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9 USING THE 420 AS A DUAL PHASE INSTRUMENT TO TAKE
ACCURATE MEASUREMENTS
This method uses the fact that the R channel output from the instrument is insensitive to the
phase relationship between the input signal and the reference signal.
(1) Connect up the lock-in amplifier.
(2) Switch the offset control (6) to off.
(3) Switch the 1F/2F switch (11) to that required.
(4) Switch the input sensitivity (1) to 1V.
(5) Switch the time constant (3) to 100mS.
(6) Switch both phase controls (9)(10) to 0°.
(7) Switch the output select switch (7) to R.
(8) Increase the sensitivity control (1) until a signal is seen at the output (13).
(9) If the output becomes noisy, increase the time constant setting (3) until the noise is
removed.
(10) Switch the output select switch (7) to X.
(11) Take a measurement of the output.
(12) Add 180° to the phase shift by turning the coarse phase control (9) by two positions.
(13) Take a measurement of the output.
(14) The two measurements taken in (11) and (13) above should be the inverse of each other. If
this is not the case, switch on the offset control switch (6) and adjust the X offset (4) and
repeat the measurements until this is the case.
(15) Switch the output select switch (7) to Y.
(16) Take a measurement of the output.
(17) Add 180° to the phase shift by turning the coarse phase control (9) by two positions.
(18) Take a measurement of the output.
(19) The two measurements taken in (16) and (18) above should be the inverse of each other. If
this is not the case, switch on the offset control switch (6) and adjust the Y offset (5) and
repeat the measurements until this is the case.
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