Sound Card LCR Meter

B

Bob Masta

Guest
[SHAMELESS PLUG]
Possibly relevant to "On old electrolytic caps" thread, but
also for general interest, I've just released Daqarta v7.60
which includes a macro "mini-app" to use your sound card as
an LCR meter. (Many thanks to Robert A. Macy for
inspiration and advice on this!)

The basic idea is that the sound card outputs a sine wave
(about 1 kHz default) that drives a voltage divider made of
a known reference resistor (say, 1k) in series with the DUT.
The sound card stereo inputs monitor the voltage at the top
of the resistor, and the top of the DUT. The (complex)
voltage across the resistor allows the determination of the
current, which also flows through the DUT, so the complex
impedance of the DUT can be found via Ohm's law.

To calibrate, you only need to know the value of the
resistor (read if from a good DMM), then there are 3 buttons
you click on one by one:

1) Resistor shorted, no DUT

2) Resistor present, no DUT

3) DUT shorted.

That allows the macro to determine the sound card input
impedance, which is otherwise in parallel with the DUT, and
compensate for that in subsequent operation. With a 1k
reference, and a typical sound card impedance of 15k, this
allows reading DUT resistance above 1 Meg.

The big (resizeable) meter shows R (ESR) and X by default,
labelled with proper units (Ohms, K, M, pF, nF, uF, uH, mH,
H). You can change one line in the macro to include DF (or
Q for inductors) as well. One button saves a reading to a
log file; another saves notes/comments.

Regarding the original question about typical ESR values, I
was puzzled for a long time about the fact that ESR was
always inversely proportional to C, such that a 47 pF cap
might show an ESR over 500k.

Turns out that the basic impedance measurement doesn't
distinguish between series and parallel connections of the
real and complex impedance components, so you have to help
it out by toggling a "Series" button to "Parallel" based on
your judgement. Typically, this turns out to be pretty
simple: Use the default Series for electrolytics, and use
Parallel for 1 uF and below. That 47 pF cap reading then
shows a parallell (leakage) resistance well over a Megohm.

Full write-up from the Help system is at
<http://www.daqarta.com/dw_0o0z.htm>, including a screen
shot, photo of a simple test fixture, and circuit diagrams.

Best regards,


Bob Masta

DAQARTA v7.60
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Sound Level Meter
Frequency Counter, Pitch Track, Pitch-to-MIDI
FREE Signal Generator, DaqMusiq generator
Science with your sound card!
 
On Fri, 20 Jun 2014 06:14:59 -0700, Bob Masta <N0Spam@daqarta.com> wrote:

...snip...to keep Aioe happy
DAQARTA v7.60
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Sound Level Meter
Frequency Counter, Pitch Track, Pitch-to-MIDI
FREE Signal Generator, DaqMusiq generator
Science with your sound card!
Click to expand...

Bob, congratulations on this one. ...thanks for mention.
 
On Fri, 20 Jun 2014 13:14:59 GMT, N0Spam@daqarta.com (Bob Masta) wrote:

Possibly relevant to "On old electrolytic caps" thread, but
also for general interest, I've just released Daqarta v7.60
which includes a macro "mini-app" to use your sound card as
an LCR meter. (Many thanks to Robert A. Macy for
inspiration and advice on this!)

The basic idea is that the sound card outputs a sine wave
(about 1 kHz default) that drives a voltage divider made of
a known reference resistor (say, 1k) in series with the DUT.
The sound card stereo inputs monitor the voltage at the top
of the resistor, and the top of the DUT. The (complex)
voltage across the resistor allows the determination of the
current, which also flows through the DUT, so the complex
impedance of the DUT can be found via Ohm's law.

To calibrate, you only need to know the value of the
resistor (read if from a good DMM), then there are 3 buttons
you click on one by one:

1) Resistor shorted, no DUT

2) Resistor present, no DUT

3) DUT shorted.

That allows the macro to determine the sound card input
impedance, which is otherwise in parallel with the DUT, and
compensate for that in subsequent operation. With a 1k
reference, and a typical sound card impedance of 15k, this
allows reading DUT resistance above 1 Meg.

The big (resizeable) meter shows R (ESR) and X by default,
labelled with proper units (Ohms, K, M, pF, nF, uF, uH, mH,
H). You can change one line in the macro to include DF (or
Q for inductors) as well. One button saves a reading to a
log file; another saves notes/comments.

Regarding the original question about typical ESR values, I
was puzzled for a long time about the fact that ESR was
always inversely proportional to C, such that a 47 pF cap
might show an ESR over 500k.

Turns out that the basic impedance measurement doesn't
distinguish between series and parallel connections of the
real and complex impedance components, so you have to help
it out by toggling a "Series" button to "Parallel" based on
your judgement. Typically, this turns out to be pretty
simple: Use the default Series for electrolytics, and use
Parallel for 1 uF and below. That 47 pF cap reading then
shows a parallell (leakage) resistance well over a Megohm.

Full write-up from the Help system is at
http://www.daqarta.com/dw_0o0z.htm>, including a screen
shot, photo of a simple test fixture, and circuit diagrams.

Best regards,


Bob Masta

DAQARTA v7.60
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Sound Level Meter
Frequency Counter, Pitch Track, Pitch-to-MIDI
FREE Signal Generator, DaqMusiq generator
Science with your sound card!
Click to expand...

A 47 pF ceramic cap will typically have a shunt resistance in the gohms and a
series resistance well under 1 ohm. Characterization of either will be about
impossible at 1 KHz, where Xc is over 3 megohms.

I use a TDR oscilloscope, or a fast pulse generator and a scope, to measure the
ESR of stuff like this.

Here's a 50 ohm square wave generator dumped into a polymer aluminum cap.

https://dl.dropboxusercontent.com/u/53724080/Parts/Caps/Polymer_ESR.JPG

The superfast spike is ESL, and the roughly 1/2 div step is ESR. The gross
slopes are C. Maybe a sound card could do something like that, square pulse
mode, and even display the waveform across the DUT. What's the usable bandwidth
on a sound card, in and out?

Even at low bandwidth, you might interpolate the line slopes and compute the
step size.





--

John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com

Precision electronic instrumentation
 
On Sat, 21 Jun 2014 08:25:06 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

A 47 pF ceramic cap will typically have a shunt resistance in the gohms and a
series resistance well under 1 ohm. Characterization of either will be about
impossible at 1 KHz, where Xc is over 3 megohms.
Click to expand...

Thanks for that, but where does one get this kind of info?
I could find nothing on the Web regarding R values of
low-value caps, either series or shunt. (And I've learned
from personal experience that "there ain't no such thing as
a resistance over 100 Meg" with real-world parts, in
real-world circuits, on real-world boards... or at least,
not for long, despite alcohol rinse and teflon stand-offs,
etc! <g>)

I use a TDR oscilloscope, or a fast pulse generator and a scope, to measure the
ESR of stuff like this.

Here's a 50 ohm square wave generator dumped into a polymer aluminum cap.

https://dl.dropboxusercontent.com/u/53724080/Parts/Caps/Polymer_ESR.JPG

The superfast spike is ESL, and the roughly 1/2 div step is ESR. The gross
slopes are C. Maybe a sound card could do something like that, square pulse
mode, and even display the waveform across the DUT. What's the usable bandwidth
on a sound card, in and out?
Click to expand...

The highest sound card sample rate I've heard of is 192 kHz,
which would imply a bandwidth under 96 kHz. The only one
I've actually tested at 192 kHz was in a laptop (not a place
where I expect high performance), and the upper cutoff was
closer to 48 kHz.

Even at low bandwidth, you might interpolate the line slopes and compute the
step size.
Click to expand...

Yes, I think if big-cap ESR is the main interest, then a
pulse-based approach is the way to go. I was attracted to
Macy's slick network analyzer approach because it gives
wide-range LCR measurements, even on el-cheapo systems.
Seemed like it might have wider appeal, especially for
beginners. (I would have loved to have had a decent LCR
meter back when I was starting out, though lack of same did
lead to some educational experiments with AC bridges and
headphone null-detectors!)

The pulse-based scheme is still on the "To Do" list,
however.

Best regards,


Bob Masta

DAQARTA v7.60
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Sound Level Meter
Frequency Counter, Pitch Track, Pitch-to-MIDI
FREE Signal Generator, DaqMusiq generator
Science with your sound card!
 
On Sun, 22 Jun 2014 06:28:15 -0700, Bob Masta <N0Spam@daqarta.com> wrote:

...snip...
Yes, I think if big-cap ESR is the main interest, then a
pulse-based approach is the way to go. I was attracted to
Macy's slick network analyzer approach because it gives
wide-range LCR measurements, even on el-cheapo systems.
Seemed like it might have wider appeal, especially for
beginners. (I would have loved to have had a decent LCR
meter back when I was starting out, though lack of same did
lead to some educational experiments with AC bridges and
headphone null-detectors!)

The pulse-based scheme is still on the "To Do" list,
however.

...snip....
Click to expand...

The 192kS/s rate works well out to 89kHz.

I ALWAYS recommend steady state vs pulsed. Because in steady state all the
components have arrived at their quiescent operating points. But in a
pulsed system, each pulse, well you get the drift. And, you'll be limited
by any inherent voltage vs characteristics in your components. Also,
pulsed is more of an 'amplitude' measurement, not a phase, or frequency
measurement and the dynamic range will ALWAYS be less.

And finally, talk about maximizing S/N ratio in your measurement. Pulsed,
you are absolutely confined to 1 1/3 1/5 etc harmonics at 1 3rd 5th
frequencies. That's a lot of energy replicated and confined within a
voltage time pulse. Instead distribute that energy 'evenly' throughout
your spectrum, in an absolutely uncorrelated manner. Now, you can get more
energy for the same voltage peak limit. [Increase S/N by at least 3dB and
often 6dB] Plus you can start to 'draw' any slope, or tendency by doing a
curve fit to your multiple data set. Thus, where a single frequency yields
noise and limits the reading, multiple frequencies tend to cancel out, now
add to that pre-knowledge of what you're looking for [the resistance is
fixed vs frequency] and you can REALLY lower your noise floor on the
measurement to much more accuracy than you could ever obtain with a pulsed
technique. Additionally, if you selected your 'distributed' freequency
energy properly, you will get only fundamentals, not a single harmonic,
thus by using a calibration sequence you have also effectively removed ALL
distortion in your measurement system, a win-win.
 
On Sun, 22 Jun 2014 13:28:15 GMT, N0Spam@daqarta.com (Bob Masta) wrote:

On Sat, 21 Jun 2014 08:25:06 -0700, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

A 47 pF ceramic cap will typically have a shunt resistance in the gohms and a
series resistance well under 1 ohm. Characterization of either will be about
impossible at 1 KHz, where Xc is over 3 megohms.

Thanks for that, but where does one get this kind of info?
I could find nothing on the Web regarding R values of
low-value caps, either series or shunt. (And I've learned
from personal experience that "there ain't no such thing as
a resistance over 100 Meg" with real-world parts, in
real-world circuits, on real-world boards... or at least,
not for long, despite alcohol rinse and teflon stand-offs,
etc! <g>)
Click to expand...

I have some sample 0805 1 Tohm resistors.

Here's a little PCB, deliberately cruddy with rosin flux and fingerprints,
pinning my meter on the 1e14 ohm range:

https://dl.dropboxusercontent.com/u/53724080/Gear/Keithley/Leak_Gloppy_Flux.JPG

https://dl.dropboxusercontent.com/u/53724080/Gear/Keithley/Leak_Test.JPG




I use a TDR oscilloscope, or a fast pulse generator and a scope, to measure the
ESR of stuff like this.

Here's a 50 ohm square wave generator dumped into a polymer aluminum cap.

https://dl.dropboxusercontent.com/u/53724080/Parts/Caps/Polymer_ESR.JPG

The superfast spike is ESL, and the roughly 1/2 div step is ESR. The gross
slopes are C. Maybe a sound card could do something like that, square pulse
mode, and even display the waveform across the DUT. What's the usable bandwidth
on a sound card, in and out?

The highest sound card sample rate I've heard of is 192 kHz,
which would imply a bandwidth under 96 kHz. The only one
I've actually tested at 192 kHz was in a laptop (not a place
where I expect high performance), and the upper cutoff was
closer to 48 kHz.

Even at low bandwidth, you might interpolate the line slopes and compute the
step size.

Yes, I think if big-cap ESR is the main interest, then a
pulse-based approach is the way to go. I was attracted to
Macy's slick network analyzer approach because it gives
wide-range LCR measurements, even on el-cheapo systems.
Seemed like it might have wider appeal, especially for
beginners. (I would have loved to have had a decent LCR
meter back when I was starting out, though lack of same did
lead to some educational experiments with AC bridges and
headphone null-detectors!)

The pulse-based scheme is still on the "To Do" list,
however.
Click to expand...

LCR meters tend to not agree on things like big caps and power inductors. The
time-domain step response, like in my ESR pic, could be displayed as a waveform,
to help separate series and shunt effects that confuse sinewave LCR meters.
There's just a lot more information in that picture than in the amplitude and
phase of one sine wave.


--

John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com

Precision electronic instrumentation
 
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