lock-amplifier Error -200278

[johnsold]

jmoya,

Lock In amplifiers use averaging, so it may be possible to measure signals smaller than the quantization levels.  Suppose your singal was 20 uV from a nominal digitizer transition point and that your signal contains some (zero mean) noise.  Then on average about one sample in eight will be digitzed at one count higher (or lower) than the other seven.  The average of those eight samples generates the result which appears to be between quantization levels. Vave =(1/8)*Vres + Vnominal.

If you signal frequency and phase is stable over the time it takes for the scope/ELVIS to take several sets of readings, you may be able to get away with triggering the acquistion and using a simulated signal with the same sampling rate for the reference.  I have never tried this, but in principle it should work as the triggers are always at the same phase with respect to the reference.  If your signals are unstable, the lock in probably would not work very well anyway.

Lynn

[jmoya]

Thank you very much Jacob and Lynn for your answers.

I forgot to say that I am practically a beginner in Labview, but I guess that you realize that, form the kind of questions I made.

Having said that, I ask you, Lynn, why do you put as an example the average of 8 signals. It has nothing to do with my 8 bits DAC, right? On the other hand, I understand and agree with the general idea of the average signals, thank you very much.

With respect to the simulation of the reference signal, ok, I will keep trying the way to go on. And if I have not success, may be that my signal is not very stable in frequency and phase, or…, that I should learn more Labview.

Javier Moya

[johnsold]

Javier,

I chose 8 samples because that was the smallest number which wold approximately produce the 20 uV / 156 uV signal to resolution ratio you reported.  Nothing to do with the 8-bit A/D converter.

All of us were beginners at some time.  Keep working on it and thinking about what you are trying to accomplish and before long you will not be a beginner any more.

Lynn