Frequently Asked Questions

Our light choppers can be used in any orientation.

Our light choppers can be used in any orientation.

Normally you want the time constant to be several 10's or 100's of periods of the reference cycle. So with a typical 2 kHz chopper frequency you might use a time constant (TC) between 10 and 100 ms. Remember that it is the averaging caused by taking many cycles of the reference waveform multiplied by the signal input that allows the noise to be rejected. The TC setting should really be defined by how long you are prepared to wait for a valid output reading following a step change in the input signal. So if you have a scanned system that takes say 10 points of data per second, you need the measured output to have settled within a 100 ms period. This in turn defines a TC of 10 or 20 ms, on the basis that you need between two and five time-constants to get a valid reading. 

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. 

Our light choppers can be used in any orientation.

Normally you want the time constant to be several 10's or 100's of periods of the reference cycle. So with a typical 2 kHz chopper frequency you might use a time constant (TC) between 10 and 100 ms. Remember that it is the averaging caused by taking many cycles of the reference waveform multiplied by the signal input that allows the noise to be rejected. The TC setting should really be defined by how long you are prepared to wait for a valid output reading following a step change in the input signal. So if you have a scanned system that takes say 10 points of data per second, you need the measured output to have settled within a 100 ms period. This in turn defines a TC of 10 or 20 ms, on the basis that you need between two and five time-constants to get a valid reading. 

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. 

Our light choppers can be used in any orientation.

The lock-in calculates the magnitude output by using the following equation: 
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.

We are really sorry about this bug but there is an easy solution. Turn off the instrument and then press the front panel SET key. Keep this pressed while you turn on the instrument and, once the main display has appeared, release it. This process causes the internal memory to be reset and enables Spectral Display. It is only necessary to do this process once - the mode will work properly thereafter. 

Our light choppers can be used in any orientation.