Stanford Research Systems SR844, User Manual

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3-28 Reference Section
SR844 RF Lock-In Amplifier
Freq
This key displays the reference frequency. If the reference mode is
EXTERNAL
, then
the measured external reference frequency is displayed. Measurements are made 6–12
times a second and the display is updated at 5.7 Hz. The display will be erroneous if the
instrument is
UNLOCK
ed or the frequency is
OUT OF RANGE
. Indicators on the front
panel show both of these error conditions. In external reference mode, the knob serves no
function with this display.
In INTERNAL reference mode, the internal reference frequency is displayed. The
internal reference frequency is adjusted using the knob. The SR844 offers 3 digits of
resolution in specifying the internal reference frequency, and 4 digits of accuracy. For
example, frequencies of 1.23 and 1.24 MHz may be selected. When 1.23 MHz is
selected, the actual reference frequency will be in the range 1.229 to 1.231 MHz.
Use
PRECISE FREQ
to display the frequency with more resolution (in either reference
mode).
PRECISE
FREQ
[Shift–Freq] This key sequence shows the reference frequency with 6 or 7 digits of resolution using
the CH2 and Reference displays together. Read the two displays as if they were one
single display. The
FREQ
indicator within the CH2 display turns on whenever the
Precise Frequency is displayed.
To cancel this display mode, choose another reference display (
Freq
or
Phase
for
example).
While the internal reference frequency is set to 3 digits resolution, the actual frequency
generated in internal mode may be slightly different (within
±
1 in the 4th digit).
I.F.
[Shift–
AuxOut] This key sequence shows the IF frequency on the Reference Display. See Chapter 2,
Sources of Error , for more information about the IF (chop frequency). This display is
provided as a user convenience. The instrument has weak spurious responses at offsets
of
±
2
×
IF,
±
4
×
IF, etc. from the reference frequency. Some users may wish to set up their
experiments to avoid specific IF frequencies.
When the reference is in internal mode and the IF frequency is displayed, the knob may
be used to adjust the internal
reference frequency while showing the IF frequency.
Important!
The SR844 covers the operating frequency range in octaves bands. Users can check the
IF frequency to determine whether the instrument is at the high end of an octave band or
the low end of the next band. At the high end of an octave, the IF frequency will be close
to 3 kHz (12 kHz for time constants
≤
300
µ
s), while at the low end it will be close to 2
kHz (8 kHz for time constants
≤
300
µ
s). The IF frequency affects the output update rate
for the analog CH1 and CH2 OUTPUTs. The fastest update rates occur at the high end
of each octave band (where the IF is the highest).
AuxOut This key shows the two rear panel Aux output values. Pressing AuxOut alternates
between AUX OUT 1 and AUX OUT 2. The selected output is indicated by AxOut1 or
AxOut2
within the display. The knob is used to adjust the selected output voltage within
the range
±
10.500 Volts.
Phase This key shows the phase, relative to the reference, currently being used for signal
detection. The phase is displayed in degrees (–179.99
°
to +180.00
°
). The knob is used to
adjust the phase. See below for more details.