I've been able to use TDR/TDT to measure in the single picoseconds = (though I haven't done a formal MCA - Measurement Capability Analysis). = Here are notes I put together for myself when I was doing this ~3 years = ago - nothing formal, and no guarantees whatsoever. And figures won't = get through. But, it might help. It's written for the TEK TDR, but I = don't think there's anything unique - similar technique should work for = Agilent's. Good luck. ___________________________________________________________ Differential, 4 Port TDR/TDT Deskew Procedure TEK TDS8000 w/ 2 Dual TDR Heads (4 channels) Jeff Loyer Disclaimer: This procedure is provided "as-is", with no claims made to its accuracy = or viability. Board Design: To facilitate these measurements, the board design should incorporate = differential "thru" connections, including very short structures void of = any skew (no fiberweave effects, etc.). The launch structure should be = designed to minimize the possibility of skew introduced by probe = positioning ("probe" portion of launch structures minimized). Instrument Settings: Each waveform should be taken with an average of at least 32 = measurements, with instrument beginning measurement when "start/stop" = button is pressed, and stopping after those samples are completed.=20 Coarse Deskew: Coarse deskew does not include deskew of probes (only cables and TDR = sampling heads). Nor does it account for skew differences related to = even versus odd propagation mode. The coarse skew can be accomplished = with either mode of propagation (odd or even). The final differential = deskew is accomplished with only one mode of drive (odd or even = propagation), and includes probe or final launch skew. =20 Step a: align ch1 and ch2's (with corresponding cables) TDR outputs, = using ch3 as acquisition reference. a1) Connect ch1 + ch1's cable to ch3, with ch1 as TDR, ch3 as TDT. Save = ch3 waveform as reference_a1 a2) Connect ch2 + ch2's cable to ch3, with ch2 as TDR, ch3 as TDT. = Adjust "TDR", "Manual Step Deskew", "C1_C2" such that ch3's waveform = falls on top of reference_a1. =20 ch1 + ch1's cable output is now aligned with ch2 + ch2's cable output. = The reference waveforms can be cleared. =20 Step b: align ch2, ch3, and ch4's acquisition using ch1's TDR drive as = waveform, ch2 as reference. b1) Connect ch1 + ch1's cable + adapter to ch2's cable + ch2 with ch1 as = TDR, ch2 as TDT. Save ch2 waveform as reference_b1 b2) Connect ch1 + ch1's cable + adapter to ch3's cable + ch3 with ch1 as = TDR, ch3 as TDT. Adjust ch3's "Vertical", "Channel", "Deskew" to align = ch3 waveform to reference_b1. b3) Connect ch1 + ch1's cable + adapter to ch4's cable + ch4 with ch1 as = TDR, ch4 as TDT. Adjust ch4's "Vertical", "Channel", "Deskew" to align = ch4 waveform to reference_b1. NOTE: this may be an iterative process - you may have to coarsely align = the waveforms and then recapture the reference_b1, since the reference = waveform horizontal scale can't be adjusted. ch 2, ch3, and ch4's acquisition are now aligned. The reference = waveforms can be cleared. =20 Step c: align ch1's acquisition to ch2, using ch3's TDR as waveform ch2 = as reference. c1) Connect ch3 + ch3's cable + adapter to ch2's cable + ch2 with ch3 = as TDR, ch2 as TDT. Save ch2 waveform as reference_c1. c2) Connect ch3 + ch3's cable + adapter to ch1's cable + ch1 with ch3 as = TDR, ch1 as TDT. Adjust ch1's "Vertical", "Channel", "Deskew" to align = ch1 waveform to reference_c1. =20 ch1 & ch2's acquisitions are now aligned with cables only. The = reference waveforms can be cleared. Final Differential Deskew: Includes and compensates for probe skew and provides fine deskew when a = 4 port system is driven in odd mode with channel 1,2 driving and channel = 3,4 acquiring. Compensation must be run first such that ch1 and ch2 = amplitude is exactly the same, or within the measurement resolution of = the TDS8000 screen ("compensation" calibrates the full scale output of = the TDR driver and receiver. Run "Utilities", "Compensation"). TDS 8000 Set-up (must be maintained through steps d,e, and f) for Odd = propagation mode: =09 TDR Step Polarity Acq Units Channel 1 On Up, rising On V Channel 2 On Down, falling On V Channel 3 Off n.a. On V =09 Channel 4 Off n.a. On V Deskew of Channel 1-2 and 3-4 can be more easily accomplished by = multiplying the channel by (-1) in math mode and display this as M1, M2, = etc., the following illustrates: =20 Channel 1, 2 in Odd mode propagation with apparent skew. All graphs are = 20psec/division horizontal scale. =20 Channel 1, 2 in odd mode, with Channel*(1) using math function in = TDS8000mainframe. Measurement of skew can now be accomplished (skew is = measured to be 16.4psec). =20 After C1-C2 Manual Deskew, channels are deskewed although there is = apparent asymmetry between magnitudes of TDR stimulus when run in = different modes. Running compensation before deskew minimizes this, = however. Skew is measured at the 50% threshold of all signals. Step d: Connect probes, compensate for probe skew from step a. Align = ch1 and ch2's (with cables and associated probe) TDR outputs, using ch3 = as acquisition reference. d1) Connect ch1 + ch1's probe to ch3 using a short Thru, with ch1 as = TDR, ch3 as TDT. Save ch3 waveform as reference_d1 d2) Connect ch2 + ch2's probe to ch3 using a short Thru, with ch2 as = TDR, ch3 as TDT. Adjust "TDR", "Manual Step Deskew", "C1_C2" such that = ch3's waveform falls on top of reference_d1. =20 ch1 + ch1's probe output is now aligned with ch2 + ch2's probe output. = The reference waveform can be cleared. Step e: With probes, align ch3, and ch4's acquisition using ch1's TDR = drive as waveform, ch2 as reference. e1) Connect ch1 probe to ch3's probe using short Thru, with ch1 as TDR = and ch3 as TDT. Adjust ch3's "Vertical", "Channel", "Deskew" to align = ch3 waveform to reference_e1. e2) Connect ch1 probe to ch4's probe using short Thru with ch1 as TDR = and ch4 as TDT. Adjust ch4's "Vertical", "Channel", "Deskew" to align = ch4 waveform to reference_e1. Step f: align ch1's acquisition to ch2, using ch3's TDR, both probes = open, open reflected TDR f1) Raise ch1 and ch2 probe such that there is open reflected TDR only = (not TDT) from the probe tips. f2) Adjust either C1 or C2 vertical deskew. Should be very small = value (<several psec's) representing only probe skew. ch1 & ch2's = acquisitions are now aligned including the probes. Final "Even Mode" Deskew: Similar to differential deskew, but both channels drive in same = direction. The timing within the TDR head changes between odd and even = modes, so deskew performed for the odd mode is not valid for the even = mode. Jeff Loyer -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] = On Behalf Of changyifeng Sent: Wednesday, February 28, 2007 5:43 PM To: problem Subject: [SI-LIST] Testing method of differential intra-pair skew Importance: High Hi all, =20 I am looking for accurate testing methods(or apparatus) of differential = intra-pair skew. =20 Because of the resolving power of TDR and the limited bandwidth of VNA, = there are some defects in some methods such as TDR, TDT and VNA testing. =20 What is the best solution when the intra-pair skew is between 1ps to = 5ps.(or 10ps) =20 Thanks. Regards Chang Yifeng Email=A3=BAchangyifeng@xxxxxxxxxx *************************************************************************= ************** =A1=A1=A1=A1This e-mail and its attachments contain confidential = information from HUAWEI, which is intended only for the person or entity = whose address is listed above. Any use of the information contained = herein in any way (including, but not limited to, total or partial = disclosure, reproduction, or dissemination) by persons other than the = intended recipient(s) is prohibited. If you receive this e-mail in = error, please notify the sender by phone or email immediately and delete = it! *************************************************************************= ************** ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List technical documents are available at: http://www.si-list.net List archives are viewable at: =20 //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu =20 ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List technical documents are available at: http://www.si-list.net List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu