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Old 20th May 2011, 11:21 PM   #41
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Sy yes it is a château Cardboard 13.5%, but quite a drinkable one

Conrad, thanks for the link! gives me some more to think about. especially interesting that the inductance of the LM317 goes down as the current goes up, explains SY's comment (in another thread) about aiming to have at least 100mA of current draw. Also of interest is that a 0.47uF cap will produce a 29Khz resonant peak I'm assuming the load was 500mA (as per the previous figure) PS I like the idea of using copper clad and using a dremel to remove material to make the circuit connections!

In my tests, input voltage is 15V out is 10V load is around 360mA (though I was testing originally at 180mA), and I DO have a 0.47uF cap (with no series resistance) in parallel with the 1000uF (that has 0r33) series resistance. Weird thing though is that the same freq noise spike is present with the scope probe disconnected and shorted..

On the tracking down noise sources, I have observed that some times are quieter than others. Most have been done at night, and all the lights in the house are compact fluorescents. I also forgot that when measuring with the scope that disconnecting the TV aerial made a big difference (we are in a unit with a common arial and amplifier for everyone), I'll have to try that again.

zigaflux, there is also a wireless router on the other side of the room, and certainly switching power supplies in the PC and also the HTPC (and probably in other components), plenty of places for noise to come from, and who knows what is in the unit upstairs .

George I might even try turning off the computer monintor and using the laptop to remote control with VNC to start and stop the measurements!

Tony.
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Last edited by wintermute; 20th May 2011 at 11:30 PM.
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Old 21st May 2011, 01:24 AM   #42
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OK the plot thickens with respect to the 50Hz. The preamp is definitely compromising the results in the 50Hz region. Also I opened it up and checked the chip, and it would appear that I have upgraded it at some point (even though I don't recall doing so) as it has a motorola LF347N in it, which is a relatively low noise opamp with 4Mhz bandwidth, compared to the original very pedestrian, LM324 I also found I have a TL074CN from ST micro in my junk box...

Anyway below are a series of measurements of the preamp with the scope probe switch in the ref position (shorted).

1st pic (left) no connection to sound card, (right) preamp connected 10X gain
2nd pic (left) 10X gain position, right 1X gain position.
3rd pic (left) 1X gain position, right 0.1X gain position (interestingly with this one the noise is worse in the 0.1X gain position)
4th pic (left) 0.1X gain position, (right) 0.01X gain position.
5th pic (left) 0.01X gain position, (right) no connection to sound card.

The very interesting thing about this is that 0.1 and 0.01 gain positions just attenuate the input from the scope probe.... The opamp runs in unity gain.

I'm thinking that there may be 50Hz coming in on the shield from the sound card connection and poluting the ground connection. The preamp itself runs of a 12V gell cell Lead acid battery.

The good thing is I have plenty of bandwidth to increase the gain, the bad news is, if I increase the gain to 100X I'm going to have a big problem with the 50Hz noise.... sigh...

Tony.
Attached Images
File Type: png no_connection_vs_10Xgain.png (23.5 KB, 442 views)
File Type: png preamp_10Xgain_vs_1Xgain.png (24.6 KB, 437 views)
File Type: png preamp_1Xgain_vs_0.1Xgain.png (24.0 KB, 431 views)
File Type: png preamp_0.1Xgain_vs_0.01Xgain.png (23.9 KB, 423 views)
File Type: png 0.01Xgain_vs_no_connection.png (23.8 KB, 415 views)
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Last edited by wintermute; 21st May 2011 at 01:28 AM.
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Old 21st May 2011, 01:54 AM   #43
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This makes far more sense. The ripple current will have a 100 Hz fundamental.

The sad reality of high sensitivity measurement is that it takes almost no time to do the measurement, but a LONG time to set things up properly!
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Old 21st May 2011, 04:41 AM   #44
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If it was a 100Hz fundamental then why is the 50 Hz dominant? Yes there will be 100Hz harmonics as a result of the 50Hz problem. Do note however that all the previous measurements were done with the on board sound card which doesn't seem to have as much of an issue at 50Hz (another reason I think the noise is coming down the shield from the input of the soundcard, to the output of the preamp.

I unfortunately forgot my comment from last night about disconnecting the TV aerial, and the soundcard preamp is in bits right now (cant see the chip without desoldering the BNC connectors). I will re-try with the TV aerial disconnected, I know I DID get some 50Hz free measurements early on, and I suspect I had disconnected the TV aerial.

Also whilst there will be some contamination of the 100Hz measurements those that show 20db or more at 100Hz have the actual 100Hz noise completely dominating any polution.

I will of course endeavour to get some clean measurements. I think whilst the preamp is in bits I might swap a few parts as well... perhaps up the 68n input cap to 100n (or possibly bigger if I have one handy). Also might up the decoupling cap to 47uF or swap it for a nichicon FG 10uF...

Also I just checked and I didn't change the voltage divider for the 2V reference when I changed the PS to 12V (from the design spec of 5V) I'n getting 4.4V instead of 2V. Will fix that too, I've got 0.66V dc on the output.......

Tony.
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Old 21st May 2011, 05:22 AM   #45
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While you are in "fix" mode, why don't you put an IC socket for not to solder-desolder ICs in the (near ) future?
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Old 21st May 2011, 06:20 AM   #46
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Hi George, there is already one there (though it is a low quality one). I've just run into a problem though.... I must have stuffed up the measurement of the dc on the output, I was completely not understanding what is going on there, looking at the circuit diagram again it should have had the full voltage of the divider (ie 4.4V) on the output... I changed the resistors to get the "reference 2v" and got 2V on the output... thought that was a bit odd, thought maybe it needed to be half the supply voltage (makes sense if it is a single rail the input should be half way between, if the input is half way then the output should be too! Why on earth they didn't include a dc blocking cap on the output I'm not sure, but I think I will add one in! I've potentially been feeding my sound card input with 4.4V DC, and if I change the divider to be even then 6V DC (which hasn't been connected)... I thought maybe I'd cooked the LF467N so I swapped in the TL074 and same result. I put back the original divider network and also get 4.4V on the output...

I assume if I put a few micro farads on the output all should be ok.....

Am I looking at this the right way?

Tony.
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Old 21st May 2011, 01:57 PM   #47
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well now I'm just confused... I decided to take a measurement without the battery connected. The 50Hz is still present. If I just put an extension cable on the sound card input without the preamp plugged in there is a small amount of 50Hz but not as much.

Where it gets really weird is that if I change the gain from 10X to 1X the 50Hz gets lower.

However I just turned everything off (including the monitor) The only thing running in the vicinity is the ethernet switch, and I have a much more acceptable result.

below shows comparison of 10X gain setting on preamp, compared to nothing attached to the soundcard input. scopte probe set to ref (shorted) position for the 10X measurement.

Mods I made to the preamp were:

replaced feedback caps with 47uF nichicon FG's
replaced 100uF filter cap with a rubycon YXA
replaced 10uF reference voltage cap with a 10uF nichicon FG
replaced input caps (oring 68nf wima polyester) with 100nF vishay MKP1387's
changed both resistors in the voltage divider network (for ref voltage) with 22K
added 1uF axon polyprop caps on the output (otherwise there would be 6V DC)

All this from stuff I had in my junk box,.

I don't think I'm going to completely eliminate the 50Hz problem, but I do think that this is an improvement, and If I can replicate later future results should be just that bit more "reliable"

edit: added 2nd pic to show previous 10X gain measurement compared to current. left is latest measurement


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Attached Images
File Type: png 10Xvs_no_input_modified_all_off.png (23.2 KB, 64 views)
File Type: png new_vs_old.png (20.7 KB, 62 views)
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Last edited by wintermute; 21st May 2011 at 02:11 PM.
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Old 21st May 2011, 02:39 PM   #48
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This is why I don't like PC sound card based measurements! Whenever I do PC based stuff, I first do a reality check with something like a signal generator and a resistive divider. Once I've confirmed that the analysis gives the expected levels over the frequency range, and isn't contaminated with mystery artifacts, I have at least some confidence in what it tells me about the DUT. My experience (with an EMU-0202) is that incredibly sensitive measurements can be made, but the input circuitry is a bit weird. In my case having a DC path from the input to ground is critical to making the thing work properly. There are also so many places one can change the gain settings (and I don't know what controls what) that calibration is required every time I set it up. That said, the PC can still do measurements that are difficult or impossible with all the other test equipment on my bench. Just for fun you might try measuring the noise output of a battery. Use a fully charged SLA or a fresh alkaline. The noise level should be down at the floor of your soundcard, or darn close to it.
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Old 21st May 2011, 03:03 PM   #49
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The noise level of an SLA can be measured if you have a 2 channel FFT which does cross-correlation. (Look for Boggs, Doak "Measurement of Voltage Noise in Chemical Batteries").

I am not Conrad's level of expertise, but have found as well that simple things like the layout of power supply wiring to the measurement amplifier can have a profound effect upon measurements! Shielded twisted pair, and a differential amplifier, short links, and a perusal of Ralph Morrison's "Grounding and Shielding Techniques in Instrumentation" are all helpful.

I keep coming back to the SSM2017 amplifier which WJ used in his 1995 TAA articles. The SSM2017 has been discontinued, and supplanted by the SSM2019 which is advertised as a 1nV/RtHz amplifier and available in DIP-8. A=100 gives the best bandwidth. I have the front end capacitor coupled with some low leakage back-to-back electrolytics, 49.9 ohm resistors to limit current inrush and clamp diodes. I had used batteries to drive the amplifier, but always forgot to turn the thing off, so now drive the amplifier with a Tektronix power supply, followed by the Pooge 5.51 rail drivers and 7805/7905 local regulators. It does better than the advertised spec!

You can always cheat, and do a "table sweep" omitting the line frequency and its harmonics.

Oh, turn off the flourescent lights too, and choose your time of day carefully -- electrical noise here in NNJ really seems to perk up between 5 and 8 in the evening.
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Old 21st May 2011, 05:15 PM   #50
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I am not Conrad's level of expertise
Definitely one of the more humorous things I've read today- you're definitely far beyond me with a long history of excellent posts to prove it!
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