Stanford Research Systems SR844, User Manual

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Reference Section 3-29
SR844 RF Lock-In Amplifier
The SR844 is calibrated such that an input signal that is in phase with the rising edge of
the External Reference input is measured as having X=R, Y=0 and
θ
=0
°
when the
Reference Phase is set to 0
°
. If the unit is in Internal Reference mode, there is no external
reference signal; in this case an input signal in phase with REF OUT yields X=R, Y=0
and
θ
=0
°
.
The Reference Phase may be changed to detect the signal at any phase relative to the
reference. The Reference Phase control is applied inside the DSP (a simple coordinate
rotation), which means that changing the Reference Phase does not change any of the RF,
IF or reference signals inside the SR844. The phase control is applied before the output
time-constant filtering.
+90
°
This key adds 90
°
to the Reference Phase. 360
°
is subtracted from the phase if necessary
to keep it within range –179.99
°
to +180.00
°
. This key essentially exchanges the
measured in-phase (X) and quadrature (Y) components of the signal.
–90
°
[Shift– +90
°
]
This key adds –90
°
to the Reference Phase. 360
°
is added to the phase if necessary to
keep it within range –179.99
°
to +180.00
°
. This key essentially exchanges the measured
in-phase (X) and quadrature (Y) components of the signal.
Zero
This key resets the Reference Phase to 0
°
.
AUTO
[Shift–Phase]
This key sequence automatically selects a Reference Phase that matches the phase of the
input signal. This results in a measured phase of the input signal that is close to zero.
Note that if the measured phase of the input signal is not settled or is noisy at the time
Shift-Phase is pressed, the measured phase will not settle to exactly 0
°
.
Auto phase is executed once at the time the keys are pressed. The Reference Phase will
not track changes in the phase of the input signal. However the R function always
provides the magnitude of the input signal, even as the phase moves, as long as the phase
moves slowly compared to the measurement time constant.
Reference
Mode
In
EXTERNAL
Reference mode, the SR844 locks to the signal present on the External
Reference Input.
In INTERNAL Reference mode, the SR844 generates the reference frequency using an
internal synthesizer. It is recommended to leave the External Reference input
disconnected when the unit is in internal mode.
REF OUT
In both reference modes, REF OUT provides a 1 Vpp signal (into 50
Ω
) in phase with the
reference. This signal can be used to provide the modulation necessary in the experiment.
Source
This key sets the Reference mode of the SR844, either EXTERNAL or INTERNAL .
The selected mode is shown by indicators above the key.
2F
[Shift-Source]
This key sequence toggles 2F harmonic detection.
2F
mode is shown by the indicator
above the
EXTERNAL
and
INTERNAL
indicators.
2F detection is available for both External and Internal Reference modes. In both cases,
the displayed frequency is the 2F detection frequency. The frequency of the REF OUT
signal is at F or half of the displayed detection frequency. In External mode, the
frequencies of REF IN and REF OUT are both F.