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Scan - Rel
3-39
Scan and Rel Example
In this example we will make transfer function measurements of a device–under–test, or DUT, using the
SR844 REF OUT signal in Internal Reference mode. For this example, we assume that the DUT attenuates
the REF OUT signal. We will make measurements at several frequencies using the Scan Mode. We will
use Rel Mode to store the measured values when the DUT is by-passed (using STORE R[dBm]θ).
First, we set the frequency scan parameters to select the scan frequencies. Then we step through the scan
with the DUT by-passed, storing Rel Configurations and Values at each frequency (using
STORE R[dBm]θ). This measures the DUT input. Next we pass the signal through the DUT and step
through the frequency scan again. This measures the transfer response of the DUT directly in dB and
degrees.
If the DUT amplifies the signal, we would need to make the scan with the DUT in the signal path first,
storing Rel Values and Configurations. The second scan by-passes the DUT and measures the transfer
response (negate both the dB and phase readings). The order is determined by which configuration
(through the DUT or by-pass the DUT) has the larger output signal. This determines the required
measurement configuration at each frequency.
1
Connect the REF OUT signal (appropriately attenuated and terminated) to the input of the DUT. The
DUT output should be connected to the SIGNAL IN of the SR844.
Now by-pass the DUT, leave as much of the apparatus in place as possible. It is important to use the
same cabling for both setups (by-pass and through the DUT) in order to preserve phase delays
through the system.
2
Set the frequency scan parameters using the Scan Set key and knob as described previously in the
section Using Frequency Scans. Press CLEAR ALL [Shift– Rel Mode] to discard any previously
stored Rel Values.
3
Press Start/Step once to begin/continue the frequency scan.
4
Adjust the instrument configuration to measure the signal appropriately. Set the displays to show
R[dBm] on CH1 and θ on CH2. This measures the DUT input.
5
Press STORE R[dBm]θ. This saves the signal phase and the measured value of R[dBm] as well as
the measurement configuration to be used in the next scan.
6
Repeat steps 3–5 until the Scan is done.
7
Now pass the signal through the DUT.
8
Press Start/Step once more to turn Scan Mode off at the Stop frequency.
9
Press Rel Mode to turn on REL MODE and use the stored Rel Values. The next scan will display
R[dBm] and phase relative to the original scan taken with the DUT by-passed. This is exactly the
transfer function we started out to measure.
10
Press Start/Step once to begin/continue the frequency scan.
11
When the measurement is stable, note the readings. In dB and degrees, these give the value of the
transfer function of the DUT at the current frequency.
12
Repeat steps 10–11 until the Scan is done.
SR844 RF Lock-In Amplifier