Author Topic: LCR Meter Accuracy  (Read 13287 times)

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Offline bugi

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Re: LCR Meter Accuracy
« Reply #50 on: September 04, 2018, 05:14:35 pm »
The are still the same meter manufactured by DER EE (not DER). I don't think they would downgrade it under their own name.

IET did not manufacture the IET DE-5000 (or DE-6000), DER EE did.
While DER EE manufactured it for IET, IET could have asked for tighter quality than what DER EE is doing now. How good a reputation DER EE is wanting compared to IET?

The missing and changed components in DE-5000 is not about what I (or you or anyone) thinks, it is pretty much a fact, based on what people have found inside in different units. Whether those differences count as "downgrade" or "inferior" is another thing.

The first example are the supply input "protection" diodes. That is, imho originally slightly badly designed thing, they could clamp to low enough voltage and to break to (hopefully) short if a battery or DC-input was connected the wrong way, and thus (hopefully) save the rest of circuitry, though still needing a repair for the broken diode. But my unit had no such diode at all for battery, only the unpopulated pads for it, and the DC input side diode had been moved to replace a larger bulk supply capacitor (the spot still having capacitor markings!). The battery diode certainly does not break now, but it won't be saving the rest of the circuitry, either. I think that counts as a minor downgrade; failure result changed from "potentially slightly broken with easy fix" to "totally broken". (The DC input "protection" diode was also changed to a larger one, I'm not sure that was a good idea, either.)

The following example is something I don't know if it is a change or was it always so: missing capacitors on supply rails (which has been measured to have quite some ripple without those caps), only unpopulated pads waiting. At least that does not improve anything. So, if that was a change done after IET times, that is likely also a minor downgrade. (Whether that brings it out of spec is something I can not say.)
 

Offline David Hess

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Re: LCR Meter Accuracy
« Reply #51 on: September 04, 2018, 05:51:56 pm »
A had some fun a couple months ago investigating ways to provide input protection to 4 wire LCR meter designs without compromising performance.

One thing about input protection diodes and circuits is that even if they protect the rest of the instrument, if they fail it is still usually a hard kill unless the user can replace them like with accessible fuses.  I find this crop up in poorly designed RF receivers all the time.  If the input shunt protection which guards the RF amplifier shorts out due to an overload, the fact that it saved the RF amplifier is irrelevant to the user.
 

Offline JohnPi

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Re: LCR Meter Accuracy
« Reply #52 on: September 04, 2018, 06:25:40 pm »
Quote
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Yes, it autoranges. One nice feature of the Rigol is it has arbitrary values for the y-scale. I basically set the y V/div to make the measurement span +/- 3 divs. If the Q isn't too high and levels don't change too much between frequency steps, this works well. In fact the Rigol only uses +/- 100 counts for full screen display, so there is some over range available. Later I'll add a check for over range and rescale if needed. For the 1st frequency measurement, I do it twice -- once to get a good autorange value.

Autoranging comes with its own set of problems. Now the inaccuracies from range switching is part of the equation. It puts you squarely in the territory of real LCR meters with this - various "range resistors" and receiver gain ranges, all with their own freq and level trims, to cover a wide Z range. OSL does not compensate well for this...

I intend to do a 2-D cal -- over frequency and voltage(to account for possible different voltage scaling errors on different ranges); both on a log scale. I need to determine what types of errors the scope introduces when it switches ranges, and when it uses the vernier range. Likely separately on each channel.

I'll store the data vs log(f) and log(V) and use 2-D interpolation between points when actually measuring. So that calibration doesn't introduce additional (random) errors, it needs to be performed many times and averaged.
 

Online ogden

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Re: LCR Meter Accuracy
« Reply #53 on: September 04, 2018, 06:41:49 pm »
If the input shunt protection which guards the RF amplifier shorts out due to an overload, the fact that it saved the RF amplifier is irrelevant to the user.

If you ignore possibly countless other incidents when protection saved RF amplifier w/o failing/shorting - then maybe yes ;) Anyway repair shops shall take in account difference between repairing protection components compared to frontend amplifier (LNA) repair.
« Last Edit: September 04, 2018, 06:44:05 pm by ogden »
 

Offline MrW0lf

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Re: LCR Meter Accuracy
« Reply #54 on: September 05, 2018, 07:52:03 am »
Your SNR and dynamic range (30 dB only ?) will be horrible with a noise source and FFT. The scope doesn't have enough ADC resolution (8 bit) to remove the noise (in a reasonable time).

BTW back in the day I did some wild experiments with single-formula (no programming) FRA to work directly in scope software using external signal gen just outputting freq sweep (also no programming). Range was indeed about 30...35dB however some interesting results like measuring general shape of response at 100MHz on 25MHz analog BW scope :D Main issue with method was that phase plot could not be signed.

https://www.eevblog.com/forum/testgear/picoscope-2000/msg1295493/#msg1295493
 
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Offline Szybkijanek

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Re: LCR Meter Accuracy
« Reply #55 on: August 13, 2020, 10:04:57 pm »
Hi, I have question, is new 670$ Pintek LCR-900 good choice for home use?
« Last Edit: August 13, 2020, 10:10:38 pm by Szybkijanek »
 

Offline rdsi

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Re: LCR Meter Accuracy
« Reply #56 on: August 13, 2020, 11:54:23 pm »
Hi, I have question, is new 670$ Pintek LCR-900 good choice for home use?

I'm not familiar with that brand but I did pickup a BK Precision 891 on ebay for $800 - new open box.
This unit operates at DC or from 20 Hz to 300 kHz with 0.05% best accuracy.
I thought it was a good compromise between cheap & high end models.
I'm not a big fan of BK but it fit my needs & has been accurate/reliable.
 

Offline Szybkijanek

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Re: LCR Meter Accuracy
« Reply #57 on: August 14, 2020, 03:42:09 am »
Thank You, this model is much better.
 

Offline rodpp

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Re: LCR Meter Accuracy
« Reply #58 on: August 14, 2020, 04:20:18 am »
Look carefully at the specifications, because the accuracy can vary according with the DUT impedance value. Sometimes orders of magnitude.

Did you considered used ones? For example the digibridges from genrad. I have the digibridge 1657 an 1689, both are very good and you can find complete manuals with schematics. The 1657 is more limited but less expensive, and the 1689 is much more accurate and flexible in test configurations.


 

Offline Szybkijanek

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Re: LCR Meter Accuracy
« Reply #59 on: August 14, 2020, 04:47:04 am »
I did not consider used ones because I am afraid that it will be damaged.
 

Offline rodpp

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Re: LCR Meter Accuracy
« Reply #60 on: August 14, 2020, 05:22:24 am »
Yes, that is a valid concern.

But for home use, maybe it is part of the fun...

Buying used equipment sometimes is the only reasonable way to have good tools for home use. I always choose an used better specified equipment over a new one with less performance at the same price range.

Another point is that after the warranty of new equipment expires, it is very hard to fix an eventual problem. While with old ones chances are that you can find full schematics and the problem is easy to fix.

But if the equipment use will be other than hobby, it is a different story.
 


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