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R&S NGE100 voltage jumping around

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RBBVNL9:
@justanothername

I now did the measurements you asked for. They are done with the NGE103B set to 4V output and the following conditions:
-   No load
-   1mA load (using a 3.9 kΩ resistor as load)
-   100mA load (using three 120 Ω resistors in parallel as load)

Measurements are done with a Siglent SDM3045X DMM. That is only a 4.5 digit DMM, but given the voltage differences we are interested in, it seems fit for this task. I also repeated the measurements with a set of 3 new AA battery as voltage source, to check there is no short-term drift in the DMM itself, or inaccuracy in repeated measurements (there is not).

This DMM also has an AC filter. According the manufacturer: “The analog filter of the Multimeter can be used to reduce the influence of the AC component in DC measurement. Generally the filter may not be required, but sometimes it can improve the DC measurement. For example, if the measured DC electrical source has a big AC ripple, the analog filter can reduce the influence.”

Every measurement I did consists of 120 samples, with an in-between delay set to 1.0 second. Because the measurement itself also takes some time, the total time span is a bit longer than 2 minutes (2 min 26 seconds to be precise).

The data file is attached at the end of this post, as well as some graphs.

My observations are:
-   The AC filter does not seem to affect measurements a lot, but it does seem to need some time to stabilize. So for PP calculations, I’ll ignore the first 20 samples (where this stabilisation takes place).
-   Overall, in all my measurements (of 2.5 minutes each), the maximum PP difference is 20 mV or lower.
-   In two cases, I do observe a voltage step (see the red arrows in the graphs), both of about 10mV. Both times this happens during measurements with the AC filter on, but I take that as coincidence (N is way too low to make any inferences).

As discussed above, R&S only has published specs for ripple and noise for 20Hz to 20MHz, and not for slower voltage drifts. During the measurements I did (which are not infinitive or exhaustive), the average drift over 2.5 minute periods was 12mV, and the maximum drift I saw was 20mV. I guess this is good enough for my purposes, but others may have other needs.

Yet, the steps I observe are fascinating, and they would be compatible with the idea of the design (a switching regulator followed by a linear control circuitry) indeed has discrete steps when doing its work. But of course this is speculation, not evidence.

rf-messkopf:

--- Quote from: RBBVNL9 on October 09, 2020, 06:47:05 pm ---If the issue is longer term voltage drifting, then I really would be fascinated to see whether this is somehow related to the design of having a switching regulator first and a a linear control circuitry next. I don't know the details of the NGE design, but I wonder whether it involves some steps in the process, and these could possibly explain the behaviour you seem to notice. If so, then such issues may occur at certain (threshold) voltages and not at others. This all is pure speculation, however.

--- End quote ---

I don't know whether this is of interest in the present thread, but by coincidence I noticed a similar jumping of the output voltage on an Agilent E3646A. See the attached picture: The output is set to 5V and directly connected to the multimeter (no load). It does jumps of about 500µV. When I noticed that I just shrugged my shoulders as it is within spec (<0.01% of output voltage + 3mV), even though it's not exactly nice.

The Agilent E3646A has no switching regulator, and the jumps are present on both channels. I haven't seen any jumps on a R&S HMP4040 that is sitting next to it, which does have a switching pre-regulator, but is somewhat more noisy than the Agilent.

justanothername:
Thank you for the measurements RBBNVL9!
Your measurements seem to be way better than mine!
You have 7.7mV at no load min to max, the worst value is 11mV at 100mA. This is the performance I'm expecting.
My NGE has around 30mV at all loads to 100mA, so it is definitively worse! I will return the device and lets see what happens.

Fixpoint:
I am skeptical that all this is really meaningful.

(a) Firstly, keep in mind that your measurements are not necessarily comparable with each other because you don't have the same conditions. For example, the performance of your power supply is influenced by your mains characteristics. If somewhere in your house a fridge, a vacuum cleaner, or an LED lamp turns on, this could change the behavior of the power supply. Some of you may have a very clean mains, others a very noisy or "ripply" one. If one of you measures "30 mV" and another "10 mV", what does that mean? I don't think that it means much as long as we don't know all the parameters. In my opinion, performance measurements must be done under very well-controlled conditions, otherwise they are of very limited value because we don't really know how to interpret them. We can only use them as ballpark indicators.

(b) Secondly, keep in mind that we don't buy equipment "just because" (except we are collectors) but for using it for something. I am using the NGE103B on a daily basis for hours (and yes, I am doing precision stuff with VCOs), it does its job very well, and I never think about 10 mV or 20 mV ripple just because I have better things to do. So, is all this talk really worth the time? Just build something ...

justanothername:
Fixpoint, we bought this PSU as replacement for the old analog Hameg 3ch PSUs. As such it was presented to us by R&S when we visited them. Whatever my coworker was trying to do, he searched for an error in his circuit awhile before pinning it down to the new PSU and throwing it back to me. Hes is using the old one again.
And I don't know why a properly built PSU should jump 30mV around at 4V constantly, even when mains is unstable (which is not, of course). I just needed to know if others have the same flaws, seems not, so I will send it back!
I would overlook this when it would be a cheap aliexpress PSU but not for a >1000€ unit.

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