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Difficulty using Q(Tg) for NISM on DC-DC POL's where impedance peak is <~30mOhm
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Picotest
Jun 30, 2022
Precisely! One tip I often give is for cleaner plots, use fewer datapoints. I think I mentioned this in my online classes too. This does trade-off definition, but you’ll lose some definition in the peaks (that you didn’t want) and also fewer datapoints mean more interpolation, which includes smoothing. Q(Tg) is related to the phase slope, so the higher the Q the steeper the slope, or the steeper the slope the higher the Q. Often these noise blips are due to external noise – for example maybe the noise from LED lighting or fluorescent lights, noise from other circuits in the system, etc. In rare cases, they are because the system is oscillating. Even poor phase margin does not create such a narrow “spur” but an oscillation can. Most of these noise blips are external, they may also be there when the power is OFF, confirming you can ignore it. In any case, store the OFF measurement in memory and then look at the ON measurement. You are interested in where the two curves converge, and in the case of poor stability, the ON impedance will be greater than the OFF impedance. It IS possible for these blips to be stability related, so I am NOT telling you to ignore them, but to be cautious and look for additional signs that it isn’t just passive noise. If it IS stability-related it generally looks smoother and broader, not like this. You can also create the Bode plot from these two traces, since BODE=1-OFF/ON. This is from small signal theory closed loop=open loop/(1-T) and solving for T, where closed loop is ON and open loop is OFF. I have a few papers on this, but see attached. We do have one customer that insists on 1600 datapoints, so there isn’t really a guide, but for mine I usually like to see 20points/decade UNLESS I am looking at a sharp peak. For example, xtal is very high Q, so needs a lot of datapoints for good definition. The series resonance of a ceramic cap also might require more points to accurately produce the sharp impedance dip. One more tip is to use LOG magnitude for the Qtg curve (and the impedance curve) and it won't get as “blown out.” Steve
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Difficulty using Q(Tg) for NISM on DC-DC POL's where impedance peak is <~30mOhm
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Picotest
Jun 27, 2022
Thanks for this question, it is a good one, and probably of general interest. I looked at your settings, and you did all the right things to manage noise. Which is great. There are few things to be aware of here. First, the best method for this impedance measurement, voltage permitting is the 2-port shunt through. That is the lowest noise and is optimum for measuring very low impedance. NISM uses Qtg, which is a derivative, making it very susceptible to noise, but this is generally overcome by good noise management technique, which you have applied well here. This is not about the magnitude of the impedance, but about the Q. The measurement itself is very low Q, so we don’t actually expect to see a peak, and the NISM would generally report >71 degrees. Pat yourself on the back, this is a keeper. It is also generally recommended that you measure with power both OFF and ON, again making the 2-port shunt through measurement ideal. This will clearly show where the ON impedance is greater than the OFF impedance, and that is the peak we want to measure. You do have a very sharp peak at around 10kHz. This could be noise from your board, noise from external sources (we have seen LED lamps, monitors, etc. create such noise) and it can also be control loop related. Measuring with power OFF and ON would help determine that. You could use the traditional 3dB cursors (Bode 100 offers these) to determine the Q, which looks to be below 0.5. This is a very stable loop, and that is why you are having trouble finding the peak – there isn’t one!! If you see this situation generally, congratulations, you are doing well. If you perform a step load on this VRM you will also note that there is no ringing, since this is overdamped. Well done! Steve
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Battery impedance testing with your J2111A Current Injector
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Picotest
Mar 10, 2022
The MSO6B with the latest power software is capable of both 2 port (VNA) and three port (FRA) frequency domain measurements, though the FRA doesn’t provide impedance units (yet). You don’t say what your battery voltage is, but for low voltage batteries, the 2-port shunt through is more accurate, though requires an attenuation resistor, attenuating probe, such as P2102A or DC Blocks. The measurement also requires the J2162A active splitter and either J2102B coaxial transformer or J2113A active splitter to measure low impedance at frequency below about 1MHz, depending on the cables. The FRA method requires the J2111A, which includes a 1Ohm current monitor, and with a low noise scope probe, can also measure impedance. While we don’t usually discuss it, there is also a similar ground loop in this measurement, and in fact, in most measurements. Since this measurement is a current sink, it’s a floating measurement. Either of these measurement techniques can measure 1mOhm or lower with care. I attached a verification of the J2111A impedance measurement, performed by Keysight, using their E5061B VNA. This VNA has an advantage in this it has a differential input for the voltage measurement, eliminating the ground loop. A standard differential probe is generally pretty noisy, so not quite as good for low impedance measurement. Using a J2113A or J2102B with one of our P2104A or P2105A probes or will yield the best results if the impedance is very low. We have most of these in the demo pool, if you would like to borrow one. We can use the measurements to create an application note for the MSO6B. Email info@picotest.com for any demo equipment requests.
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Lesson 4 question on TDR measurement setup
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Picotest
Mar 24, 2021
I haven’t ever derived or really discussed the equation, though I have shown it and discussed the method frequently. If you look at this 70GHz image, taken by Mike Martin for us, you can see the perfect edge of our J2151A TDR. We even trademarked PerfectPulse. The other 2 images show the dVdt of the edge. This is a bandwidth limited impulse. The bandwidth is 0.35/edge speed. Looking at the FFT of an impulse it is flat response up to this bandwidth. You can see that in the FFT. This is of course only looking at the 1 channel that is the generator. Through probes, cables, etc, this edge can be degraded, but we can measure what is at this “port”. If I connected this to a second channel through a perfect cable, the second channel would see the same thing as the first and dividing them would provide the transfer function. If we present the signal to the DUT through 50 Ohms (our TDR splitter does) we’ll measure the S-parameter, S21. The second channel also looks at the derivative, since it is also a response of the signal comb, though the scope connects these points with smoothing for us. This is a division, but since the measurements are in dB we subtract these to get the division. This is a neat trick. We can use this to measure uWave frequency response. I attached a few measurements using this “VNA”. You’ll see 1.57GHz and 4GHz bandpass filters and also a 20dB attenuator for verification of the math. If we turn the splitter around so we DON’T present 50 Ohms, we can also measure the transfer function of probes, cables, etc without the 50 Ohm damping. See the attached PML probe measurement and the Tek005 (power rail probe). Of course this also works to measure the scope transfer function and the bandwidth limiters and the step vs flat response selections, etc. It’s quite a handy tool that fits in your pocket. We do have setup files for all of these, since we showed them in our Tektronix events. FYI, this is also how the calibration labs validate our 10.5GHz specification for our TDR. Using the same method as I showed here. It just requires a scope with much more bandwidth than our 10.5GHz edge. Using the signal generator for TDR, just looks at the same signal, but a bit differently, evaluating mostly the reflections. One more highlight. Looking at the attached Tek003_001, you can see ringing on the edges. Both edges. Interestingly, you can also see the ringing BEFORE the transition from low to hi and hi to low. Hmmm, that says there is a pre-response or a non-causal response. How can that be? That is a perfect picture of the Gibbs phenomenon 😉 So much fun in one pocket sized gadget… Also see this helpful article Measuring a Scope Probe Requires Two Oscilloscope Channels and a Very Flat Signal Source
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Do you have any waveforms of the J2152A High Speed Probe Calibrator with the associated load information of the probes being measured?
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J2180A Ultra Low Noise Preamps
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