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| Usefulness of different TDR designs? |
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| rhb:
While most TDRs seem to use a square wave, if an impulse is used the energy is distributed across a very broad spectrum. I have one of Leo's 100 ps impulse units. The output spectrum of that is approximately a sinc(f)**2 with the first zero at 50 GHz. Unfortunately, a DSO with that BW is rather more expensive than an FDR. I plan to build some pulsers in an attempt to have something capable of testing my SD-32 which at 7 ps is quite a challenge. |
| FriedLogic:
--- Quote from: rhb on April 25, 2019, 12:19:57 am --- 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. --- End quote --- Hi, The point of RIS etc. is more to fill in the trace with actual measurements rather than a calculated interpolation, and so get around the artefacts and errors that come with it - and in the case of older scopes like that it's needed even more because the ADC's used could not sample fast enough, which was common at the time. I wouldn't have expected it to change the rise time much, except to the extent that the interpolation was wrong. I have an old 100MHz Metrix that samples at 50MS/s, and that can get interesting.... |
| rhb:
I have an SD-32 sampling head in transit which should arrive on Saturday. I am very interested in suggestions of how to produce an edge which is faster than the 7 ps rise time of the SD-32. |
| David Hess:
--- Quote from: rhb on April 25, 2019, 12:19:57 am ---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. --- End quote --- From the manual: Repetitive signals can be acquired and stored at a Random Interleaved Sampling (RIS) rate of 10 GS/s for all models except LC564, LC584, AND LC684 SERIES, whose RIS rate is 25 GS/s. --- Quote ---The DDA-125 will dither the clock to give 40 ps sample intervals and claims a measurement resolution of 5 ps. --- End quote --- The 40 picosecond sampling resolution in RIS mode is consistent with the given 25 GS/s sampling rate in RIS mode. 5 picoseconds probably represents the interpolated measurement resolution which is not unrealistic. --- Quote ---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. --- End quote --- Why would it? The sampling method has nothing to do with input bandwidth. --- Quote ---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. --- End quote --- Sampling oscilloscopes do not act or operate as you describe. Their bandwidth extends from DC to whatever their -3 dB point is. The -3 dB point exists for a sampling input however it does not really have the same meaning because the frequency response curve is non-linear with a null at the reciprocal of the sampling gate time and every harmonic past that. For this reason they do not obey the 0.35 rule but their transition time can definitely be measured or calculated based on the sampling gate width. This actually provides an easy way to measure sampling input bandwidth; find the first null and work backwards using the sin(x)/x response; a leveled signal source is not required. (1) One thing not commonly realized is that sampling oscilloscopes often have trigger bandwidths considerably in excess of their sampler bandwidth and their trigger bandwidth can be extended using injection locking. Injection locking a sweep used to be pretty common. Tektronix made normal oscilloscopes which included the same type of triggering (HF SYNC) through the 1980s for special applications. (1) This breaks down somewhere above 20 GHz where samplers get weird. |
| rhb:
Write out the acquisition equation in the time domain and then apply an analytic Fourier transform. |
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