# FAQ - Hardware

If you are using the curve buffer in our models 7225, 7265 or 7280 to log the output readings then this time of 100 ms also defines the sampling rate. If you wanted to see just the data at each scan point you would use a 100 ms time per point, but if you wanted to see how data changed during the scan you might use 10 or 20 ms.

MAG = SquareRoot( (X * X) + ( Y * Y) )

and the signal phase using a form of Arctangent algorithm:

PHA = ArcTangent( Y/X) when X is positive and

PHA = ArcTangent( Y/X) + 180 when X is negative

Phase (when using the non floating-pont PHA command, i.e. without a following dot) is reported in centidegrees, so a value of say -4523 is equal to -45.23 degrees.

Using the above formula and the reported values of X and Y gives:

MAG = SquareRoot( (-9169 * -9169) + ( 3993 * 3993) )

= 10000

which agrees with the MAG value from the lock-in, and

PHA = ArcTangent(3993/-9169) + 180 (because X is negative)

= ArcTangent(3993/-9169) + 180

= 156.47 degrees

which agrees with the PHA value from the lock-in.

When using Excel to perform signal phase calculations on raw X, Y values, use the function ATAN2( X, Y) rather than ATAN(Y/X). The former works in the same way as the algorithm in the lock-in. To convert the response from these functions, which is in radians, into degrees, multiply by 180/pi

Our light choppers can be used in any orientation.

If you increase your chopping rate to say 500 Hz, then you can increase the sampling rate. Of course you are limited in the end by the mechanics of the blade and the frequency response of the detector. The maximum rate at which the lock-in can output measurements depends on the computer interface, but in the case of the 7265 is 200 Hz max (5 ms per point) when using GPIB and somewhat lower when using RS232 (maybe 80 - 90 Hz). If you store data to the instrument's 32,768 point curve buffer first and then transfer it, the maximum rate is 800Hz.

In a dual phase instrument, when the Y-output is maximised the DC component of the Y output is zero. Hence if the Y output is passed to a true RMS - DC converter then the output of this will be proportional to the noise on the Y output, and thus also to that present at the input within the bandwidth set by the time constant. The value needs correcting for the measurement bandwidth to derive a noise reading in volts or amps per root hertz. The 7265 can be set to display a noise reading directly, so you can directly measure the noise at any frequency (up to 60kHz) simply by setting the lock-in to internal reference mode and the frequency as required. The measurement bandwidth can be changed by changing the time constant, but if the noise spectral density is reasonably uniform then the actual bandwidth (typically at most a few hertz) is not significant.

If you want to measure a noise spectrum (rather than noise about one frequency) then you need to arrange to step the oscillator frequency over the required range and record the noise at each step. Our Acquire data acquisition software can do this - try the free demo copy from this website.