[time-nuts] Tektronix Sample Heads

Bill Byrom time at radio.sent.com
Mon May 4 03:08:34 UTC 2015 

Yes, that's about right, Magnus. I believe it depends on how many
sampling channels are in use, since there is a factor of two
interleaving in some cases (to the best of my memory).  That is really
the holdoff rate between the acceptance of trigger edges. If the trigger
signal rate is lower (such as 10 kHz), that will be the sampling rate.
If the trigger signal is at a higher frequency (such as 400 MHz), only
one out of every 1,000 trigger edges might be accepted.  Each accepted
trigger event causes a single sample of the input voltage (relative to
the feedback voltage) to be taken. The sampled voltage is significantly
attenuated due to sampler loss, so the sampler output is multiplied by
the assumed correction factor to change the feedback voltage (which is
then measured by a low speed high resolution A/D).  If the gain is set
correctly, the step response is pretty good, but due to nonlinearity
there is a small digital feedback error resulting in slight ringing at
the sampling rate. This ringing is often not noticed unless the scope
time/div is set to a low value.

Sampling oscilloscopes are only undersampled with respect to signals
which are not harmonically related to the trigger signal. The sequential
equivalent time sampling rate is often several THz/sec.

Basic TDR/TDT doesn't need any additional software to get an estimate of
the reflected impedance. But the iConnect software (which was developed
by a small company which Tektronix purchased many years ago) allows much
higher accuracy and removes most of the ambiguity of multiple
reflections. It also allows S-parameters to be extracted and SPICE
models to be verified.  This software isn't free - we have many more
software engineers than hardware engineers these days!  

-- 
Bill Byrom N5BB

On Sun, May 3, 2015, at 04:28 PM, Magnus Danielson wrote:
> Bill,
> 
> As I recall it, it has a 400 kS/s rate.
> 
> Undersampling as it is, if properly used, it is a marvelous tool.
> The TDR capability had such a live component to it that I miss in newer 
> (but butter calibrated) systems.
> 
> No wonder I have a system myself these days.
> 
> The one thing I would love to have, is to be able to run the TDR/TDT 
> post-processing software. I haven't seen any free alternative either.
> 
> Cheers,
> Magnus
> 
> On 04/26/2015 06:26 AM, Bill Byrom wrote:
> > I still work for Tektronix, but not in Service or the sampling scope
> > product line. I'm a Tektronix field RF Application Engineer.
> >
> > You can find the service manual for the SD-24 at:
> > http://www.tek.com/oscilloscope/sd24-manual/sd-24-service-manual But
> > it's not user repairable, so there are no schematics (just block
> > diagrams).
> >
> > The SD-24 was introduced about 25 years ago for the 11801/11802 family
> > of sampling scopes. The SD-24 is a dual TDR sampling head, so it can
> > generate a fast risetime step from either or both outputs. The steps can
> > be the same polarity (for common mode testing) or opposite polarity (for
> > differential mode testing). The sampling bridges measure both the
> > incident (forward) and reflected pulses.
> >
> > The SD-26 is basically the same product without the TDR pulse sources.
> >
> > The SD-22 is a lower noise (and lower bandwidth) version of the SD-26.
> >
> > As pointed out by others, these heads aren't useful without a 11800 or
> > CSA800 family mainframe. The SD-series measure signals using sequential
> > equivalent-time sampling.
> >   * Single events can't be measured. Only repeating signals with a
> >     low-jitter trigger source can be measured. The trigger must be an
> >     externally input signal (unless you use the SD-24 with the internal
> >     TDR step source or an external signal pickoff transformer).
> >   * Each trigger edge which is accepted by the mainframe is delayed by a
> >     precise amount and then used to create a sampling strobe which is
> >     sent by the mainframe to the sampling head.
> >   * The sampling head (SD-24/26/22) actually measures the error
> >     difference between an internal feedback loop and the sampled input
> >     voltage. Since the sampling bridge has a high loss, the error
> >     voltage is multiplied by the assumed bridge loss to create the new
> >     feedback loop voltage. A high resolution low-noise A/D converter
> >     measures the loop voltage for the microprocessor-created raster scan
> >     display on the CRT.
> >   * The sampling system takes around 10 microseconds to reset between
> >     triggers. So no more than about 100K triggers and samples can be made
> >     per second. It might be a little slower than this - I'm remembering
> >     this from my experience over 20 years ago.
> >   * The delay from the trigger input to the sampling strobe (sent to the
> >     SD-xx sampling head) is sequentially delayed by slightly increasing
> >     time delays to create a time domain display. The delay increment
> >     between samples can be less than 1 ps (down in the 100 fs range).
> >   * Since the signal is not actually sampled in real time, this is called
> >     equivalent-time sampling. In this case, the sampling strobe is
> >     sequentially advanced in time upon trigger signal acceptance. This
> >     results in very high time accuracy with low jitter (a couple of ps
> >     RMS jitter in these older products).
> >   * The voltage measurement range is usually a few hundred millivolts
> >     peak-peak, while the damage level is at around 3 volts.
> >
> > --
> > Bill Byrom N5BB
> >
> >
> >
> > On Fri, Apr 24, 2015, at 10:42 PM, Ivan Cousins wrote:
> >> Since I was working at Tektronix at the time, I still remember the
> >> first instruments that were in the family.
> >>
> >> Like the main frames 11801, 11802, CSA801, CSA802, CSA803, etc and
> >> sampling heads SD-20, SD-22, SD-24, etc.
> >>
> >> You can try a google search like "Tektronix 11801 filetype:pdf". You
> >> can also try a google search like "Tektronix SD-24 filetype:pdf".
> >>
> >> If you want to know more about google filetypes, enter "google
> >> filetype search" into a google search.
> >>
> >> To find out more about sampling heads you can look for information on
> >> the instruments they connect to.
> >>
> >> w140.com has a lot of information on both the mainframes and the
> >>       sampling heads.
> >>
> >> http://w140.com/tekwiki/wiki/Main_Page#11000_Series
> >> http://w140.com/tekwiki/wiki/SD-24
> >>
> >> It is good to know more google-fu. It is even better to be able to
> >> still remember about any of this. :)
> >>
> >> Ivan Cousins
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