So "peak detection" substitutes the peak value during the sample period for the value at the time of the sample clock? If so, that has some very *interesting* effects. None of which seems to me desirable.
That is not how it is implemented.
During decimation which would normally produce a fractional number of points in the acquisition record by dropping samples, instead the maximum and minimum samples during the duration of two points of the acquisition record are saved which necessarily halves the acquisition record length. So an envelope of the input signal is captured during a single acquisition, within the limits of the maximum sample rate.
CCD based digitizers also did peak detection but in the analog domain and I am not sure how; it sure was not with analog comparators or diode peak detectors. I suspect Asgard technology was involved.
If that is a correct interpretation I cannot understand why anyone would want to do it except for lack of comprehension of sampling theory. Aliasing is a consequence of regular sampling, so there is *no* possibility of "peak detection" preventing aliasing. This is dead obvious if you simply draw the time domain multiplication and the corresponding frequency domain convolution.
What is ultimately captured is the envelope of the input signal during a single acquisition record. Under conditions where aliasing would be present because of decimation, a non-graded envelope is captured. A DPO (digital phosphor) style DSO produces a similar display but captures a histogram instead of an envelope so it may be index graded; DPO operation requires an order of magnitude more acquisition memory.
As far as the practicality of the display, in most cases it looks normal but with all peak-to-peak noise displayed unless you are Tektronix who managed to implement a noise reduction algorithm to produce a display which looks like a sample display but also shows detected peaks. Luckily this was Tektronix of the past and this feature was configurable.
Below is an example of what peak detection allows. The display shows 30+ kHz switching noise on an analog control signal for a switching power supply controller during startup so it was a single shot acquisition. A coaxial connection was required to get a clean signal. If peak detection was not used, then the sample rate would have been 50 kHz instead of 100 MHz and the shaded area of the trace would be entertaining but questionable.
This image was how I recognized that Tektronix had implemented a noise reduction algorithm for use during peak detection. I "knew" it in the sense that I had read about it when I read the manual cover to cover but had not made the connection to what it actually did until examining this photograph later and thinking, "That does not look right. Where is the added noise from peak detection? The readout says peak detection." Since then I have never found their noise reduction algorithm lying, but I still never trust it. Honestly though it has revealed details which would normally be missed. I might trust it if Tektronix had described how it works.
The 100 ps pulse I've been using is only 10% of the 1 ns sample interval, but it still shows up.
Mathematically even a 1 ps spike should show up at slower sampling rates. However, I find that a 5 ns impulse at 1 second intervals does not consistently show up unless I set peak detection mode. Why I still do not understand, but clearly it is useful.
Narrow pulses get spread and attenuated by the limited bandwidth before sampling so you end up with a probably to capture a given pulse width to a specific accuracy. On older DSOs, Tektronix actually listed these probabilities and accuracies but later DSOs had sample rates high enough to essential capture the results of narrow pulse which made it through the input amplifiers.
So my question for David Hess, is which model Rigol?
This thread is intended to be an antidote to rumor and insinuation as much as a goad to fix issues.
It has been a couple years but I can check my notes; it was the DS1000D and DS1000E series. From page 2-56 of the manual:
Peak Detect Aqusition: Peak Detect mode captures the maximum and minimum values of a signal. Finds highest and lowest record points over many acquisitions.That is not peak detection as it was commonly understood and it contradicts later Rigol documentation for their DSOs which do support peak detection. I went to the manual after informally evaluating a DS1000D.
What I find a little ironic is that the ancient Tektronix 2440 series of DSOs are advertised as having peak detection, and they do, but have no such acquisition mode; the designers were a little too clever and implemented peak detection as envelope mode with the number of acquisitions set to the minimum of 1.