Gentec-EO T-Rad-USB (LEMO) - Manual (Page 12)

T-RAD-LEMO-USB Instruction Manual Version 2.0 June 2012 12
There are two BNC’s and one Lemo connector on the instrument. The probe in use is plugged
into the Lemo connector and must be plugged in before connecting the USB cable. One BNC is
labeled Detector Output, the other is labeled External Trigger In.
The Detector Output BNC supplies a buffered signal that is the same value sampled by the
instrument’s ADC. The output impedance of this connector is 100Ω.
The External Trigger Input is the connection for the reference chopping frequency. This signal’s
frequency is measured by the instrument and used to generate the reference sine wave
described in the Theory of Operation section. This BNC should be connected to the Sync Output
BNC of the chopper controller in use.
6 Using the Instrument
To use the T-Rad-USB, connect the probe to the instrument first, then plug the instrument into the
USB connector on the PC that is running the Applications Software. Connect the chopper
frequency reference to the External Trigger Input BNC. Now start the software. A window will
appear, as shown below.
Press the Proceed button. The software will examine the available communications ports and
establish communications with the instrument. When the WAIT prompt disappears from the data
display, the software and instrument are ready to use. The T-Rad-USB is not a triggered device.
It is always taking data, but it will not send data to the application until asked to do so. Pressing
the Data Collection Button causes data to be sent to the application where the lock in algorithm is
implemented. The user need only select an appropriate range and filter time constant.
The Frequency Range indicator will display the correct chop rate to use. If the range is 24.5 HZ to
25.5HZ, set the chopper to 25Hz. The instrument will extract the signal at the reference
frequency. Note that if there is any other signal with components at the reference frequency, they
will be measure along with the desired signal. The longer the filter time constant, the longer it will
take to settle on a reading, but the more noise and interference will be rejected. The user must
make a tradeoff between measurement time and accuracy when using a lock in amplifier. It will
take 5 time constants for the reading to settle to 1%, 7 time constant for the reading to settle to
0.1%. Fluctuations in the signal after this amount of time are not due to the lock in settling time
but are real signal variations.
The subsequent sections explain in detail how to use the Applications Software to makes
measurements.