Stanford Research Systems SR844, User Manual

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Performance Tests 5-9
SR844 RF Lock-In Amplifier
3. Phase Response
This test measures the phase response vs. frequency. Due to the high frequencies involved, a test of this
type is difficult to perform without specialized (and expensive) test equipment along with complex test
procedures beyond the scope of this manual. Instead, this test is designed to use a simpler procedure
which, if passed, verifies the functionality of the SR844. If the unit passes this test, then it is very probable
that the unit meets its stated accuracy. This test is
not intended to verify the accuracy of the SR844.
Setup
We will use the RF synthesizer (Marconi 2023 or equivalent) to provide both the external reference signal
and the input signal. Use an RF 2-way power splitter (Mini-Circuits ZFRSC-2050 or equivalent) to split
the synthesizer output into 2 signals. Connect
identical short cables to the splitter outputs. Connect the 2
splitter outputs as follows:
•
One to REF IN through a feed-through 3 dB attenuator (Mini-Circuits CAT-3 or equivalent).
•
One to SIGNAL IN through a feed-through 3 dB attenuator (Mini-Circuits CAT-3 or equivalent).
Set the RF synthesizer to:
Frequency: 100.0 kHz
Amplitude: +9.0 dBm
Modulation: OFF
The synthesizer 9.0 dBm, the splitter (-6 dB) and the feed-through 3 dB attenuators provide each
input with 0 dBm signals. This is the nominal signal level for the REF IN and a good reference level for
the SIGNAL IN as well.
At high frequencies the difference in path length between the two signals contributes large amounts of
phase shift. For example, even 1” of difference between the signal path and the external reference path
contributes 6
°
of phase shift. In addition, the phase imbalance of the splitter may be as high as 3
°
. Take
care to make the two signal paths as identical as possible. The results of this test are almost entirely
determined by the experimental setup. You should be able to achieve 10
°
phase matching without
difficulty. If you are unable to achieve this level of matching, try a different setup using different
components and cables.
Procedure
1) Press
Shift
then
Recall
to restore the factory preset instrument settings.
2) Press the keys in the following sequence:
Source
Switch to
EXTERNAL
Reference mode.
CH1 Display
once
Set the CH1 display to R[V] .