Gentec-EO T-Rad (DB-15) - Manual (Page 19)

T-RAD-DB15--USB Instruction Manual Version 2.1 December. 2012 19
The External Trigger Input is the connection for the reference chopping frequency. This signa l’s
frequency is measured by the instrument and used to generate the reference sine wave described in
the Theory of Operation section. This BNC must be connected to the Sync Output BNC of the
chopper controller in use before starting the Applications Software.
1.6 Using the Instrument
To use the T-RAD, connect the probe to the instrument first, then plug the instrument into the USB connector
on the PC that is running the Applications Software. For the T-RAD Monitor with a DB-15, use the PC-T-
RAD - V2.00.00 software or the most recent. Connect the chopper frequency reference to the External
Trigger Input BNC. Do not remove the External trigger while the software is running. 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 W AIT prompt disappears from the data
display, the software and instrument are ready to use. The T-RAD 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 25.5
Hz, set the chopper to 25 Hz. Caution: For stable readings the chopping frequency must be stable to +/-
0.1 Hz or better. 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.