[time-nuts] Tektronix Sample Heads
time at radio.sent.com
Sun Apr 26 00:26:15 EDT 2015
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:
it's not user repairable, so there are no schematics (just block
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
* 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.
> 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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