| ionomolo |
I was thinking in a diy lock-in. I expect its build to be extremely time-consuming as it approaches an infinite-Q filter.
I have many experiments in progress and very little free time, but my idea is to start using a sound card as sine wave generator (at first only to try) and a small time constant to get less selectivity because it would allow for automated scanning and then improove everything to discover how far can i go keeping everything cheap.
It would be based on a 7.1 soundcard with one channel outputting the reference signal and two other channels outputting the harmonic to meter at 0 and 90 degrees.
My idea is that this would boost the soundcard sensitivity up to extraordinary levels since noise would be canceled due to long time integration and to use a parallel port adc i built to meter large electrolytics by fitting theyr discharge curve.
Uh, i forgot the worst problem. Do you thing of a good analog multiplier for that purpose? |
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| syn08 |
| quote: | Originally posted by ionomolo
Uh, i forgot the worst problem. Do you thing of a good analog multiplier for that purpose? |
This is indeed the most difficult problem if you would decide to build a lock in amplifier by yourself. I doubt that any integrated analog multiplier implementation will match the linearity requirements for a good noise rejection.
A digital switching multiplier is a much better solution, however I am not aware of any good IC or chipset that would implement such. |
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| Conrad Hoffman |
| Building a really good lock-in should be way easier than it used to be. I think there's even a circuit in the LT app notes, probably using the LT1043 switch. I'd check but my books are put away and my memory is fuzzy. Only problem is I'm not sure how fast it is. |
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| syn08 |
| quote: | Originally posted by Conrad Hoffman
Building a really good lock-in should be way easier than it used to be. I think there's even a circuit in the LT app notes, probably using the LT1043 switch. I'd check but my books are put away and my memory is fuzzy. Only problem is I'm not sure how fast it is. |
There is a 0.01% multiplier application in the LT1043 datasheet. But I doubt that's good enough and, as you said, the speed is also pretty low. |
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| ionomolo |
| This is really interesting, i will do some research on those multipliers and pray to find a good one. |
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| 1audio |
| Just to throw a curve at an old thread the Shibasoku distortion analyzer (725 series) has that lock in amp as part of it for harmonic analysis. It measures harmonics 2-5 and THD with a residual of .00005 or so. And the matching oscillators are good enough to get that low. |
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