Sherman The resolution of any measurement system is dependent on the measurement bandwidth at the DUT. Just about anything between the instrument and the DUT will cut the available bandwidth - and discontinuities can pack a big punch. And in many cases you will have two, one at the input and one at the output of the DUT for insertion loss type measurements and once going in and once going out for return loss measurements. It is well known that TDR's ability to resolve features is related to the risetime of the TDR edge at the DUT, and that the TDR edge has to go through the system twice, once getting to the DUT and once getting back to the scope. The other thing you will run into the reflections in the system will make the downstream measurements inaccurate. This can be compensated for with IConnect from Tek. At every discontinuity some of the energy is reflected back to the scope. This implies there is less energy propagating past the discontinuity. If you look at the equations for calculating the impedance based on the amount of reflected voltage you will see that both the system Zo and the incident voltage are used. Vreflected = Vincident(Zx-Zo)/(Zx+Zo) Thus for accurate measurements the incident voltage must be known at the input to every discontinuity. The peeling algorithm in IConnect does this computation to accurately calculate the impedance. Tom Dagostino Teraspeed Labs 9999 SW Wilshire Street Suite 102 Portland, OR 97225 tom@xxxxxxxxxxxxxxxxx www.teraspeedlabs.com 971-279-5325 office 503-430-1065 cell -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Chen, Sherman Sent: Tuesday, January 20, 2015 8:25 AM To: tom@xxxxxxxxxxxxxxxxx; Charles.Grasso@xxxxxxxxxxxx; heidi_barnes@xxxxxxxxxxxx; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: tips of using TDR probe Hi Tom, Thanks for sharing the valuable experiences. To verify my measurement results, I did numerous simulation in ADS. Here is what I got: 1. an inductive peak or a capacitive dip at the launch will smear the peaks and dips at the following discontinuities. But looks they don't distort the average impedance much. 2. a lossy tline also has the similar impact. So the point here is: when there exist peaks or dips, or a segment of lossy tline in front of the UUT, the peaks and dips displayed on the TDR profile are very likely distorted. Meanwhile the average impedance of the UUT mostly is still trustable. As I said in my last mail, to accurately evaluate the impedance of a UUT, I will first try to set the launch point closest to the UUT. If it's not possible to do so, above conclusion can be used to estimate the average impedance of the far off UUT. Best Regards, Sherman Chen Signal Integrity EMC Global Hardware Engineering Tel: +86 21 60951100-3329 -----Original Message----- From: Tom Dagostino [mailto:tom@xxxxxxxxxxxxxxxxx] Sent: Tuesday, January 20, 2015 9:42 AM To: Charles.Grasso@xxxxxxxxxxxx; Chen, Sherman; heidi_barnes@xxxxxxxxxxxx; si-list@xxxxxxxxxxxxx Subject: RE: [SI-LIST] Re: tips of using TDR probe Hi Sherman and others I've worked with TDR and all kinds of interfacing to the DUT. Coaxial and microprobes tend to be best IF the launches into the test board are well designed. If not you have created a low pass filter ahead of your DUT. This low pass filter will either be an RC like filter if your launch if capacitive or a series inductance if your launch is inductive. This filter does two things, first it limits the insertion loss measurement. Secondly is can severely limit the return loss performance. At higher frequencies you are seeing the launch not the DUT. With a coaxial interface you can calibrate out the effects of the launch if the launches have consistent performance, i.e., they have identical TDR responses. If you apply the rule of thumb of insertion loss minus return loss should be greater than 15 dB for a quality measure at a given frequency you can see how important the return loss effect of a poor launch can impact your measurements. I ran some experiments with some launches we have to illustrate this effect. There were three cases, a very good launch designed by Teraspeed, a published launch by the connector vendor and a "hand soldered" launch typical of someone trying to use a piece of coax soldered to the board. The discontinuity cause by the launches were +2 Ohms, +15 Ohms and +40 Ohms inductive respectfully if my memory serves me. The corresponding effective bandwidths were about 20 GHz, 6 GHz and 1 GHz. I defined effective bandwidth as a 15 dB spread between the insertion and return loss. With hand held probes you will run into a consistency issue and unless you design a consistent and effective return path you will have an inductive launch into your DUT. And if you have a differential probe the bandwidth of that probe is determined by the spacing of the probe tips. The wider the spacing the larger the inductive loop is at the probe/DUT interface. For single ended probes the launch will also have an inductive characteristic caused by the loop area of the probe/ground return path. And who knows what the characteristics of the DUT's interface looks like. Unlike a designed launch you have with the coax interface you may be probing a couple of test points 0.100" apart. They will in all likelihood look inductive or if the pads are large and use large drill sizes - capacitive. If you used something to keep the impedance constant between the probe and the DUT then you will have a better shot at a meaningful measurement. But this has to be consistent from test to test and during any de-embedding calibration you do. If they do not then the difference between the two measurements (calibration and measurement) will be placed between your measurement system and the DUT. Many of the TDR probes are measure for differential measurements and don't really use a ground return. These can do an excellent job of making differential insertion or return loss measurements if the interface to the DUT is electrically clean. And they are do a very respectful job of differential Zo measurements. I've made test jigs to hold the hand held probes so that I got consistent placement of the probe onto the circuit board. This helps, the geometry is the same from measurement to measurement and it frees up the hands to push buttons and capture the waveforms. And trying to hold a probe on a test point consistently while waiting for a VNA sweep or multiple average TDR measurement is a pain. As Heidi points out for quality measurements you need quality cables. The same things applies to the interface between the probe and the DUT, it has to be consistent. Hope this helps. Tom Dagostino Teraspeed Labs 9999 SW Wilshire Street Suite 102 Portland, OR 97225 tom@xxxxxxxxxxxxxxxxx www.teraspeedlabs.com 971-279-5325 office 503-430-1065 cell -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Grasso, Charles Sent: Monday, January 19, 2015 1:09 PM To: sherman.chen@xxxxxxx; heidi_barnes@xxxxxxxxxxxx; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: tips of using TDR probe Hello Chris - I am intrigued. It sounds like you solved a tricky problem. Are you permitted to expand on your " enhancing the shielding" solution? I am wondering how you avoided changing the impedance of the probe. Best Regards Charles Grasso Compliance Engineer Echostar Communications (w) 303-706-5467 (c) 303-204-2974 (t) 3032042974@xxxxxxxxx (e) charles.grasso@xxxxxxxxxxxx (e2) chasgrasso@xxxxxxxxx -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Chen, Sherman Sent: Monday, January 19, 2015 1:06 PM To: heidi_barnes@xxxxxxxxxxxx; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: tips of using TDR probe Hi Heidi, Thanks for sharing the tips. I was able to figure out the way how to accurately do TDR measurement by enhancing the shielding around the very ends of the probe tips. Now we are using this method for correlating simulation to measurement even on very short structures such as vias. And it works fine. For the method of using VNA to measure the sparam of the UUT, I think although theoretically any types of probe can be used with ISS (impedance substrate standard) to calibrate themselves out, despite they are highly reflective or not, due to the calibration error, a highly reflective probe may not deliver a satisfying calibration result. I will do some experiments to verify this when I get a chance to play with ISS. Best Regards, Sherman Chen Signal Integrity EMC Global Hardware Engineering Tel: +86 21 60951100-3329 -----Original Message----- From: heidi_barnes@xxxxxxxxxxxx [mailto:heidi_barnes@xxxxxxxxxxxx] Sent: Saturday, January 17, 2015 5:45 AM To: Chen, Sherman; si-list@xxxxxxxxxxxxx Subject: RE: tips of using TDR probe Hi Sherman, Investing time in set-up and training to use probes can pay off when it comes to making measurements, but it does require a methodical process for repeatability. 1) Connecting to probes with an instrument tends to leave the connecting cables unsupported with multiple bends. This means that high performance phase stable cables (and new ones that have not yet been damaged) are needed to avoid errors in the calibration due to phase changes from cable movement. Investing in the setup to minimize cable movement and provide support for the cables can reduce the phase errors which should help with a cleaner de-embedding. 2) Effervescent waves love to travel on the outside of a coax cable ( or probe tip) so getting the fields to go into the PCB and not back up the cable does require as continuous a ground as possible. GSG probes are better than GS, and if you look at some of the vertical launch SMA's that are compression mount, they could be considered the ideal probe with a continuous 360 ground and no possibility for fields to travel on the outside jacket. Some manufacturers add polyiron at the probe tip to reduce this problem, others try to improve the coax to planar transition of the probe. 4) Clean surfaces are also important to achieve repeatable contacts with the same force and flexing of the probe tip. Ideally, one should verify each probe landing by looking at the TDR to confirm the impedance profile is the same as that used for calibration. 5)Using a 2-Tier calibration with NIST traceable coaxial calibrations to the end of the cables and then de-embedding of the probes is my preferred method. This way I always have the NIST traceable calibrated data set, and I can always come back later and improve the probe model for de-embedding if needed. De-Embedding probes is not easy since the probe is low loss and measured S-Parameters can suffer from passivity and causality problems (usually cable movement). However, one can also look at creating deconstructed measurement based models in simulation to provide adjustable probe S-parameters to match with a given measurement. Goodluck with the probing, Heidi Barnes EDA Software for SI/PI Applications Keysight Technology -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Chen, Sherman Sent: Thursday, January 15, 2015 8:42 AM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] tips of using TDR probe Hi, We're evaluating the performance of our TDR probe to determine if it can be used in VNA measurement. Here is some observations: 1. Although in most people's opinion the contact quality of probe is far less reliable & trustable compared to SMA, based on my experience, it's feasible to achieve a stable contact with probe, at least for the bandwidth up to 20GHz. 2. Besides measuring the sparam of the tline or other components on PCB without the trouble of fabricating the test fixture, another benefit of directly probing is the masking effect due to the fanout traces on the fixture is avoided-more details will be shown on the TDR profile. 3. Experiments showed that the contact of ground collar on the TDR probe can reduce the inductive peak at the very beginning of the TDR curve. It seems the ground conductor must be surrounding the signal - a single ground wire won't have much effect. 4. The deembeding of the probe is challenging - lots of glitches appeared on the deembedded sparam. We're still working on that trying to find out the best method. I would appreciate if you can share your experience with using TDR probe either for TDR measurement, or for sparam measurement. Best Regards, Sherman Chen Signal Integrity EMC Global Hardware Engineering Tel: +86 21 60951100-3329 ------------------------------------------------------------------ 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 forum is accessible at: http://tech.groups.yahoo.com/group/si-list List archives are viewable at: //www.freelists.org/archives/si-list Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu ------------------------------------------------------------------ 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 forum is accessible at: http://tech.groups.yahoo.com/group/si-list List archives are viewable at: //www.freelists.org/archives/si-list Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu ------------------------------------------------------------------ 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 forum is accessible at: http://tech.groups.yahoo.com/group/si-list List archives are viewable at: //www.freelists.org/archives/si-list Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu ------------------------------------------------------------------ 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 forum is accessible at: http://tech.groups.yahoo.com/group/si-list List archives are viewable at: //www.freelists.org/archives/si-list Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu ------------------------------------------------------------------ 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 forum is accessible at: http://tech.groups.yahoo.com/group/si-list List archives are viewable at: //www.freelists.org/archives/si-list Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu