The Tektronix SD-24 is a dual channel TDR/sampling head with 17.5 ps rise time (equiv. 20 GHz bandwidth) for the 11800 series scopes.
Each channel is also capable of generating a fast rising step for use in Time Domain Reflectometry (TDR). In TDR mode, the acquisition portion of the sampling head monitors the incident step and any reflected energy. The reflected rise time of the TDR step is 35 ps or less. The polarity of each channel's TDR step can be selected independently of the other channel. This allows for differential or common-mode testing of two coupled lines, in addition to the independent testing of isolated lines. The SD-24 can be used to characterize crosstalk by using the TDR step to drive one line while monitoring a second with the other channel.
The "filter" function on the CSA803C/11801C can be used with TDR or crosstalk measurements to characterize a system at a slower rise time.
Picosecond Pulse Labs Application Note AN-2a says:
TEK 11802/SD-24: TEK introduced this new scope in the fall of 1989. It is a completely new design and is not interchangeable with any of the previous "S" series TEK sampling scopes. The TEK design team included: Stan Kaveckis, Agoston Agoston, John Carlson, John Rettig, Jon Lucker and Roy Lewallen. The 11802 is a completely digital mainframe. It can support two SD-24 dual-channel samplers. It is IEEE-488 programmable. The time base accuracy was excellent. The trigger jitter was 2.5 ps RMS.
The SD-24 sampler uses a six-diode sampling bridge similar in concept to the "trapped charge", traveling wave S-4 sampler. The SD-24 is a dual-channel sampler which includes a built-in TDR pulser. The SD-24 sampler is internally terminated in 50 Ohms. The risetime is 17.5 ps as specified. The picosecond domain transient performance was found to track well with waveforms measured on the HYPRES scope. It did show a few extra small (4%) perturbations in the 200 ps to 500 ps region that were not seen on the HYPRES or HP-54121A samplers. After 1 ns, the settling time transient performance was excellent. It was far better than any of the other samplers tested.
|Rise time||17.5 ps|
|Sampling repetition rate||100 Hz to 200 kHz|
|Time coincidence||10 ps between channels|
|Maximum input voltage||1 Vp-p, 1.6 Vp-p operating, ±3 V non-destructive|
|Pulser rise time||28 ps|
|Pulser voltage||250 mV|
|Input||3.5 mm connectors|
Thanks to John Larkin, Highland Technology, for the internal photos of the SD-24.