SRS Labs SR850 - Manual (Page 47)

2-35
TRACE MATH
This example is designed to familiarize the user with the trace math functions in the lock-in. You will need
BNC cables and a 50
Ω
terminator.
Specifically, you will record the input noise in a trace and perform various calculations with the trace. The
internal oscillator will also be used to provide signal data for curve fits.
There are a few points to remember about the trace math functions.
Math functions may only be performed on trace data which is stored in a trace buffer and displayed in
the active chart display. Data which are not within the time window of the chart are not operated upon. Use
the horizontal chart scale to select the size of the time window and move the cursor to pan the window to the
desired portion of the trace buffer. Data which exceeds the upper or lower display range of the graph are, of
course, operated upon.
The amount of time a math function takes to complete depends upon its complexity and the number of data
points which are operated on. For example, 25-point smoothing takes longer than 5-point smoothing while
each type of smoothing takes longer the more points there are. Do not operate on more points than
necessary.
Math functions can only be performed while data acquisition is stopped, either by pausing or finishing a scan.
Some math operations change the stored trace data. Resuming a scan after smoothing, for example, will
result in a trace where a region of the trace is smoothed and other regions are not.
1. Disconnect all cables from the lock-in. Turn
the power on while holding down the [
←
]
(backspace) key. Wait until the power-on tests
are completed.
2. Connect a 50
Ω
terminator to the A input.
3. Press [GAIN/TC]
Press <Sensitivity>
When the power is turned on with the backspace
key depressed, the lock-in returns to its default
settings. See the Default Settings list in the Menu
section for a complete listing of the settings.
The display is the "normal" lock-in display. The
lock-in setup is displayed across the top of the
screen. The upper numeric readout and bar graph
shows the value of X (Rcos
θ
) and the lower graph
shows the the value of Y (Rsin
θ
).
The input impedance of the lock-in is 10 M
Ω
. The
terminator effectively provides a short at the input
so that the input noise is predominantly the volt-
age noise of the input transistors.
The lock-in defaults to the internal oscillator refer-
ence set at 1.000 kHz. All measurements will be
made at 1 kHz in this example.
Display the Gain and Time Constant menu.
Highlight the sensitivity.
Trace Math