious.  Note what the Agilent reports at 152 feet vs. the end of the line at 176 feet.  That is what the Tek and RB report at a slightly closer length.  That is a connection point between the end of the LMR400 and the more flexible line used in the rotator loop.  Note the "noisy" return loss of that rotator loop cable.  You only see that on the Agilent.


--- Quote ---Also what is the frequency step on the Agilent?  Does the Agilent correct for transmission loss?
--- End quote ---

As it shows on the Agilent display, I left it in the auto-frequency mode to automatically chose the best frequency range for the best length resolution.  It chose 375 - 672.485 MHz.  As the Agilent shows I selected the mostly proper LMR400 cable.  The Agilent takes the velocity factor and attenuation of the cable into account when preparing its measurements.

The receive antenna is a special one which I made for TV channel 6 through the lower half of the FM broadcast band (80 - 98 MHz).  It is a ten wide spaced element Yagi antenna almost 100 feet high.  That's why the return loss of the antenna was so high--it wasn't made for those higher measurement frequencies.  My concern was line evaluation at a higher frequency where its limitations are more obvious with greater location resolution.
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