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| Usefulness of different TDR designs? |
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| Per Hansson:
--- Quote from: David Hess on April 22, 2019, 10:41:59 pm ---In a TDR, random sampling would allow the TDR to measure the reflected pulse immediately after or even before it is sent (1) but since a TDR can always generate its own pretrigger signal of whatever duration is required, random sampling is never needed. (1) A must when doing maintenance on a TARDIS. This explains the rarity and high price of 7T11 and 7T11A plug-ins; those who still do TARDIS maintenance can pay unearthly (Gallifreyan) prices on the Terran market and can rebid on lost auctions if they choose. --- End quote --- Hmm, I don't think I'm as deeply versed in Doctor Who as you are ;) A time machine would be useful though, will put it on my to-do list :D --- Quote from: rhb on April 21, 2019, 12:53:57 am ---Leo will provide 1 MHz square wave units on request for an extra 10 pounds. I have ordered two for TDR with a DSO. --- End quote --- Yes having Leo's and maybe also Mr Carlson's TDR for a comparison would be nice. Would be interesting to see how they stack up for TDR use. |
| rhb:
I'm planning to play with fast edge circuits when my SD-32 arrives. |
| David Hess:
--- Quote from: rhb on April 23, 2019, 12:47:31 pm ---Those provide a better idea of what RIS/ETS was about. I was taught that ETS implied that you had to provide an external band pass filter to limit the BW to the Shannon-Nyquist limit. --- End quote --- That sounds more like an under-sampling application where the bandwidth of the input signal must be limited to below the Nyquist frequency as oppose to a baseband application like an oscilloscope where the input bandwidth must extend to DC. IF sampling receivers do this to extract say a 15 kHz passband from a 15 kHz IF stage using a 100 kSample/second ADC at 10.7 MHz or whatever the IF frequency is. The right kind of sampling oscilloscope can do the same thing using an external clock. Oddly enough, some old DSOs can do it also because they support an external clock input. This has the effect of using the ADC as the final mixer stage. With a Costas loop to recover the carrier, such an oscilloscope can directly demodulate AM from an IF within its bandwidth. I think Linear Technology published an application note doing this using one of their sampling ADCs. --- Quote ---I was a bit disturbed when I discovered that the EE community were not being very scrupulous about the mathematical niceties. My LeCroy samples at 2 GSa/s with a 1.5 GHz BW. So everything above 500 MHz is aliased. Gad! --- End quote --- That sure does not seem right. Exactly what LeCroy model DSO is it? Typically a DSO of that caliber would deliver a real time sample rate of 2 GSamples/second but deliver 20 Gsamples/second or higher using ETS or what LeCroy called RIS in the application note I linked. My Tektronix 2440 is like that. The full 300 MHz bandwidth is not supported by its real time sample rate of 500 MSamples/second but the sample rate in ETS mode is 25 GSamples/second. --- Quote from: Per Hansson on April 24, 2019, 02:32:30 pm ---Hmm, I don't think I'm as deeply versed in Doctor Who as you are ;) A time machine would be useful though, will put it on my to-do list :D --- End quote --- One of the Tektronix 7B92A timebases I bought produced a sweep which went backwards in time. Suggestively, it was prominently marked "Physics Department" in red ink on the side. Unfortunately, I fixed it before considering what I had carefully enough. |
| rhb:
The Shannon-Nyquist limit is the BW of the signal, not the maximum frequency. I'll leave Shannon's paper to the interested reader. The LeCroy in question is a DDA-125/LC684DLX. It's 2 GSa/s on 4 channels, 4 GSa/s on 2 channels and 8 GSa/S on one channel with an external adapter to combine 2 & 3. I don't have the adapter unfortunately. But it has a <250 ps rise time whether it is in 2 channel or 4 channel mode. It also has 20% overshoot which is actually stated in the datasheet. At least they are honest enough to include it. The rise time is the same in RIS mode. The DDA-120 doesn't need the external adapter to merge 2&3. So it looks like a good candidate for a set of switched low pass filters with 750, 1500 and 3000 MHz corner frequencies. And a VGA LCD to replace the CRT and provide room for the filters and SMA relays. The DDA-125 will dither the clock to give 40 ps sample intervals and claims a measurement resolution of 5 ps. However, I've not observed it to be useful for anything. It certainly does not give you a faster rise time measurement unless there is some menu toggle I've not found. I had great hopes of 11801 level performance when I bought it. But at least I can do hysteresis plots without requiring an external analog integrator. As far as I'm concerned, the EE sampling jargon is an egregious abuse of the English language and the mathematics put forth by Wiener and Shannon. Sampling scopes do *not* evade the Shannon-Nyquist constraints any more than wavelets evade the time-frequency resolution limits of the Fourier transform. A sampling scope is only sampling a very narrow range of frequencies. One need only consider the integral transform of the sampling window to see this. Or as I used to put it when people would breathlessly tell me that wavelets had better resolution than short window Fourier transforms, "If a statement is true in French, then it is still true correctly translated into any other language." |
| ocw:
There are a number of factors influencing the use of a TDR or Frequency Domain Reflectometry (FDR) measurements. While I have used a Tektronix 1503B and various Riser Bond TDR's to locate and then correct many cable problems, FDR's are generally replacing TDR's since they do a better job of locating most faults. See: https://www.anritsu.com/en-US/test-measurement/solutions/en-us/distance-to-fault Examples of this are shown in my attachments. I am comparing a Tektronix 1503B to a Agilent E7495A. The best example is probably comparing an inexpensive Chinese 2M BNC to BNC RG58 cable. The end of the cable is connected to a 6 GHz 50 ohm load. The Tektronix TDR display indicates that it is perhaps not the best cable in the world, while the Agilent FDR display clearly demonstrates that besides the connectors being marginal, the cable is not the best quality either. The RG214 cable comparison shows the differences when measuring a better quality cable. Additional cables are shown on just the Agilent FDR since the differences are more easily seen on that instrument. |
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