Stanford Research Systems SR844, User Manual

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Performance Tests 5-7
SR844 RF Lock-In Amplifier
2. Amplitude Response
This test measures the amplitude 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 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). We
will need 2 feed-through 20 dB attenuators (Mini-Circuits CAT-20 or equivalent) for portions of this
test.
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.
The synthesizer output amplitude accuracy is
±
0.8 dB. The attenuators and splitters have a typical
accuracy of about
±
0.5 dB. Depending upon the number of attenuators used in the signal path, the total
error in the signal input is between
±
1.8 dB and
±
2.8 dB. Coupled with the SR844 accuracy, the worst
case error in the reading may be as large as
±
2.3 and
±
3.8 dB (0.25 dB less below 50 MHz). This is
typical of RF measurements. Your measurements will typically be quite a bit more accurate.
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
twice
Set the CH1 display to R[dBm] .