[time-nuts] Tektronix Sample Heads

Daniel Mendes dmendesf at gmail.com
Sun Apr 26 19:25:52 EDT 2015


Thanks all for your info about these modules.

Daniel

On 26/04/2015 01:26, 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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