Pulse Aberration Verification

Question: I wrote you concerning the use of automatic measurements a couple of months ago and you were able to answer my question quite handedly.  I have another question, this time concerning the verification of pulse aberrations on fast Risetime pulse sources.

We are calibrating a Tunnel Diode Pulser that has a Risetime of 125 ps and pulse aberrations of <1%.  In the past we have used an Tektronix 11801 Sampling Oscilloscope with an SD-26 Sampling Head.   The Tunnel Diode Pulser is feed through a D-11 Delay Line so the Risetime of the initial pulse can be viewed.  The Delay Line will contribute to a decrease in the Risetime.  Can the pulse aberrations be accurately quantified and certified to a known value using the Tektronix 11801 Sampling Oscilloscope with SD-26 Sampling Head and D-11 Delay Line?  Would it be better to feed the Tunnel Diode Pulse directly to the input of the Sampling Head?

Answer:  The Tunnel Diode Pulser specifications described in your question match those of the Tektronix 067-0681-01, most recently offered by Tegam with an aberration specification of <1 % (typical) in a 1 GHz (350 ps rise time) system.  TD Pulsers were originally developed at least 40 years ago, before there were NIST traceable aberration standards.  A TD Pulser was assumed to have by design an inherently flat pulse response, suitable as a reference waveform for fairly low bandwidth oscilloscopes and pulse generators with relatively slow rise times such as the Tektronix PG506 (<1 ns rise time). 

While modern sampling oscilloscopes have better response characteristics than the models of 40 years ago, the aberration specifications, at least for Tektronix models, are typical (not guaranteed).  The aberration specifications may be higher than 1 %, depending on the defined region (time interval after the transition) for the aberration specification.   As a result, unless a sampling oscilloscope response is specially calibrated, the oscilloscope is not suitable as a traceable aberration standard.  For example, SD-26 aberration specifications are typically +/-3 % from 300 ps to 5 ns after the step, and +/-1 % from 5 ns to 100 ns after the step.  These specifications, even without a delay line, are not adequate for verifying a 1 % aberration specification.

Traceability for aberrations and settling parameters is now possible.  NIST offers the service, “Repetitive Pulse Waveform Measurements, Including Settling Parameters (65250S)”.  Learn more here.  

Returning to the question about the delay line, the effect of the DL-11 may be measured by comparing waveforms with and without the DL-11, using a system that doesn’t need a trigger pick-off.  One such system is a sampling mainframe and TDR module such as a Tektronix 11801 and SD-24, or a Tektronix DSA8200 and 80E04.   To help answer this question, measurements were made on a DL-11 using a DSA8200, 80E04, and SMA cables with combined length of about 1 m.  The DSA8200 was set to display a Math waveform having a 350 ps rise time filter, for a 1 GHz system bandwidth.  A horizontal setting of  2 ns/div was used, since that is the defined setting when a TD Pulser is used in PG506 calibration.  The DL-11 Trigger output was terminated in 50 ohm.   The DL-11 showed about 1.2 % overshoot at 4 ns after the edge, when compared with the reference waveform.  This is likely due to compensation for delay cable loss in the DL-11.  With different horizontal settings, the aberrations would measure differently relative to the level at the right edge of the screen, since it takes at least 500 ns for the DL-11 pulse response to settle to a constant level.

In summary, the DL-11 does affect the pulse response enough to be significant for a 1 % aberration measurement.  It is possible to measure and account for the DL-11 pulse response under defined conditions.  However the 11801/SD-26 sampling system pulse response may also have significant aberrations, according to the sampling system specifications.  The sampling system specifications are not guaranteed or traceable without a special calibration.  It is necessary to have a traceable aberration and settling parameter standard if traceable aberration measurements are desired.

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