Stanford Research Systems SR844, User Manual

Page: 113 / 246

Reference and Phase 4-11
SR844 RF Lock-In Amplifier
Reference and Phase Commands
FMOD(?){i}
The
FMOD
command sets or queries the Reference Mode. The parameter
i
selects Internal (
i=1
) or External (
i=0
) reference.
HARM(?){i}
The
HARM
command sets or queries 2F detect mode. The parameter
i
selects
OFF, (detect at F,
i=0
) or ON, (detect at 2F,
i=1
).
The 2F detect frequency is limited to 50 kHz to 200 MHz. The
HARM 1
command
will generate an error if the Reference Mode is Internal and the reference
frequency is below 50 kHz. The
HARM 1
command will result in an OUT OF
RANGE
indication if the Reference Mode is External and the REF IN frequency
is below 25 kHz.
FREQ(?){f}
The
FREQ
command sets or queries the detection (reference) frequency. When 2F
mode is Off, the detection frequency is the same as REF IN and REF OUT. When
2F mode is On, the detection frequency is twice REF IN and REF OUT.
The
FREQ?
query will return the detection frequency in Internal or External mode.
In Internal mode the frequency is returns the displayed reference frequency (3 digit
resolution), whereas in External mode frequency is returned to the measurement
resolution (6 or 7 digits). The instrument measures the frequency at 6–12 times a
second.
The
FREQ f
command sets the frequency of the internal reference oscillator. This
command is allowed only if the Reference Mode is Internal. The parameter
f
is a
real number that specifies the frequency in Hz. The value of
f
will be rounded to
the nearest available internal frequency. The value of
f
must be in the range 2.5E4
≤
f
≤
2.0E8 (2F Off) and 5.0E4
≤
f
≤
2.0E8 (2F On)
FRAQ? The
FRAQ?
command queries the reference frequency. The frequency is returned
with the measurement resolution (6 or 7 digits) in both External and Internal mode.
FRIQ?
The
FRIQ?
command queries the IF frequency. The returned value is an integer
with units of Hz.
The IF frequency will be in the range of approximately 2–3 kHz, for all time
constants 1 ms and longer. The SR844 uses a higher IF, approximately 8–12 kHz,
for faster time constants.
PHAS(?){x} The
PHAS
command sets or queries the detection phase, in degrees, relative to the
reference. The parameter
x
is the phase (real number of degrees) and may be
specified from
–360
≤
x
≤
360
. The value specified is rounded to 0.01
°
and
adjusted to the interval [
-
179.99 , 180.00 ].
APHS
The
APHS
command performs the Auto Phase function. This command is the same
as pressing Shift
−
Phase . This command adjusts the reference phase so that the
current measurement has a Y value of zero and an X value equal to the signal
magnitude, R.
REFZ(?){i}
The
REFZ
command sets or queries the Reference Input impedance. The
parameter
i
selects 50
Ω
(
i=0
) or 10 k
Ω
(
i=1
).