Zoom ChallengeSome folks have the need for a high dynamic range, i.e. the ability to inspect small details in a signal. To accomplish this, they usually increase the vertical gain of the DSO and use the position control to center the region of interest on the screen. This way, even 8-bit oscilloscopes can display some detail – as long as the signal distortions, caused by overdriving the oscilloscope frontend, don’t affect the displayed portion of the signal too much. The distortions are especially bad with general purpose oscilloscopes, as they use the well-known split path input buffer with its problematic overload recovery behavior.
Now let’s examine our options with the Siglent SDS800X HD.
First, we could try to use the traditional technique in overloading the scope. Without too much thinking, we can just connect a strong signal and then “zoom in” by increasing the vertical gain of the oscilloscope.
In the following example we have a 2 Mbps PRBS-signal with 3 V amplitude connected directly, hence a 1x probe factor applies.
SDS824X HD_PRBS-4_A3V_V1V_P1
Now we try to take a closer look at the pulse tops and increase the sensitivity. This works reasonably well down to 200 mV/div, but at 100 mV/div we hear a relay clicking and the signal gets distorted:
SDS824X HD_PRBS-4_A3V_V100mV_P1
With a distorted signal like this, it makes no sense to try to look at any details in the signal. So, this obviously is the wrong approach.
For most applications, it is not the overload recovery of the semiconductor devices, like clamping diodes and transistors, which cause the problem. The overload recovery time of these devices is usually in the low (or even sub-) nanoseconds and is only really of concern in multi-GHz instruments.
Our problem is the clamping in the split-path input buffer, which causes clean clipping in the LF-path, but a differentiation of the waveform in the HF-path. When the clipped LF-path is recombined again with the both offset- and phase-shifted HF-path, the result is heavily distorted and has little similarity with the original signal.
Knowing all this, we are able to find a solution: just don’t drive the input buffer so hard that the clamps get activated. Keep the input signal well below 1 Vpp by using 100x probes if necessary. This also has the advantage of a much lower capacitive load at the probe tip and the low noise of the SDS824X HD makes the use of x100 probes unproblematic.
The next screenshot demonstrates a 1 MHz square wave with 5V amplitude and a 10 mVpp 40 MHz sine riding on it, using a ten times probe.
SDS824X HD_OVD_5V_10mV_P10
Yes, the trace is noisy. It would be much better if we could use the 20 MHz bandwidth limiter – but unfortunately, this would also affect the 40 MHz signal we are interested in. Averaging would help a lot, but we want to be able to watch dynamic signals, hence it is not an option either.
We can still see the 40 MHz sine clear enough to know it is there – and that for a signal amplitude ratio of 1:500! That’s what a low noise high resolution DSO can do for you…
There might be situations, when we just cannot get that low – maybe because the signal levels are so high that the output of even x100 probes would still exceed ~500 mVpp. Then a combination of (moderately!) overdriving the scope and vertical zoom could be the best solution.
Consider a 1 MHz Square wave with 5 V amplitude – maybe as output of a x100 probe, so the original signal would be 500V - unbelievable, isn’t it? It could be some 625 watt transmitter – but these wouldn’t output a square wave and hopefully there wouldn’t be any subtle signal details to observe, which would not be better analyzed by using the FFT, but I digress…
Here is that familiar 40 MHz sine wave again, riding on the square wave:
SDS824X HD_Ref_5V_10mVpp
First step is to increase the vertical gain, i.e. dial in lower numbers, just before the relay would click. We could use the fine adjust to get 102 mV/div (because this is the highest gain we can get without changing the attenuator setting), but this shouldn’t be necessary for now. So we finally end up with 200 mV/div:
SDS824X HD_OVD_limit_5V_10mVpp
We can already see the little wiggles on the top of the square, it is much smaller than the overshoot and ringing at the rising edge. Yet now we engage the Zoom mode to take a closer look:
SDS824X HD_OVD_limit_5V_10mVpp_Z20mV_ERES2.0
The above screenshot demonstrates two things: first is the ERES2.0 math trace in the main window, that lets us look at the 40 MHz sine at 10 mV/div. It is ugly, because ERES cannot get rid of the 1/f noise, so there’s no use displaying it in more detail in the zoom window. But secondly, we have the regular trace in the zoom window at 20 mV/div, which is at least as clear as the overdrive zoom before.
One more time it should be remembered that we have a signal ratio of 500:1 here.